{-# LANGUAGE CPP             #-}
{-# LANGUAGE MultiWayIf      #-}
{-# LANGUAGE TupleSections   #-}

{-# OPTIONS_GHC -Wno-incomplete-record-updates #-}
{-
(c) The University of Glasgow 2006
(c) The GRASP/AQUA Project, Glasgow University, 1992-1998

-}

-- | Monadic type operations
--
-- This module contains monadic operations over types that contain mutable type
-- variables.
module GHC.Tc.Utils.TcMType (
  TcTyVar, TcKind, TcType, TcTauType, TcThetaType, TcTyVarSet,

  --------------------------------
  -- Creating new mutable type variables
  newFlexiTyVar,
  newNamedFlexiTyVar,
  newFlexiTyVarTy,              -- Kind -> TcM TcType
  newFlexiTyVarTys,             -- Int -> Kind -> TcM [TcType]
  newOpenFlexiTyVar, newOpenFlexiTyVarTy, newOpenTypeKind,
  newOpenBoxedTypeKind,
  newMetaKindVar, newMetaKindVars, newMetaTyVarTyAtLevel,
  newAnonMetaTyVar, cloneMetaTyVar,
  newCycleBreakerTyVar,

  newMultiplicityVar,
  readMetaTyVar, writeMetaTyVar, writeMetaTyVarRef,
  newTauTvDetailsAtLevel, newMetaDetails, newMetaTyVarName,
  isFilledMetaTyVar_maybe, isFilledMetaTyVar, isUnfilledMetaTyVar,

  --------------------------------
  -- Creating new evidence variables
  newEvVar, newEvVars, newDict,
  newWanted, newWanteds, cloneWanted, cloneWC,
  emitWanted, emitWantedEq, emitWantedEvVar, emitWantedEvVars,
  emitDerivedEqs,
  newTcEvBinds, newNoTcEvBinds, addTcEvBind,
  emitNewExprHole,

  newCoercionHole, fillCoercionHole, isFilledCoercionHole,
  unpackCoercionHole, unpackCoercionHole_maybe,
  checkCoercionHole,

  newImplication,

  --------------------------------
  -- Instantiation
  newMetaTyVars, newMetaTyVarX, newMetaTyVarsX,
  newMetaTyVarTyVarX,
  newTyVarTyVar, cloneTyVarTyVar,
  newPatSigTyVar, newSkolemTyVar, newWildCardX,

  --------------------------------
  -- Expected types
  ExpType(..), ExpSigmaType, ExpRhoType,
  mkCheckExpType, newInferExpType, tcInfer,
  readExpType, readExpType_maybe, readScaledExpType,
  expTypeToType, scaledExpTypeToType,
  checkingExpType_maybe, checkingExpType,
  inferResultToType, fillInferResult, promoteTcType,

  --------------------------------
  -- Zonking and tidying
  zonkTidyTcType, zonkTidyTcTypes, zonkTidyOrigin,
  tidyEvVar, tidyCt, tidyHole, tidySkolemInfo,
    zonkTcTyVar, zonkTcTyVars,
  zonkTcTyVarToTyVar, zonkInvisTVBinder,
  zonkTyCoVarsAndFV, zonkTcTypeAndFV, zonkDTyCoVarSetAndFV,
  zonkTyCoVarsAndFVList,

  zonkTcType, zonkTcTypes, zonkCo,
  zonkTyCoVarKind, zonkTyCoVarKindBinder,
  zonkEvVar, zonkWC, zonkImplication, zonkSimples,
  zonkId, zonkCoVar,
  zonkCt, zonkSkolemInfo,

  ---------------------------------
  -- Promotion, defaulting, skolemisation
  defaultTyVar, promoteMetaTyVarTo, promoteTyVarSet,
  quantifyTyVars, isQuantifiableTv,
  skolemiseUnboundMetaTyVar, zonkAndSkolemise, skolemiseQuantifiedTyVar,
  doNotQuantifyTyVars,

  candidateQTyVarsOfType,  candidateQTyVarsOfKind,
  candidateQTyVarsOfTypes, candidateQTyVarsOfKinds,
  CandidatesQTvs(..), delCandidates,
  candidateKindVars, partitionCandidates,

  ------------------------------
  -- Levity polymorphism
  ensureNotLevPoly, checkForLevPoly, checkForLevPolyX, formatLevPolyErr
  ) where

#include "HsVersions.h"

-- friends:
import GHC.Prelude

import {-# SOURCE #-}   GHC.Tc.Utils.Unify( unifyType {- , unifyKind -} )

import GHC.Core.TyCo.Rep
import GHC.Core.TyCo.Ppr
import GHC.Tc.Utils.TcType
import GHC.Core.Type
import GHC.Core.TyCon
import GHC.Core.Coercion
import GHC.Core.Class
import GHC.Types.Var
import GHC.Core.Predicate
import GHC.Tc.Types.Origin

-- others:
import GHC.Tc.Utils.Monad        -- TcType, amongst others
import GHC.Tc.Types.Constraint
import GHC.Tc.Types.Evidence
import GHC.Types.Id as Id
import GHC.Types.Name
import GHC.Types.Var.Set
import GHC.Builtin.Types
import GHC.Types.Var.Env
import GHC.Types.Name.Env
import GHC.Utils.Misc
import GHC.Utils.Outputable
import GHC.Utils.Panic
import GHC.Data.FastString
import GHC.Data.Bag
import GHC.Data.Pair
import GHC.Types.Unique.Set
import GHC.Driver.Session
import GHC.Driver.Ppr
import qualified GHC.LanguageExtensions as LangExt
import GHC.Types.Basic ( TypeOrKind(..) )

import Control.Monad
import GHC.Data.Maybe
import qualified Data.Semigroup as Semi

{-
************************************************************************
*                                                                      *
        Kind variables
*                                                                      *
************************************************************************
-}

newMetaKindVar :: TcM TcKind
newMetaKindVar :: TcM Kind
newMetaKindVar
  = do { TcTyVarDetails
details <- MetaInfo -> TcM TcTyVarDetails
newMetaDetails MetaInfo
TauTv
       ; Name
name    <- FastString -> TcM Name
newMetaTyVarName (String -> FastString
fsLit String
"k")
                    -- All MetaKindVars are called "k"
                    -- They may be jiggled by tidying
       ; let kv :: TyVar
kv = Name -> Kind -> TcTyVarDetails -> TyVar
mkTcTyVar Name
name Kind
liftedTypeKind TcTyVarDetails
details
       ; String -> SDoc -> TcRn ()
traceTc String
"newMetaKindVar" (TyVar -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyVar
kv)
       ; Kind -> TcM Kind
forall (m :: * -> *) a. Monad m => a -> m a
return (TyVar -> Kind
mkTyVarTy TyVar
kv) }

newMetaKindVars :: Int -> TcM [TcKind]
newMetaKindVars :: Int -> TcM [Kind]
newMetaKindVars Int
n = Int -> TcM Kind -> TcM [Kind]
forall (m :: * -> *) a. Applicative m => Int -> m a -> m [a]
replicateM Int
n TcM Kind
newMetaKindVar

{-
************************************************************************
*                                                                      *
     Evidence variables; range over constraints we can abstract over
*                                                                      *
************************************************************************
-}

newEvVars :: TcThetaType -> TcM [EvVar]
newEvVars :: [Kind] -> TcM [TyVar]
newEvVars [Kind]
theta = (Kind -> IOEnv (Env TcGblEnv TcLclEnv) TyVar)
-> [Kind] -> TcM [TyVar]
forall (t :: * -> *) (m :: * -> *) a b.
(Traversable t, Monad m) =>
(a -> m b) -> t a -> m (t b)
mapM Kind -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
forall gbl lcl. Kind -> TcRnIf gbl lcl TyVar
newEvVar [Kind]
theta

--------------

newEvVar :: TcPredType -> TcRnIf gbl lcl EvVar
-- Creates new *rigid* variables for predicates
newEvVar :: forall gbl lcl. Kind -> TcRnIf gbl lcl TyVar
newEvVar Kind
ty = do { Name
name <- OccName -> TcRnIf gbl lcl Name
forall gbl lcl. OccName -> TcRnIf gbl lcl Name
newSysName (Kind -> OccName
predTypeOccName Kind
ty)
                 ; TyVar -> TcRnIf gbl lcl TyVar
forall (m :: * -> *) a. Monad m => a -> m a
return (Name -> Kind -> Kind -> TyVar
mkLocalIdOrCoVar Name
name Kind
Many Kind
ty) }

newWanted :: CtOrigin -> Maybe TypeOrKind -> PredType -> TcM CtEvidence
-- Deals with both equality and non-equality predicates
newWanted :: CtOrigin -> Maybe TypeOrKind -> Kind -> TcM CtEvidence
newWanted CtOrigin
orig Maybe TypeOrKind
t_or_k Kind
pty
  = do CtLoc
loc <- CtOrigin -> Maybe TypeOrKind -> TcM CtLoc
getCtLocM CtOrigin
orig Maybe TypeOrKind
t_or_k
       TcEvDest
d <- if Kind -> Bool
isEqPrimPred Kind
pty then CoercionHole -> TcEvDest
HoleDest  (CoercionHole -> TcEvDest)
-> IOEnv (Env TcGblEnv TcLclEnv) CoercionHole
-> IOEnv (Env TcGblEnv TcLclEnv) TcEvDest
forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
<$> Kind -> IOEnv (Env TcGblEnv TcLclEnv) CoercionHole
newCoercionHole Kind
pty
                                else TyVar -> TcEvDest
EvVarDest (TyVar -> TcEvDest)
-> IOEnv (Env TcGblEnv TcLclEnv) TyVar
-> IOEnv (Env TcGblEnv TcLclEnv) TcEvDest
forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
<$> Kind -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
forall gbl lcl. Kind -> TcRnIf gbl lcl TyVar
newEvVar Kind
pty
       CtEvidence -> TcM CtEvidence
forall (m :: * -> *) a. Monad m => a -> m a
return (CtEvidence -> TcM CtEvidence) -> CtEvidence -> TcM CtEvidence
forall a b. (a -> b) -> a -> b
$ CtWanted { ctev_dest :: TcEvDest
ctev_dest = TcEvDest
d
                         , ctev_pred :: Kind
ctev_pred = Kind
pty
                         , ctev_nosh :: ShadowInfo
ctev_nosh = ShadowInfo
WDeriv
                         , ctev_loc :: CtLoc
ctev_loc = CtLoc
loc }

newWanteds :: CtOrigin -> ThetaType -> TcM [CtEvidence]
newWanteds :: CtOrigin -> [Kind] -> TcM [CtEvidence]
newWanteds CtOrigin
orig = (Kind -> TcM CtEvidence) -> [Kind] -> TcM [CtEvidence]
forall (t :: * -> *) (m :: * -> *) a b.
(Traversable t, Monad m) =>
(a -> m b) -> t a -> m (t b)
mapM (CtOrigin -> Maybe TypeOrKind -> Kind -> TcM CtEvidence
newWanted CtOrigin
orig Maybe TypeOrKind
forall a. Maybe a
Nothing)

----------------------------------------------
-- Cloning constraints
----------------------------------------------

cloneWanted :: Ct -> TcM Ct
cloneWanted :: Ct -> TcM Ct
cloneWanted Ct
ct
  | ev :: CtEvidence
ev@(CtWanted { ctev_pred :: CtEvidence -> Kind
ctev_pred = Kind
pty }) <- Ct -> CtEvidence
ctEvidence Ct
ct
  = do { CoercionHole
co_hole <- Kind -> IOEnv (Env TcGblEnv TcLclEnv) CoercionHole
newCoercionHole Kind
pty
       ; Ct -> TcM Ct
forall (m :: * -> *) a. Monad m => a -> m a
return (CtEvidence -> Ct
mkNonCanonical (CtEvidence
ev { ctev_dest :: TcEvDest
ctev_dest = CoercionHole -> TcEvDest
HoleDest CoercionHole
co_hole })) }
  | Bool
otherwise
  = Ct -> TcM Ct
forall (m :: * -> *) a. Monad m => a -> m a
return Ct
ct

cloneWC :: WantedConstraints -> TcM WantedConstraints
-- Clone all the evidence bindings in
--   a) the ic_bind field of any implications
--   b) the CoercionHoles of any wanted constraints
-- so that solving the WantedConstraints will not have any visible side
-- effect, /except/ from causing unifications
cloneWC :: WantedConstraints -> TcM WantedConstraints
cloneWC wc :: WantedConstraints
wc@(WC { wc_simple :: WantedConstraints -> Bag Ct
wc_simple = Bag Ct
simples, wc_impl :: WantedConstraints -> Bag Implication
wc_impl = Bag Implication
implics })
  = do { Bag Ct
simples' <- (Ct -> TcM Ct) -> Bag Ct -> IOEnv (Env TcGblEnv TcLclEnv) (Bag Ct)
forall (m :: * -> *) a b.
Monad m =>
(a -> m b) -> Bag a -> m (Bag b)
mapBagM Ct -> TcM Ct
cloneWanted Bag Ct
simples
       ; Bag Implication
implics' <- (Implication -> IOEnv (Env TcGblEnv TcLclEnv) Implication)
-> Bag Implication
-> IOEnv (Env TcGblEnv TcLclEnv) (Bag Implication)
forall (m :: * -> *) a b.
Monad m =>
(a -> m b) -> Bag a -> m (Bag b)
mapBagM Implication -> IOEnv (Env TcGblEnv TcLclEnv) Implication
cloneImplication Bag Implication
implics
       ; WantedConstraints -> TcM WantedConstraints
forall (m :: * -> *) a. Monad m => a -> m a
return (WantedConstraints
wc { wc_simple :: Bag Ct
wc_simple = Bag Ct
simples', wc_impl :: Bag Implication
wc_impl = Bag Implication
implics' }) }

cloneImplication :: Implication -> TcM Implication
cloneImplication :: Implication -> IOEnv (Env TcGblEnv TcLclEnv) Implication
cloneImplication implic :: Implication
implic@(Implic { ic_binds :: Implication -> EvBindsVar
ic_binds = EvBindsVar
binds, ic_wanted :: Implication -> WantedConstraints
ic_wanted = WantedConstraints
inner_wanted })
  = do { EvBindsVar
binds'        <- EvBindsVar -> TcM EvBindsVar
cloneEvBindsVar EvBindsVar
binds
       ; WantedConstraints
inner_wanted' <- WantedConstraints -> TcM WantedConstraints
cloneWC WantedConstraints
inner_wanted
       ; Implication -> IOEnv (Env TcGblEnv TcLclEnv) Implication
forall (m :: * -> *) a. Monad m => a -> m a
return (Implication
implic { ic_binds :: EvBindsVar
ic_binds = EvBindsVar
binds', ic_wanted :: WantedConstraints
ic_wanted = WantedConstraints
inner_wanted' }) }

----------------------------------------------
-- Emitting constraints
----------------------------------------------

-- | Emits a new Wanted. Deals with both equalities and non-equalities.
emitWanted :: CtOrigin -> TcPredType -> TcM EvTerm
emitWanted :: CtOrigin -> Kind -> TcM EvTerm
emitWanted CtOrigin
origin Kind
pty
  = do { CtEvidence
ev <- CtOrigin -> Maybe TypeOrKind -> Kind -> TcM CtEvidence
newWanted CtOrigin
origin Maybe TypeOrKind
forall a. Maybe a
Nothing Kind
pty
       ; Ct -> TcRn ()
emitSimple (Ct -> TcRn ()) -> Ct -> TcRn ()
forall a b. (a -> b) -> a -> b
$ CtEvidence -> Ct
mkNonCanonical CtEvidence
ev
       ; EvTerm -> TcM EvTerm
forall (m :: * -> *) a. Monad m => a -> m a
return (EvTerm -> TcM EvTerm) -> EvTerm -> TcM EvTerm
forall a b. (a -> b) -> a -> b
$ CtEvidence -> EvTerm
ctEvTerm CtEvidence
ev }

emitDerivedEqs :: CtOrigin -> [(TcType,TcType)] -> TcM ()
-- Emit some new derived nominal equalities
emitDerivedEqs :: CtOrigin -> [(Kind, Kind)] -> TcRn ()
emitDerivedEqs CtOrigin
origin [(Kind, Kind)]
pairs
  | [(Kind, Kind)] -> Bool
forall (t :: * -> *) a. Foldable t => t a -> Bool
null [(Kind, Kind)]
pairs
  = () -> TcRn ()
forall (m :: * -> *) a. Monad m => a -> m a
return ()
  | Bool
otherwise
  = do { CtLoc
loc <- CtOrigin -> Maybe TypeOrKind -> TcM CtLoc
getCtLocM CtOrigin
origin Maybe TypeOrKind
forall a. Maybe a
Nothing
       ; Bag Ct -> TcRn ()
emitSimples ([Ct] -> Bag Ct
forall a. [a] -> Bag a
listToBag (((Kind, Kind) -> Ct) -> [(Kind, Kind)] -> [Ct]
forall a b. (a -> b) -> [a] -> [b]
map (CtLoc -> (Kind, Kind) -> Ct
mk_one CtLoc
loc) [(Kind, Kind)]
pairs)) }
  where
    mk_one :: CtLoc -> (Kind, Kind) -> Ct
mk_one CtLoc
loc (Kind
ty1, Kind
ty2)
       = CtEvidence -> Ct
mkNonCanonical (CtEvidence -> Ct) -> CtEvidence -> Ct
forall a b. (a -> b) -> a -> b
$
         CtDerived { ctev_pred :: Kind
ctev_pred = Kind -> Kind -> Kind
mkPrimEqPred Kind
ty1 Kind
ty2
                   , ctev_loc :: CtLoc
ctev_loc = CtLoc
loc }

-- | Emits a new equality constraint
emitWantedEq :: CtOrigin -> TypeOrKind -> Role -> TcType -> TcType -> TcM Coercion
emitWantedEq :: CtOrigin -> TypeOrKind -> Role -> Kind -> Kind -> TcM Coercion
emitWantedEq CtOrigin
origin TypeOrKind
t_or_k Role
role Kind
ty1 Kind
ty2
  = do { CoercionHole
hole <- Kind -> IOEnv (Env TcGblEnv TcLclEnv) CoercionHole
newCoercionHole Kind
pty
       ; CtLoc
loc <- CtOrigin -> Maybe TypeOrKind -> TcM CtLoc
getCtLocM CtOrigin
origin (TypeOrKind -> Maybe TypeOrKind
forall a. a -> Maybe a
Just TypeOrKind
t_or_k)
       ; Ct -> TcRn ()
emitSimple (Ct -> TcRn ()) -> Ct -> TcRn ()
forall a b. (a -> b) -> a -> b
$ CtEvidence -> Ct
mkNonCanonical (CtEvidence -> Ct) -> CtEvidence -> Ct
forall a b. (a -> b) -> a -> b
$
         CtWanted { ctev_pred :: Kind
ctev_pred = Kind
pty, ctev_dest :: TcEvDest
ctev_dest = CoercionHole -> TcEvDest
HoleDest CoercionHole
hole
                  , ctev_nosh :: ShadowInfo
ctev_nosh = ShadowInfo
WDeriv, ctev_loc :: CtLoc
ctev_loc = CtLoc
loc }
       ; Coercion -> TcM Coercion
forall (m :: * -> *) a. Monad m => a -> m a
return (CoercionHole -> Coercion
HoleCo CoercionHole
hole) }
  where
    pty :: Kind
pty = Role -> Kind -> Kind -> Kind
mkPrimEqPredRole Role
role Kind
ty1 Kind
ty2

-- | Creates a new EvVar and immediately emits it as a Wanted.
-- No equality predicates here.
emitWantedEvVar :: CtOrigin -> TcPredType -> TcM EvVar
emitWantedEvVar :: CtOrigin -> Kind -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
emitWantedEvVar CtOrigin
origin Kind
ty
  = do { TyVar
new_cv <- Kind -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
forall gbl lcl. Kind -> TcRnIf gbl lcl TyVar
newEvVar Kind
ty
       ; CtLoc
loc <- CtOrigin -> Maybe TypeOrKind -> TcM CtLoc
getCtLocM CtOrigin
origin Maybe TypeOrKind
forall a. Maybe a
Nothing
       ; let ctev :: CtEvidence
ctev = CtWanted { ctev_dest :: TcEvDest
ctev_dest = TyVar -> TcEvDest
EvVarDest TyVar
new_cv
                             , ctev_pred :: Kind
ctev_pred = Kind
ty
                             , ctev_nosh :: ShadowInfo
ctev_nosh = ShadowInfo
WDeriv
                             , ctev_loc :: CtLoc
ctev_loc  = CtLoc
loc }
       ; Ct -> TcRn ()
emitSimple (Ct -> TcRn ()) -> Ct -> TcRn ()
forall a b. (a -> b) -> a -> b
$ CtEvidence -> Ct
mkNonCanonical CtEvidence
ctev
       ; TyVar -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
forall (m :: * -> *) a. Monad m => a -> m a
return TyVar
new_cv }

emitWantedEvVars :: CtOrigin -> [TcPredType] -> TcM [EvVar]
emitWantedEvVars :: CtOrigin -> [Kind] -> TcM [TyVar]
emitWantedEvVars CtOrigin
orig = (Kind -> IOEnv (Env TcGblEnv TcLclEnv) TyVar)
-> [Kind] -> TcM [TyVar]
forall (t :: * -> *) (m :: * -> *) a b.
(Traversable t, Monad m) =>
(a -> m b) -> t a -> m (t b)
mapM (CtOrigin -> Kind -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
emitWantedEvVar CtOrigin
orig)

-- | Emit a new wanted expression hole
emitNewExprHole :: OccName         -- of the hole
                -> Type -> TcM HoleExprRef
emitNewExprHole :: OccName -> Kind -> TcM HoleExprRef
emitNewExprHole OccName
occ Kind
ty
  = do { Unique
u <- TcRnIf TcGblEnv TcLclEnv Unique
forall gbl lcl. TcRnIf gbl lcl Unique
newUnique
       ; TcRef EvTerm
ref <- EvTerm -> TcRnIf TcGblEnv TcLclEnv (TcRef EvTerm)
forall a gbl lcl. a -> TcRnIf gbl lcl (TcRef a)
newTcRef (String -> SDoc -> EvTerm
forall a. HasCallStack => String -> SDoc -> a
pprPanic String
"unfilled unbound-variable evidence" (Unique -> SDoc
forall a. Outputable a => a -> SDoc
ppr Unique
u))
       ; let her :: HoleExprRef
her = TcRef EvTerm -> Kind -> Unique -> HoleExprRef
HER TcRef EvTerm
ref Kind
ty Unique
u

       ; CtLoc
loc <- CtOrigin -> Maybe TypeOrKind -> TcM CtLoc
getCtLocM (OccName -> CtOrigin
ExprHoleOrigin OccName
occ) (TypeOrKind -> Maybe TypeOrKind
forall a. a -> Maybe a
Just TypeOrKind
TypeLevel)

       ; let hole :: Hole
hole = Hole { hole_sort :: HoleSort
hole_sort = HoleExprRef -> HoleSort
ExprHole HoleExprRef
her
                         , hole_occ :: OccName
hole_occ  = OccName
occ
                         , hole_ty :: Kind
hole_ty   = Kind
ty
                         , hole_loc :: CtLoc
hole_loc  = CtLoc
loc }
       ; Hole -> TcRn ()
emitHole Hole
hole
       ; HoleExprRef -> TcM HoleExprRef
forall (m :: * -> *) a. Monad m => a -> m a
return HoleExprRef
her }

newDict :: Class -> [TcType] -> TcM DictId
newDict :: Class -> [Kind] -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
newDict Class
cls [Kind]
tys
  = do { Name
name <- OccName -> TcM Name
forall gbl lcl. OccName -> TcRnIf gbl lcl Name
newSysName (OccName -> OccName
mkDictOcc (Class -> OccName
forall a. NamedThing a => a -> OccName
getOccName Class
cls))
       ; TyVar -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
forall (m :: * -> *) a. Monad m => a -> m a
return (HasDebugCallStack => Name -> Kind -> Kind -> TyVar
Name -> Kind -> Kind -> TyVar
mkLocalId Name
name Kind
Many (Class -> [Kind] -> Kind
mkClassPred Class
cls [Kind]
tys)) }

predTypeOccName :: PredType -> OccName
predTypeOccName :: Kind -> OccName
predTypeOccName Kind
ty = case Kind -> Pred
classifyPredType Kind
ty of
    ClassPred Class
cls [Kind]
_ -> OccName -> OccName
mkDictOcc (Class -> OccName
forall a. NamedThing a => a -> OccName
getOccName Class
cls)
    EqPred {}       -> FastString -> OccName
mkVarOccFS (String -> FastString
fsLit String
"co")
    IrredPred {}    -> FastString -> OccName
mkVarOccFS (String -> FastString
fsLit String
"irred")
    ForAllPred {}   -> FastString -> OccName
mkVarOccFS (String -> FastString
fsLit String
"df")

-- | Create a new 'Implication' with as many sensible defaults for its fields
-- as possible. Note that the 'ic_tclvl', 'ic_binds', and 'ic_info' fields do
-- /not/ have sensible defaults, so they are initialized with lazy thunks that
-- will 'panic' if forced, so one should take care to initialize these fields
-- after creation.
--
-- This is monadic to look up the 'TcLclEnv', which is used to initialize
-- 'ic_env', and to set the -Winaccessible-code flag. See
-- Note [Avoid -Winaccessible-code when deriving] in "GHC.Tc.TyCl.Instance".
newImplication :: TcM Implication
newImplication :: IOEnv (Env TcGblEnv TcLclEnv) Implication
newImplication
  = do TcLclEnv
env <- TcRnIf TcGblEnv TcLclEnv TcLclEnv
forall gbl lcl. TcRnIf gbl lcl lcl
getLclEnv
       Bool
warn_inaccessible <- WarningFlag -> TcRnIf TcGblEnv TcLclEnv Bool
forall gbl lcl. WarningFlag -> TcRnIf gbl lcl Bool
woptM WarningFlag
Opt_WarnInaccessibleCode
       Implication -> IOEnv (Env TcGblEnv TcLclEnv) Implication
forall (m :: * -> *) a. Monad m => a -> m a
return (Implication
implicationPrototype { ic_env :: TcLclEnv
ic_env = TcLclEnv
env
                                    , ic_warn_inaccessible :: Bool
ic_warn_inaccessible = Bool
warn_inaccessible })

{-
************************************************************************
*                                                                      *
        Coercion holes
*                                                                      *
************************************************************************
-}

newCoercionHole :: TcPredType -> TcM CoercionHole
newCoercionHole :: Kind -> IOEnv (Env TcGblEnv TcLclEnv) CoercionHole
newCoercionHole Kind
pred_ty
  = do { TyVar
co_var <- Kind -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
forall gbl lcl. Kind -> TcRnIf gbl lcl TyVar
newEvVar Kind
pred_ty
       ; String -> SDoc -> TcRn ()
traceTc String
"New coercion hole:" (TyVar -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyVar
co_var)
       ; IORef (Maybe Coercion)
ref <- Maybe Coercion
-> IOEnv (Env TcGblEnv TcLclEnv) (IORef (Maybe Coercion))
forall a env. a -> IOEnv env (IORef a)
newMutVar Maybe Coercion
forall a. Maybe a
Nothing
       ; CoercionHole -> IOEnv (Env TcGblEnv TcLclEnv) CoercionHole
forall (m :: * -> *) a. Monad m => a -> m a
return (CoercionHole -> IOEnv (Env TcGblEnv TcLclEnv) CoercionHole)
-> CoercionHole -> IOEnv (Env TcGblEnv TcLclEnv) CoercionHole
forall a b. (a -> b) -> a -> b
$ CoercionHole { ch_co_var :: TyVar
ch_co_var = TyVar
co_var, ch_ref :: IORef (Maybe Coercion)
ch_ref = IORef (Maybe Coercion)
ref } }

-- | Put a value in a coercion hole
fillCoercionHole :: CoercionHole -> Coercion -> TcM ()
fillCoercionHole :: CoercionHole -> Coercion -> TcRn ()
fillCoercionHole (CoercionHole { ch_ref :: CoercionHole -> IORef (Maybe Coercion)
ch_ref = IORef (Maybe Coercion)
ref, ch_co_var :: CoercionHole -> TyVar
ch_co_var = TyVar
cv }) Coercion
co
  = do {
#if defined(DEBUG)
       ; cts <- readTcRef ref
       ; whenIsJust cts $ \old_co ->
         pprPanic "Filling a filled coercion hole" (ppr cv $$ ppr co $$ ppr old_co)
#endif
       ; String -> SDoc -> TcRn ()
traceTc String
"Filling coercion hole" (TyVar -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyVar
cv SDoc -> SDoc -> SDoc
<+> String -> SDoc
text String
":=" SDoc -> SDoc -> SDoc
<+> Coercion -> SDoc
forall a. Outputable a => a -> SDoc
ppr Coercion
co)
       ; IORef (Maybe Coercion) -> Maybe Coercion -> TcRn ()
forall a gbl lcl. TcRef a -> a -> TcRnIf gbl lcl ()
writeTcRef IORef (Maybe Coercion)
ref (Coercion -> Maybe Coercion
forall a. a -> Maybe a
Just Coercion
co) }

-- | Is a coercion hole filled in?
isFilledCoercionHole :: CoercionHole -> TcM Bool
isFilledCoercionHole :: CoercionHole -> TcRnIf TcGblEnv TcLclEnv Bool
isFilledCoercionHole (CoercionHole { ch_ref :: CoercionHole -> IORef (Maybe Coercion)
ch_ref = IORef (Maybe Coercion)
ref }) = Maybe Coercion -> Bool
forall a. Maybe a -> Bool
isJust (Maybe Coercion -> Bool)
-> IOEnv (Env TcGblEnv TcLclEnv) (Maybe Coercion)
-> TcRnIf TcGblEnv TcLclEnv Bool
forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
<$> IORef (Maybe Coercion)
-> IOEnv (Env TcGblEnv TcLclEnv) (Maybe Coercion)
forall a gbl lcl. TcRef a -> TcRnIf gbl lcl a
readTcRef IORef (Maybe Coercion)
ref

-- | Retrieve the contents of a coercion hole. Panics if the hole
-- is unfilled
unpackCoercionHole :: CoercionHole -> TcM Coercion
unpackCoercionHole :: CoercionHole -> TcM Coercion
unpackCoercionHole CoercionHole
hole
  = do { Maybe Coercion
contents <- CoercionHole -> IOEnv (Env TcGblEnv TcLclEnv) (Maybe Coercion)
unpackCoercionHole_maybe CoercionHole
hole
       ; case Maybe Coercion
contents of
           Just Coercion
co -> Coercion -> TcM Coercion
forall (m :: * -> *) a. Monad m => a -> m a
return Coercion
co
           Maybe Coercion
Nothing -> String -> SDoc -> TcM Coercion
forall a. HasCallStack => String -> SDoc -> a
pprPanic String
"Unfilled coercion hole" (CoercionHole -> SDoc
forall a. Outputable a => a -> SDoc
ppr CoercionHole
hole) }

-- | Retrieve the contents of a coercion hole, if it is filled
unpackCoercionHole_maybe :: CoercionHole -> TcM (Maybe Coercion)
unpackCoercionHole_maybe :: CoercionHole -> IOEnv (Env TcGblEnv TcLclEnv) (Maybe Coercion)
unpackCoercionHole_maybe (CoercionHole { ch_ref :: CoercionHole -> IORef (Maybe Coercion)
ch_ref = IORef (Maybe Coercion)
ref }) = IORef (Maybe Coercion)
-> IOEnv (Env TcGblEnv TcLclEnv) (Maybe Coercion)
forall a gbl lcl. TcRef a -> TcRnIf gbl lcl a
readTcRef IORef (Maybe Coercion)
ref

-- | Check that a coercion is appropriate for filling a hole. (The hole
-- itself is needed only for printing.)
-- Always returns the checked coercion, but this return value is necessary
-- so that the input coercion is forced only when the output is forced.
checkCoercionHole :: CoVar -> Coercion -> TcM Coercion
checkCoercionHole :: TyVar -> Coercion -> TcM Coercion
checkCoercionHole TyVar
cv Coercion
co
  | Bool
debugIsOn
  = do { Kind
cv_ty <- Kind -> TcM Kind
zonkTcType (TyVar -> Kind
varType TyVar
cv)
                  -- co is already zonked, but cv might not be
       ; Coercion -> TcM Coercion
forall (m :: * -> *) a. Monad m => a -> m a
return (Coercion -> TcM Coercion) -> Coercion -> TcM Coercion
forall a b. (a -> b) -> a -> b
$
         ASSERT2( ok cv_ty
                , (text "Bad coercion hole" <+>
                   ppr cv <> colon <+> vcat [ ppr t1, ppr t2, ppr role
                                            , ppr cv_ty ]) )
         Coercion
co }
  | Bool
otherwise
  = Coercion -> TcM Coercion
forall (m :: * -> *) a. Monad m => a -> m a
return Coercion
co

  where
    (Pair Kind
t1 Kind
t2, Role
role) = Coercion -> (Pair Kind, Role)
coercionKindRole Coercion
co
    ok :: Kind -> Bool
ok Kind
cv_ty | EqPred EqRel
cv_rel Kind
cv_t1 Kind
cv_t2 <- Kind -> Pred
classifyPredType Kind
cv_ty
             =  Kind
t1 Kind -> Kind -> Bool
`eqType` Kind
cv_t1
             Bool -> Bool -> Bool
&& Kind
t2 Kind -> Kind -> Bool
`eqType` Kind
cv_t2
             Bool -> Bool -> Bool
&& Role
role Role -> Role -> Bool
forall a. Eq a => a -> a -> Bool
== EqRel -> Role
eqRelRole EqRel
cv_rel
             | Bool
otherwise
             = Bool
False

{- **********************************************************************
*
                      ExpType functions
*
********************************************************************** -}

{- Note [ExpType]
~~~~~~~~~~~~~~~~~
An ExpType is used as the "expected type" when type-checking an expression.
An ExpType can hold a "hole" that can be filled in by the type-checker.
This allows us to have one tcExpr that works in both checking mode and
synthesis mode (that is, bidirectional type-checking). Previously, this
was achieved by using ordinary unification variables, but we don't need
or want that generality. (For example, #11397 was caused by doing the
wrong thing with unification variables.) Instead, we observe that these
holes should

1. never be nested
2. never appear as the type of a variable
3. be used linearly (never be duplicated)

By defining ExpType, separately from Type, we can achieve goals 1 and 2
statically.

See also [wiki:typechecking]

Note [TcLevel of ExpType]
~~~~~~~~~~~~~~~~~~~~~~~~~
Consider

  data G a where
    MkG :: G Bool

  foo MkG = True

This is a classic untouchable-variable / ambiguous GADT return type
scenario. But, with ExpTypes, we'll be inferring the type of the RHS.
We thus must track a TcLevel in an Inferring ExpType. If we try to
fill the ExpType and find that the TcLevels don't work out, we fill
the ExpType with a tau-tv at the low TcLevel, hopefully to be worked
out later by some means -- see fillInferResult, and Note [fillInferResult]

This behaviour triggered in test gadt/gadt-escape1.
-}

-- actual data definition is in GHC.Tc.Utils.TcType

newInferExpType :: TcM ExpType
newInferExpType :: TcM ExpType
newInferExpType
  = do { Unique
u <- TcRnIf TcGblEnv TcLclEnv Unique
forall gbl lcl. TcRnIf gbl lcl Unique
newUnique
       ; TcLevel
tclvl <- TcM TcLevel
getTcLevel
       ; String -> SDoc -> TcRn ()
traceTc String
"newInferExpType" (Unique -> SDoc
forall a. Outputable a => a -> SDoc
ppr Unique
u SDoc -> SDoc -> SDoc
<+> TcLevel -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcLevel
tclvl)
       ; IORef (Maybe Kind)
ref <- Maybe Kind -> IOEnv (Env TcGblEnv TcLclEnv) (IORef (Maybe Kind))
forall a env. a -> IOEnv env (IORef a)
newMutVar Maybe Kind
forall a. Maybe a
Nothing
       ; ExpType -> TcM ExpType
forall (m :: * -> *) a. Monad m => a -> m a
return (InferResult -> ExpType
Infer (IR { ir_uniq :: Unique
ir_uniq = Unique
u, ir_lvl :: TcLevel
ir_lvl = TcLevel
tclvl
                           , ir_ref :: IORef (Maybe Kind)
ir_ref = IORef (Maybe Kind)
ref })) }

-- | Extract a type out of an ExpType, if one exists. But one should always
-- exist. Unless you're quite sure you know what you're doing.
readExpType_maybe :: ExpType -> TcM (Maybe TcType)
readExpType_maybe :: ExpType -> TcM (Maybe Kind)
readExpType_maybe (Check Kind
ty)                   = Maybe Kind -> TcM (Maybe Kind)
forall (m :: * -> *) a. Monad m => a -> m a
return (Kind -> Maybe Kind
forall a. a -> Maybe a
Just Kind
ty)
readExpType_maybe (Infer (IR { ir_ref :: InferResult -> IORef (Maybe Kind)
ir_ref = IORef (Maybe Kind)
ref})) = IORef (Maybe Kind) -> TcM (Maybe Kind)
forall a env. IORef a -> IOEnv env a
readMutVar IORef (Maybe Kind)
ref

-- | Same as readExpType, but for Scaled ExpTypes
readScaledExpType :: Scaled ExpType -> TcM (Scaled Type)
readScaledExpType :: Scaled ExpType -> TcM (Scaled Kind)
readScaledExpType (Scaled Kind
m ExpType
exp_ty)
  = do { Kind
ty <- ExpType -> TcM Kind
readExpType ExpType
exp_ty
       ; Scaled Kind -> TcM (Scaled Kind)
forall (m :: * -> *) a. Monad m => a -> m a
return (Kind -> Kind -> Scaled Kind
forall a. Kind -> a -> Scaled a
Scaled Kind
m Kind
ty) }

-- | Extract a type out of an ExpType. Otherwise, panics.
readExpType :: ExpType -> TcM TcType
readExpType :: ExpType -> TcM Kind
readExpType ExpType
exp_ty
  = do { Maybe Kind
mb_ty <- ExpType -> TcM (Maybe Kind)
readExpType_maybe ExpType
exp_ty
       ; case Maybe Kind
mb_ty of
           Just Kind
ty -> Kind -> TcM Kind
forall (m :: * -> *) a. Monad m => a -> m a
return Kind
ty
           Maybe Kind
Nothing -> String -> SDoc -> TcM Kind
forall a. HasCallStack => String -> SDoc -> a
pprPanic String
"Unknown expected type" (ExpType -> SDoc
forall a. Outputable a => a -> SDoc
ppr ExpType
exp_ty) }

-- | Returns the expected type when in checking mode.
checkingExpType_maybe :: ExpType -> Maybe TcType
checkingExpType_maybe :: ExpType -> Maybe Kind
checkingExpType_maybe (Check Kind
ty) = Kind -> Maybe Kind
forall a. a -> Maybe a
Just Kind
ty
checkingExpType_maybe (Infer {}) = Maybe Kind
forall a. Maybe a
Nothing

-- | Returns the expected type when in checking mode. Panics if in inference
-- mode.
checkingExpType :: String -> ExpType -> TcType
checkingExpType :: String -> ExpType -> Kind
checkingExpType String
_   (Check Kind
ty) = Kind
ty
checkingExpType String
err ExpType
et         = String -> SDoc -> Kind
forall a. HasCallStack => String -> SDoc -> a
pprPanic String
"checkingExpType" (String -> SDoc
text String
err SDoc -> SDoc -> SDoc
$$ ExpType -> SDoc
forall a. Outputable a => a -> SDoc
ppr ExpType
et)

scaledExpTypeToType :: Scaled ExpType -> TcM (Scaled TcType)
scaledExpTypeToType :: Scaled ExpType -> TcM (Scaled Kind)
scaledExpTypeToType (Scaled Kind
m ExpType
exp_ty)
  = do { Kind
ty <- ExpType -> TcM Kind
expTypeToType ExpType
exp_ty
       ; Scaled Kind -> TcM (Scaled Kind)
forall (m :: * -> *) a. Monad m => a -> m a
return (Kind -> Kind -> Scaled Kind
forall a. Kind -> a -> Scaled a
Scaled Kind
m Kind
ty) }

-- | Extracts the expected type if there is one, or generates a new
-- TauTv if there isn't.
expTypeToType :: ExpType -> TcM TcType
expTypeToType :: ExpType -> TcM Kind
expTypeToType (Check Kind
ty)      = Kind -> TcM Kind
forall (m :: * -> *) a. Monad m => a -> m a
return Kind
ty
expTypeToType (Infer InferResult
inf_res) = InferResult -> TcM Kind
inferResultToType InferResult
inf_res

inferResultToType :: InferResult -> TcM Type
inferResultToType :: InferResult -> TcM Kind
inferResultToType (IR { ir_uniq :: InferResult -> Unique
ir_uniq = Unique
u, ir_lvl :: InferResult -> TcLevel
ir_lvl = TcLevel
tc_lvl
                      , ir_ref :: InferResult -> IORef (Maybe Kind)
ir_ref = IORef (Maybe Kind)
ref })
  = do { Maybe Kind
mb_inferred_ty <- IORef (Maybe Kind) -> TcM (Maybe Kind)
forall a gbl lcl. TcRef a -> TcRnIf gbl lcl a
readTcRef IORef (Maybe Kind)
ref
       ; Kind
tau <- case Maybe Kind
mb_inferred_ty of
            Just Kind
ty -> do { Kind -> TcRn ()
ensureMonoType Kind
ty
                            -- See Note [inferResultToType]
                          ; Kind -> TcM Kind
forall (m :: * -> *) a. Monad m => a -> m a
return Kind
ty }
            Maybe Kind
Nothing -> do { Kind
rr  <- TcLevel -> Kind -> TcM Kind
newMetaTyVarTyAtLevel TcLevel
tc_lvl Kind
runtimeRepTy
                          ; Kind
tau <- TcLevel -> Kind -> TcM Kind
newMetaTyVarTyAtLevel TcLevel
tc_lvl (Kind -> Kind
tYPE Kind
rr)
                            -- See Note [TcLevel of ExpType]
                          ; IORef (Maybe Kind) -> Maybe Kind -> TcRn ()
forall a env. IORef a -> a -> IOEnv env ()
writeMutVar IORef (Maybe Kind)
ref (Kind -> Maybe Kind
forall a. a -> Maybe a
Just Kind
tau)
                          ; Kind -> TcM Kind
forall (m :: * -> *) a. Monad m => a -> m a
return Kind
tau }
       ; String -> SDoc -> TcRn ()
traceTc String
"Forcing ExpType to be monomorphic:"
                 (Unique -> SDoc
forall a. Outputable a => a -> SDoc
ppr Unique
u SDoc -> SDoc -> SDoc
<+> String -> SDoc
text String
":=" SDoc -> SDoc -> SDoc
<+> Kind -> SDoc
forall a. Outputable a => a -> SDoc
ppr Kind
tau)
       ; Kind -> TcM Kind
forall (m :: * -> *) a. Monad m => a -> m a
return Kind
tau }

{- Note [inferResultToType]
~~~~~~~~~~~~~~~~~~~~~~~~~~~
expTypeToType and inferResultType convert an InferResult to a monotype.
It must be a monotype because if the InferResult isn't already filled in,
we fill it in with a unification variable (hence monotype).  So to preserve
order-independence we check for mono-type-ness even if it *is* filled in
already.

See also Note [TcLevel of ExpType] above, and
Note [fillInferResult].
-}

-- | Infer a type using a fresh ExpType
-- See also Note [ExpType] in "GHC.Tc.Utils.TcMType"
tcInfer :: (ExpSigmaType -> TcM a) -> TcM (a, TcSigmaType)
tcInfer :: forall a. (ExpType -> TcM a) -> TcM (a, Kind)
tcInfer ExpType -> TcM a
tc_check
  = do { ExpType
res_ty <- TcM ExpType
newInferExpType
       ; a
result <- ExpType -> TcM a
tc_check ExpType
res_ty
       ; Kind
res_ty <- ExpType -> TcM Kind
readExpType ExpType
res_ty
       ; (a, Kind) -> TcM (a, Kind)
forall (m :: * -> *) a. Monad m => a -> m a
return (a
result, Kind
res_ty) }

fillInferResult :: TcType -> InferResult -> TcM TcCoercionN
-- If co = fillInferResult t1 t2
--    => co :: t1 ~ t2
-- See Note [fillInferResult]
fillInferResult :: Kind -> InferResult -> TcM Coercion
fillInferResult Kind
act_res_ty (IR { ir_uniq :: InferResult -> Unique
ir_uniq = Unique
u, ir_lvl :: InferResult -> TcLevel
ir_lvl = TcLevel
res_lvl
                               , ir_ref :: InferResult -> IORef (Maybe Kind)
ir_ref = IORef (Maybe Kind)
ref })
  = do { Maybe Kind
mb_exp_res_ty <- IORef (Maybe Kind) -> TcM (Maybe Kind)
forall a gbl lcl. TcRef a -> TcRnIf gbl lcl a
readTcRef IORef (Maybe Kind)
ref
       ; case Maybe Kind
mb_exp_res_ty of
            Just Kind
exp_res_ty
               -> do { String -> SDoc -> TcRn ()
traceTc String
"Joining inferred ExpType" (SDoc -> TcRn ()) -> SDoc -> TcRn ()
forall a b. (a -> b) -> a -> b
$
                       Unique -> SDoc
forall a. Outputable a => a -> SDoc
ppr Unique
u SDoc -> SDoc -> SDoc
<> SDoc
colon SDoc -> SDoc -> SDoc
<+> Kind -> SDoc
forall a. Outputable a => a -> SDoc
ppr Kind
act_res_ty SDoc -> SDoc -> SDoc
<+> Char -> SDoc
char Char
'~' SDoc -> SDoc -> SDoc
<+> Kind -> SDoc
forall a. Outputable a => a -> SDoc
ppr Kind
exp_res_ty
                     ; TcLevel
cur_lvl <- TcM TcLevel
getTcLevel
                     ; Bool -> TcRn () -> TcRn ()
forall (f :: * -> *). Applicative f => Bool -> f () -> f ()
unless (TcLevel
cur_lvl TcLevel -> TcLevel -> Bool
`sameDepthAs` TcLevel
res_lvl) (TcRn () -> TcRn ()) -> TcRn () -> TcRn ()
forall a b. (a -> b) -> a -> b
$
                       Kind -> TcRn ()
ensureMonoType Kind
act_res_ty
                     ; Maybe SDoc -> Kind -> Kind -> TcM Coercion
unifyType Maybe SDoc
forall a. Maybe a
Nothing Kind
act_res_ty Kind
exp_res_ty }
            Maybe Kind
Nothing
               -> do { String -> SDoc -> TcRn ()
traceTc String
"Filling inferred ExpType" (SDoc -> TcRn ()) -> SDoc -> TcRn ()
forall a b. (a -> b) -> a -> b
$
                       Unique -> SDoc
forall a. Outputable a => a -> SDoc
ppr Unique
u SDoc -> SDoc -> SDoc
<+> String -> SDoc
text String
":=" SDoc -> SDoc -> SDoc
<+> Kind -> SDoc
forall a. Outputable a => a -> SDoc
ppr Kind
act_res_ty
                     ; (Coercion
prom_co, Kind
act_res_ty) <- TcLevel -> Kind -> TcM (Coercion, Kind)
promoteTcType TcLevel
res_lvl Kind
act_res_ty
                     ; IORef (Maybe Kind) -> Maybe Kind -> TcRn ()
forall a gbl lcl. TcRef a -> a -> TcRnIf gbl lcl ()
writeTcRef IORef (Maybe Kind)
ref (Kind -> Maybe Kind
forall a. a -> Maybe a
Just Kind
act_res_ty)
                     ; Coercion -> TcM Coercion
forall (m :: * -> *) a. Monad m => a -> m a
return Coercion
prom_co }
     }


{- Note [fillInferResult]
~~~~~~~~~~~~~~~~~~~~~~~~~
When inferring, we use fillInferResult to "fill in" the hole in InferResult
   data InferResult = IR { ir_uniq :: Unique
                         , ir_lvl  :: TcLevel
                         , ir_ref  :: IORef (Maybe TcType) }

There are two things to worry about:

1. What if it is under a GADT or existential pattern match?
   - GADTs: a unification variable (and Infer's hole is similar) is untouchable
   - Existentials: be careful about skolem-escape

2. What if it is filled in more than once?  E.g. multiple branches of a case
     case e of
        T1 -> e1
        T2 -> e2

Our typing rules are:

* The RHS of a existential or GADT alternative must always be a
  monotype, regardless of the number of alternatives.

* Multiple non-existential/GADT branches can have (the same)
  higher rank type (#18412).  E.g. this is OK:
      case e of
        True  -> hr
        False -> hr
  where hr:: (forall a. a->a) -> Int
  c.f. Section 7.1 of "Practical type inference for arbitrary-rank types"
       We use choice (2) in that Section.
       (GHC 8.10 and earlier used choice (1).)

  But note that
      case e of
        True  -> hr
        False -> \x -> hr x
  will fail, because we still /infer/ both branches, so the \x will get
  a (monotype) unification variable, which will fail to unify with
  (forall a. a->a)

For (1) we can detect the GADT/existential situation by seeing that
the current TcLevel is greater than that stored in ir_lvl of the Infer
ExpType.  We bump the level whenever we go past a GADT/existential match.

Then, before filling the hole use promoteTcType to promote the type
to the outer ir_lvl.  promoteTcType does this
  - create a fresh unification variable alpha at level ir_lvl
  - emits an equality alpha[ir_lvl] ~ ty
  - fills the hole with alpha
That forces the type to be a monotype (since unification variables can
only unify with monotypes); and catches skolem-escapes because the
alpha is untouchable until the equality floats out.

For (2), we simply look to see if the hole is filled already.
  - if not, we promote (as above) and fill the hole
  - if it is filled, we simply unify with the type that is
    already there

There is one wrinkle.  Suppose we have
   case e of
      T1 -> e1 :: (forall a. a->a) -> Int
      G2 -> e2
where T1 is not GADT or existential, but G2 is a GADT.  Then supppose the
T1 alternative fills the hole with (forall a. a->a) -> Int, which is fine.
But now the G2 alternative must not *just* unify with that else we'd risk
allowing through (e2 :: (forall a. a->a) -> Int).  If we'd checked G2 first
we'd have filled the hole with a unification variable, which enforces a
monotype.

So if we check G2 second, we still want to emit a constraint that restricts
the RHS to be a monotype. This is done by ensureMonoType, and it works
by simply generating a constraint (alpha ~ ty), where alpha is a fresh
unification variable.  We discard the evidence.

-}

{- *********************************************************************
*                                                                      *
              Promoting types
*                                                                      *
********************************************************************* -}

ensureMonoType :: TcType -> TcM ()
-- Assuming that the argument type is of kind (TYPE r),
-- ensure that it is a /monotype/
-- If it is not a monotype we can see right away (since unification
-- varibles and type-function applications stand for monotypes), but
-- we emit a Wanted equality just to delay the error message until later
ensureMonoType :: Kind -> TcRn ()
ensureMonoType Kind
res_ty
  | Kind -> Bool
isTauTy Kind
res_ty   -- isTauTy doesn't need zonking or anything
  = () -> TcRn ()
forall (m :: * -> *) a. Monad m => a -> m a
return ()
  | Bool
otherwise
  = do { Kind
mono_ty <- TcM Kind
newOpenFlexiTyVarTy
       ; let eq_orig :: CtOrigin
eq_orig = TypeEqOrigin { uo_actual :: Kind
uo_actual   = Kind
res_ty
                                    , uo_expected :: Kind
uo_expected = Kind
mono_ty
                                    , uo_thing :: Maybe SDoc
uo_thing    = Maybe SDoc
forall a. Maybe a
Nothing
                                    , uo_visible :: Bool
uo_visible  = Bool
False }

       ; Coercion
_co <- CtOrigin -> TypeOrKind -> Role -> Kind -> Kind -> TcM Coercion
emitWantedEq CtOrigin
eq_orig TypeOrKind
TypeLevel Role
Nominal Kind
res_ty Kind
mono_ty
       ; () -> TcRn ()
forall (m :: * -> *) a. Monad m => a -> m a
return () }

promoteTcType :: TcLevel -> TcType -> TcM (TcCoercionN, TcType)
-- See Note [Promoting a type]
-- See also Note [fillInferResult]
-- promoteTcType level ty = (co, ty')
--   * Returns ty'  whose max level is just 'level'
--             and  whose kind is ~# to the kind of 'ty'
--             and  whose kind has form TYPE rr
--   * and co :: ty ~ ty'
--   * and emits constraints to justify the coercion
--
-- NB: we expect that 'ty' has already kind (TYPE rr) for
--     some rr::RuntimeRep.  It is, after all, the type of a term.
promoteTcType :: TcLevel -> Kind -> TcM (Coercion, Kind)
promoteTcType TcLevel
dest_lvl Kind
ty
  = do { TcLevel
cur_lvl <- TcM TcLevel
getTcLevel
       ; if (TcLevel
cur_lvl TcLevel -> TcLevel -> Bool
`sameDepthAs` TcLevel
dest_lvl)
         then (Coercion, Kind) -> TcM (Coercion, Kind)
forall (m :: * -> *) a. Monad m => a -> m a
return (Kind -> Coercion
mkTcNomReflCo Kind
ty, Kind
ty)
         else TcM (Coercion, Kind)
promote_it }
  where
    promote_it :: TcM (TcCoercion, TcType)
    promote_it :: TcM (Coercion, Kind)
promote_it  -- Emit a constraint  (alpha :: TYPE rr) ~ ty
                -- where alpha and rr are fresh and from level dest_lvl
      = do { Kind
rr      <- TcLevel -> Kind -> TcM Kind
newMetaTyVarTyAtLevel TcLevel
dest_lvl Kind
runtimeRepTy
           ; Kind
prom_ty <- TcLevel -> Kind -> TcM Kind
newMetaTyVarTyAtLevel TcLevel
dest_lvl (Kind -> Kind
tYPE Kind
rr)
           ; let eq_orig :: CtOrigin
eq_orig = TypeEqOrigin { uo_actual :: Kind
uo_actual   = Kind
ty
                                        , uo_expected :: Kind
uo_expected = Kind
prom_ty
                                        , uo_thing :: Maybe SDoc
uo_thing    = Maybe SDoc
forall a. Maybe a
Nothing
                                        , uo_visible :: Bool
uo_visible  = Bool
False }

           ; Coercion
co <- CtOrigin -> TypeOrKind -> Role -> Kind -> Kind -> TcM Coercion
emitWantedEq CtOrigin
eq_orig TypeOrKind
TypeLevel Role
Nominal Kind
ty Kind
prom_ty
           ; (Coercion, Kind) -> TcM (Coercion, Kind)
forall (m :: * -> *) a. Monad m => a -> m a
return (Coercion
co, Kind
prom_ty) }

{- Note [Promoting a type]
~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider (#12427)

  data T where
    MkT :: (Int -> Int) -> a -> T

  h y = case y of MkT v w -> v

We'll infer the RHS type with an expected type ExpType of
  (IR { ir_lvl = l, ir_ref = ref, ... )
where 'l' is the TcLevel of the RHS of 'h'.  Then the MkT pattern
match will increase the level, so we'll end up in tcSubType, trying to
unify the type of v,
  v :: Int -> Int
with the expected type.  But this attempt takes place at level (l+1),
rightly so, since v's type could have mentioned existential variables,
(like w's does) and we want to catch that.

So we
  - create a new meta-var alpha[l+1]
  - fill in the InferRes ref cell 'ref' with alpha
  - emit an equality constraint, thus
        [W] alpha[l+1] ~ (Int -> Int)

That constraint will float outwards, as it should, unless v's
type mentions a skolem-captured variable.

This approach fails if v has a higher rank type; see
Note [Promotion and higher rank types]


Note [Promotion and higher rank types]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If v had a higher-rank type, say v :: (forall a. a->a) -> Int,
then we'd emit an equality
        [W] alpha[l+1] ~ ((forall a. a->a) -> Int)
which will sadly fail because we can't unify a unification variable
with a polytype.  But there is nothing really wrong with the program
here.

We could just about solve this by "promote the type" of v, to expose
its polymorphic "shape" while still leaving constraints that will
prevent existential escape.  But we must be careful!  Exposing
the "shape" of the type is precisely what we must NOT do under
a GADT pattern match!  So in this case we might promote the type
to
        (forall a. a->a) -> alpha[l+1]
and emit the constraint
        [W] alpha[l+1] ~ Int
Now the promoted type can fill the ref cell, while the emitted
equality can float or not, according to the usual rules.

But that's not quite right!  We are exposing the arrow! We could
deal with that too:
        (forall a. mu[l+1] a a) -> alpha[l+1]
with constraints
        [W] alpha[l+1] ~ Int
        [W] mu[l+1] ~ (->)
Here we abstract over the '->' inside the forall, in case that
is subject to an equality constraint from a GADT match.

Note that we kept the outer (->) because that's part of
the polymorphic "shape".  And because of impredicativity,
GADT matches can't give equalities that affect polymorphic
shape.

This reasoning just seems too complicated, so I decided not
to do it.  These higher-rank notes are just here to record
the thinking.
-}


{- *********************************************************************
*                                                                      *
        MetaTvs (meta type variables; mutable)
*                                                                      *
********************************************************************* -}

{- Note [TyVarTv]
~~~~~~~~~~~~~~~~~
A TyVarTv can unify with type *variables* only, including other TyVarTvs and
skolems.  They are used in two places:

1. In kind signatures, see GHC.Tc.TyCl
      Note [Inferring kinds for type declarations]
   and Note [Kind checking for GADTs]

2. In partial type signatures.  See GHC.Tc.Types
   Note [Quantified variables in partial type signatures]

Sometimes, they can unify with type variables that the user would
rather keep distinct; see #11203 for an example.  So, any client of
this function needs to either allow the TyVarTvs to unify with each
other or check that they don't. In the case of (1) the check is done
in GHC.Tc.TyCl.swizzleTcTyConBndrs.  In case of (2) it's done by
findDupTyVarTvs in GHC.Tc.Gen.Bind.chooseInferredQuantifiers.

Historical note: Before #15050 this (under the name SigTv) was also
used for ScopedTypeVariables in patterns, to make sure these type
variables only refer to other type variables, but this restriction was
dropped, and ScopedTypeVariables can now refer to full types (GHC
Proposal 29).
-}

newMetaTyVarName :: FastString -> TcM Name
-- Makes a /System/ Name, which is eagerly eliminated by
-- the unifier; see GHC.Tc.Utils.Unify.nicer_to_update_tv1, and
-- GHC.Tc.Solver.Canonical.canEqTyVarTyVar (nicer_to_update_tv2)
newMetaTyVarName :: FastString -> TcM Name
newMetaTyVarName FastString
str
  = do { Unique
uniq <- TcRnIf TcGblEnv TcLclEnv Unique
forall gbl lcl. TcRnIf gbl lcl Unique
newUnique
       ; Name -> TcM Name
forall (m :: * -> *) a. Monad m => a -> m a
return (Unique -> OccName -> Name
mkSystemName Unique
uniq (FastString -> OccName
mkTyVarOccFS FastString
str)) }

cloneMetaTyVarName :: Name -> TcM Name
cloneMetaTyVarName :: Name -> TcM Name
cloneMetaTyVarName Name
name
  = do { Unique
uniq <- TcRnIf TcGblEnv TcLclEnv Unique
forall gbl lcl. TcRnIf gbl lcl Unique
newUnique
       ; Name -> TcM Name
forall (m :: * -> *) a. Monad m => a -> m a
return (Unique -> OccName -> Name
mkSystemName Unique
uniq (Name -> OccName
nameOccName Name
name)) }
         -- See Note [Name of an instantiated type variable]

{- Note [Name of an instantiated type variable]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
At the moment we give a unification variable a System Name, which
influences the way it is tidied; see TypeRep.tidyTyVarBndr.
-}

metaInfoToTyVarName :: MetaInfo -> FastString
metaInfoToTyVarName :: MetaInfo -> FastString
metaInfoToTyVarName  MetaInfo
meta_info =
  case MetaInfo
meta_info of
       MetaInfo
TauTv          -> String -> FastString
fsLit String
"t"
       MetaInfo
TyVarTv        -> String -> FastString
fsLit String
"a"
       MetaInfo
RuntimeUnkTv   -> String -> FastString
fsLit String
"r"
       MetaInfo
CycleBreakerTv -> String -> FastString
fsLit String
"b"

newAnonMetaTyVar :: MetaInfo -> Kind -> TcM TcTyVar
newAnonMetaTyVar :: MetaInfo -> Kind -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
newAnonMetaTyVar MetaInfo
mi = FastString
-> MetaInfo -> Kind -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
newNamedAnonMetaTyVar (MetaInfo -> FastString
metaInfoToTyVarName MetaInfo
mi) MetaInfo
mi

newNamedAnonMetaTyVar :: FastString -> MetaInfo -> Kind -> TcM TcTyVar
-- Make a new meta tyvar out of thin air
newNamedAnonMetaTyVar :: FastString
-> MetaInfo -> Kind -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
newNamedAnonMetaTyVar FastString
tyvar_name MetaInfo
meta_info Kind
kind

  = do  { Name
name    <- FastString -> TcM Name
newMetaTyVarName FastString
tyvar_name
        ; TcTyVarDetails
details <- MetaInfo -> TcM TcTyVarDetails
newMetaDetails MetaInfo
meta_info
        ; let tyvar :: TyVar
tyvar = Name -> Kind -> TcTyVarDetails -> TyVar
mkTcTyVar Name
name Kind
kind TcTyVarDetails
details
        ; String -> SDoc -> TcRn ()
traceTc String
"newAnonMetaTyVar" (TyVar -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyVar
tyvar)
        ; TyVar -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
forall (m :: * -> *) a. Monad m => a -> m a
return TyVar
tyvar }

-- makes a new skolem tv
newSkolemTyVar :: Name -> Kind -> TcM TcTyVar
newSkolemTyVar :: Name -> Kind -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
newSkolemTyVar Name
name Kind
kind
  = do { TcLevel
lvl <- TcM TcLevel
getTcLevel
       ; TyVar -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
forall (m :: * -> *) a. Monad m => a -> m a
return (Name -> Kind -> TcTyVarDetails -> TyVar
mkTcTyVar Name
name Kind
kind (TcLevel -> Bool -> TcTyVarDetails
SkolemTv TcLevel
lvl Bool
False)) }

newTyVarTyVar :: Name -> Kind -> TcM TcTyVar
-- See Note [TyVarTv]
-- Does not clone a fresh unique
newTyVarTyVar :: Name -> Kind -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
newTyVarTyVar Name
name Kind
kind
  = do { TcTyVarDetails
details <- MetaInfo -> TcM TcTyVarDetails
newMetaDetails MetaInfo
TyVarTv
       ; let tyvar :: TyVar
tyvar = Name -> Kind -> TcTyVarDetails -> TyVar
mkTcTyVar Name
name Kind
kind TcTyVarDetails
details
       ; String -> SDoc -> TcRn ()
traceTc String
"newTyVarTyVar" (TyVar -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyVar
tyvar)
       ; TyVar -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
forall (m :: * -> *) a. Monad m => a -> m a
return TyVar
tyvar }

cloneTyVarTyVar :: Name -> Kind -> TcM TcTyVar
-- See Note [TyVarTv]
-- Clones a fresh unique
cloneTyVarTyVar :: Name -> Kind -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
cloneTyVarTyVar Name
name Kind
kind
  = do { TcTyVarDetails
details <- MetaInfo -> TcM TcTyVarDetails
newMetaDetails MetaInfo
TyVarTv
       ; Unique
uniq <- TcRnIf TcGblEnv TcLclEnv Unique
forall gbl lcl. TcRnIf gbl lcl Unique
newUnique
       ; let name' :: Name
name' = Name
name Name -> Unique -> Name
`setNameUnique` Unique
uniq
             tyvar :: TyVar
tyvar = Name -> Kind -> TcTyVarDetails -> TyVar
mkTcTyVar Name
name' Kind
kind TcTyVarDetails
details
         -- Don't use cloneMetaTyVar, which makes a SystemName
         -- We want to keep the original more user-friendly Name
         -- In practical terms that means that in error messages,
         -- when the Name is tidied we get 'a' rather than 'a0'
       ; String -> SDoc -> TcRn ()
traceTc String
"cloneTyVarTyVar" (TyVar -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyVar
tyvar)
       ; TyVar -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
forall (m :: * -> *) a. Monad m => a -> m a
return TyVar
tyvar }

newPatSigTyVar :: Name -> Kind -> TcM TcTyVar
newPatSigTyVar :: Name -> Kind -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
newPatSigTyVar Name
name Kind
kind
  = do { TcTyVarDetails
details <- MetaInfo -> TcM TcTyVarDetails
newMetaDetails MetaInfo
TauTv
       ; Unique
uniq <- TcRnIf TcGblEnv TcLclEnv Unique
forall gbl lcl. TcRnIf gbl lcl Unique
newUnique
       ; let name' :: Name
name' = Name
name Name -> Unique -> Name
`setNameUnique` Unique
uniq
             tyvar :: TyVar
tyvar = Name -> Kind -> TcTyVarDetails -> TyVar
mkTcTyVar Name
name' Kind
kind TcTyVarDetails
details
         -- Don't use cloneMetaTyVar;
         -- same reasoning as in newTyVarTyVar
       ; String -> SDoc -> TcRn ()
traceTc String
"newPatSigTyVar" (TyVar -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyVar
tyvar)
       ; TyVar -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
forall (m :: * -> *) a. Monad m => a -> m a
return TyVar
tyvar }

cloneAnonMetaTyVar :: MetaInfo -> TyVar -> TcKind -> TcM TcTyVar
-- Make a fresh MetaTyVar, basing the name
-- on that of the supplied TyVar
cloneAnonMetaTyVar :: MetaInfo -> TyVar -> Kind -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
cloneAnonMetaTyVar MetaInfo
info TyVar
tv Kind
kind
  = do  { TcTyVarDetails
details <- MetaInfo -> TcM TcTyVarDetails
newMetaDetails MetaInfo
info
        ; Name
name    <- Name -> TcM Name
cloneMetaTyVarName (TyVar -> Name
tyVarName TyVar
tv)
        ; let tyvar :: TyVar
tyvar = Name -> Kind -> TcTyVarDetails -> TyVar
mkTcTyVar Name
name Kind
kind TcTyVarDetails
details
        ; String -> SDoc -> TcRn ()
traceTc String
"cloneAnonMetaTyVar" (TyVar -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyVar
tyvar SDoc -> SDoc -> SDoc
<+> SDoc
dcolon SDoc -> SDoc -> SDoc
<+> Kind -> SDoc
forall a. Outputable a => a -> SDoc
ppr (TyVar -> Kind
tyVarKind TyVar
tyvar))
        ; TyVar -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
forall (m :: * -> *) a. Monad m => a -> m a
return TyVar
tyvar }

-- Make a new CycleBreakerTv. See Note [Type variable cycles]
-- in GHC.Tc.Solver.Canonical.
newCycleBreakerTyVar :: TcKind -> TcM TcTyVar
newCycleBreakerTyVar :: Kind -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
newCycleBreakerTyVar Kind
kind
  = do { TcTyVarDetails
details <- MetaInfo -> TcM TcTyVarDetails
newMetaDetails MetaInfo
CycleBreakerTv
       ; Name
name <- FastString -> TcM Name
newMetaTyVarName (String -> FastString
fsLit String
"cbv")
       ; TyVar -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
forall (m :: * -> *) a. Monad m => a -> m a
return (Name -> Kind -> TcTyVarDetails -> TyVar
mkTcTyVar Name
name Kind
kind TcTyVarDetails
details) }

newMetaDetails :: MetaInfo -> TcM TcTyVarDetails
newMetaDetails :: MetaInfo -> TcM TcTyVarDetails
newMetaDetails MetaInfo
info
  = do { IORef MetaDetails
ref <- MetaDetails -> IOEnv (Env TcGblEnv TcLclEnv) (IORef MetaDetails)
forall a env. a -> IOEnv env (IORef a)
newMutVar MetaDetails
Flexi
       ; TcLevel
tclvl <- TcM TcLevel
getTcLevel
       ; TcTyVarDetails -> TcM TcTyVarDetails
forall (m :: * -> *) a. Monad m => a -> m a
return (MetaTv { mtv_info :: MetaInfo
mtv_info = MetaInfo
info
                        , mtv_ref :: IORef MetaDetails
mtv_ref = IORef MetaDetails
ref
                        , mtv_tclvl :: TcLevel
mtv_tclvl = TcLevel
tclvl }) }

newTauTvDetailsAtLevel :: TcLevel -> TcM TcTyVarDetails
newTauTvDetailsAtLevel :: TcLevel -> TcM TcTyVarDetails
newTauTvDetailsAtLevel TcLevel
tclvl
  = do { IORef MetaDetails
ref <- MetaDetails -> IOEnv (Env TcGblEnv TcLclEnv) (IORef MetaDetails)
forall a env. a -> IOEnv env (IORef a)
newMutVar MetaDetails
Flexi
       ; TcTyVarDetails -> TcM TcTyVarDetails
forall (m :: * -> *) a. Monad m => a -> m a
return (MetaTv { mtv_info :: MetaInfo
mtv_info  = MetaInfo
TauTv
                        , mtv_ref :: IORef MetaDetails
mtv_ref   = IORef MetaDetails
ref
                        , mtv_tclvl :: TcLevel
mtv_tclvl = TcLevel
tclvl }) }

cloneMetaTyVar :: TcTyVar -> TcM TcTyVar
cloneMetaTyVar :: TyVar -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
cloneMetaTyVar TyVar
tv
  = ASSERT( isTcTyVar tv )
    do  { IORef MetaDetails
ref  <- MetaDetails -> IOEnv (Env TcGblEnv TcLclEnv) (IORef MetaDetails)
forall a env. a -> IOEnv env (IORef a)
newMutVar MetaDetails
Flexi
        ; Name
name' <- Name -> TcM Name
cloneMetaTyVarName (TyVar -> Name
tyVarName TyVar
tv)
        ; let details' :: TcTyVarDetails
details' = case TyVar -> TcTyVarDetails
tcTyVarDetails TyVar
tv of
                           details :: TcTyVarDetails
details@(MetaTv {}) -> TcTyVarDetails
details { mtv_ref :: IORef MetaDetails
mtv_ref = IORef MetaDetails
ref }
                           TcTyVarDetails
_ -> String -> SDoc -> TcTyVarDetails
forall a. HasCallStack => String -> SDoc -> a
pprPanic String
"cloneMetaTyVar" (TyVar -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyVar
tv)
              tyvar :: TyVar
tyvar = Name -> Kind -> TcTyVarDetails -> TyVar
mkTcTyVar Name
name' (TyVar -> Kind
tyVarKind TyVar
tv) TcTyVarDetails
details'
        ; String -> SDoc -> TcRn ()
traceTc String
"cloneMetaTyVar" (TyVar -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyVar
tyvar)
        ; TyVar -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
forall (m :: * -> *) a. Monad m => a -> m a
return TyVar
tyvar }

-- Works for both type and kind variables
readMetaTyVar :: TyVar -> TcM MetaDetails
readMetaTyVar :: TyVar -> TcM MetaDetails
readMetaTyVar TyVar
tyvar = ASSERT2( isMetaTyVar tyvar, ppr tyvar )
                      IORef MetaDetails -> TcM MetaDetails
forall a env. IORef a -> IOEnv env a
readMutVar (TyVar -> IORef MetaDetails
metaTyVarRef TyVar
tyvar)

isFilledMetaTyVar_maybe :: TcTyVar -> TcM (Maybe Type)
isFilledMetaTyVar_maybe :: TyVar -> TcM (Maybe Kind)
isFilledMetaTyVar_maybe TyVar
tv
 | MetaTv { mtv_ref :: TcTyVarDetails -> IORef MetaDetails
mtv_ref = IORef MetaDetails
ref } <- TyVar -> TcTyVarDetails
tcTyVarDetails TyVar
tv
 = do { MetaDetails
cts <- IORef MetaDetails -> TcM MetaDetails
forall a gbl lcl. TcRef a -> TcRnIf gbl lcl a
readTcRef IORef MetaDetails
ref
      ; case MetaDetails
cts of
          Indirect Kind
ty -> Maybe Kind -> TcM (Maybe Kind)
forall (m :: * -> *) a. Monad m => a -> m a
return (Kind -> Maybe Kind
forall a. a -> Maybe a
Just Kind
ty)
          MetaDetails
Flexi       -> Maybe Kind -> TcM (Maybe Kind)
forall (m :: * -> *) a. Monad m => a -> m a
return Maybe Kind
forall a. Maybe a
Nothing }
 | Bool
otherwise
 = Maybe Kind -> TcM (Maybe Kind)
forall (m :: * -> *) a. Monad m => a -> m a
return Maybe Kind
forall a. Maybe a
Nothing

isFilledMetaTyVar :: TyVar -> TcM Bool
-- True of a filled-in (Indirect) meta type variable
isFilledMetaTyVar :: TyVar -> TcRnIf TcGblEnv TcLclEnv Bool
isFilledMetaTyVar TyVar
tv = Maybe Kind -> Bool
forall a. Maybe a -> Bool
isJust (Maybe Kind -> Bool)
-> TcM (Maybe Kind) -> TcRnIf TcGblEnv TcLclEnv Bool
forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
<$> TyVar -> TcM (Maybe Kind)
isFilledMetaTyVar_maybe TyVar
tv

isUnfilledMetaTyVar :: TyVar -> TcM Bool
-- True of a un-filled-in (Flexi) meta type variable
-- NB: Not the opposite of isFilledMetaTyVar
isUnfilledMetaTyVar :: TyVar -> TcRnIf TcGblEnv TcLclEnv Bool
isUnfilledMetaTyVar TyVar
tv
  | MetaTv { mtv_ref :: TcTyVarDetails -> IORef MetaDetails
mtv_ref = IORef MetaDetails
ref } <- TyVar -> TcTyVarDetails
tcTyVarDetails TyVar
tv
  = do  { MetaDetails
details <- IORef MetaDetails -> TcM MetaDetails
forall a env. IORef a -> IOEnv env a
readMutVar IORef MetaDetails
ref
        ; Bool -> TcRnIf TcGblEnv TcLclEnv Bool
forall (m :: * -> *) a. Monad m => a -> m a
return (MetaDetails -> Bool
isFlexi MetaDetails
details) }
  | Bool
otherwise = Bool -> TcRnIf TcGblEnv TcLclEnv Bool
forall (m :: * -> *) a. Monad m => a -> m a
return Bool
False

--------------------
-- Works with both type and kind variables
writeMetaTyVar :: TcTyVar -> TcType -> TcM ()
-- Write into a currently-empty MetaTyVar

writeMetaTyVar :: TyVar -> Kind -> TcRn ()
writeMetaTyVar TyVar
tyvar Kind
ty
  | Bool -> Bool
not Bool
debugIsOn
  = TyVar -> IORef MetaDetails -> Kind -> TcRn ()
writeMetaTyVarRef TyVar
tyvar (TyVar -> IORef MetaDetails
metaTyVarRef TyVar
tyvar) Kind
ty

-- Everything from here on only happens if DEBUG is on
  | Bool -> Bool
not (TyVar -> Bool
isTcTyVar TyVar
tyvar)
  = ASSERT2( False, text "Writing to non-tc tyvar" <+> ppr tyvar )
    () -> TcRn ()
forall (m :: * -> *) a. Monad m => a -> m a
return ()

  | MetaTv { mtv_ref :: TcTyVarDetails -> IORef MetaDetails
mtv_ref = IORef MetaDetails
ref } <- TyVar -> TcTyVarDetails
tcTyVarDetails TyVar
tyvar
  = TyVar -> IORef MetaDetails -> Kind -> TcRn ()
writeMetaTyVarRef TyVar
tyvar IORef MetaDetails
ref Kind
ty

  | Bool
otherwise
  = ASSERT2( False, text "Writing to non-meta tyvar" <+> ppr tyvar )
    () -> TcRn ()
forall (m :: * -> *) a. Monad m => a -> m a
return ()

--------------------
writeMetaTyVarRef :: TcTyVar -> TcRef MetaDetails -> TcType -> TcM ()
-- Here the tyvar is for error checking only;
-- the ref cell must be for the same tyvar
writeMetaTyVarRef :: TyVar -> IORef MetaDetails -> Kind -> TcRn ()
writeMetaTyVarRef TyVar
tyvar IORef MetaDetails
ref Kind
ty
  | Bool -> Bool
not Bool
debugIsOn
  = do { String -> SDoc -> TcRn ()
traceTc String
"writeMetaTyVar" (TyVar -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyVar
tyvar SDoc -> SDoc -> SDoc
<+> SDoc
dcolon SDoc -> SDoc -> SDoc
<+> Kind -> SDoc
forall a. Outputable a => a -> SDoc
ppr (TyVar -> Kind
tyVarKind TyVar
tyvar)
                                   SDoc -> SDoc -> SDoc
<+> String -> SDoc
text String
":=" SDoc -> SDoc -> SDoc
<+> Kind -> SDoc
forall a. Outputable a => a -> SDoc
ppr Kind
ty)
       ; IORef MetaDetails -> MetaDetails -> TcRn ()
forall a gbl lcl. TcRef a -> a -> TcRnIf gbl lcl ()
writeTcRef IORef MetaDetails
ref (Kind -> MetaDetails
Indirect Kind
ty) }

  -- Everything from here on only happens if DEBUG is on
  -- Need to zonk 'ty' because we may only recently have promoted
  -- its free meta-tyvars (see Solver.Interact.tryToSolveByUnification)
  | Bool
otherwise
  = do { MetaDetails
meta_details <- IORef MetaDetails -> TcM MetaDetails
forall a env. IORef a -> IOEnv env a
readMutVar IORef MetaDetails
ref;
       -- Zonk kinds to allow the error check to work
       ; Kind
zonked_tv_kind <- Kind -> TcM Kind
zonkTcType Kind
tv_kind
       ; Kind
zonked_ty      <- Kind -> TcM Kind
zonkTcType Kind
ty
       ; let zonked_ty_kind :: Kind
zonked_ty_kind = HasDebugCallStack => Kind -> Kind
Kind -> Kind
tcTypeKind Kind
zonked_ty
             zonked_ty_lvl :: TcLevel
zonked_ty_lvl  = Kind -> TcLevel
tcTypeLevel Kind
zonked_ty
             level_check_ok :: Bool
level_check_ok  = Bool -> Bool
not (TcLevel
zonked_ty_lvl TcLevel -> TcLevel -> Bool
`strictlyDeeperThan` TcLevel
tv_lvl)
             level_check_msg :: SDoc
level_check_msg = TcLevel -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcLevel
zonked_ty_lvl SDoc -> SDoc -> SDoc
$$ TcLevel -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcLevel
tv_lvl SDoc -> SDoc -> SDoc
$$ TyVar -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyVar
tyvar SDoc -> SDoc -> SDoc
$$ Kind -> SDoc
forall a. Outputable a => a -> SDoc
ppr Kind
ty
             kind_check_ok :: Bool
kind_check_ok = Kind -> Bool
tcIsConstraintKind Kind
zonked_tv_kind
                          Bool -> Bool -> Bool
|| HasDebugCallStack => Kind -> Kind -> Bool
Kind -> Kind -> Bool
tcEqKind Kind
zonked_ty_kind Kind
zonked_tv_kind
             -- Hack alert! tcIsConstraintKind: see GHC.Tc.Gen.HsType
             -- Note [Extra-constraint holes in partial type signatures]

             kind_msg :: SDoc
kind_msg = SDoc -> Int -> SDoc -> SDoc
hang (String -> SDoc
text String
"Ill-kinded update to meta tyvar")
                           Int
2 (    TyVar -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyVar
tyvar SDoc -> SDoc -> SDoc
<+> String -> SDoc
text String
"::" SDoc -> SDoc -> SDoc
<+> (Kind -> SDoc
forall a. Outputable a => a -> SDoc
ppr Kind
tv_kind SDoc -> SDoc -> SDoc
$$ Kind -> SDoc
forall a. Outputable a => a -> SDoc
ppr Kind
zonked_tv_kind)
                              SDoc -> SDoc -> SDoc
<+> String -> SDoc
text String
":="
                              SDoc -> SDoc -> SDoc
<+> Kind -> SDoc
forall a. Outputable a => a -> SDoc
ppr Kind
ty SDoc -> SDoc -> SDoc
<+> String -> SDoc
text String
"::" SDoc -> SDoc -> SDoc
<+> (Kind -> SDoc
forall a. Outputable a => a -> SDoc
ppr Kind
zonked_ty_kind) )

       ; String -> SDoc -> TcRn ()
traceTc String
"writeMetaTyVar" (TyVar -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyVar
tyvar SDoc -> SDoc -> SDoc
<+> String -> SDoc
text String
":=" SDoc -> SDoc -> SDoc
<+> Kind -> SDoc
forall a. Outputable a => a -> SDoc
ppr Kind
ty)

       -- Check for double updates
       ; MASSERT2( isFlexi meta_details, double_upd_msg meta_details )

       -- Check for level OK
       ; MASSERT2( level_check_ok, level_check_msg )

       -- Check Kinds ok
       ; MASSERT2( kind_check_ok, kind_msg )

       -- Do the write
       ; IORef MetaDetails -> MetaDetails -> TcRn ()
forall a env. IORef a -> a -> IOEnv env ()
writeMutVar IORef MetaDetails
ref (Kind -> MetaDetails
Indirect Kind
ty) }
  where
    tv_kind :: Kind
tv_kind = TyVar -> Kind
tyVarKind TyVar
tyvar

    tv_lvl :: TcLevel
tv_lvl = TyVar -> TcLevel
tcTyVarLevel TyVar
tyvar


    double_upd_msg :: MetaDetails -> SDoc
double_upd_msg MetaDetails
details = SDoc -> Int -> SDoc -> SDoc
hang (String -> SDoc
text String
"Double update of meta tyvar")
                                Int
2 (TyVar -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyVar
tyvar SDoc -> SDoc -> SDoc
$$ MetaDetails -> SDoc
forall a. Outputable a => a -> SDoc
ppr MetaDetails
details)

{-
************************************************************************
*                                                                      *
        MetaTvs: TauTvs
*                                                                      *
************************************************************************

Note [Never need to instantiate coercion variables]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
With coercion variables sloshing around in types, it might seem that we
sometimes need to instantiate coercion variables. This would be problematic,
because coercion variables inhabit unboxed equality (~#), and the constraint
solver thinks in terms only of boxed equality (~). The solution is that
we never need to instantiate coercion variables in the first place.

The tyvars that we need to instantiate come from the types of functions,
data constructors, and patterns. These will never be quantified over
coercion variables, except for the special case of the promoted Eq#. But,
that can't ever appear in user code, so we're safe!
-}


newMultiplicityVar :: TcM TcType
newMultiplicityVar :: TcM Kind
newMultiplicityVar = Kind -> TcM Kind
newFlexiTyVarTy Kind
multiplicityTy

newFlexiTyVar :: Kind -> TcM TcTyVar
newFlexiTyVar :: Kind -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
newFlexiTyVar Kind
kind = MetaInfo -> Kind -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
newAnonMetaTyVar MetaInfo
TauTv Kind
kind

-- | Create a new flexi ty var with a specific name
newNamedFlexiTyVar :: FastString -> Kind -> TcM TcTyVar
newNamedFlexiTyVar :: FastString -> Kind -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
newNamedFlexiTyVar FastString
fs Kind
kind = FastString
-> MetaInfo -> Kind -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
newNamedAnonMetaTyVar FastString
fs MetaInfo
TauTv Kind
kind

newFlexiTyVarTy :: Kind -> TcM TcType
newFlexiTyVarTy :: Kind -> TcM Kind
newFlexiTyVarTy Kind
kind = do
    TyVar
tc_tyvar <- Kind -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
newFlexiTyVar Kind
kind
    Kind -> TcM Kind
forall (m :: * -> *) a. Monad m => a -> m a
return (TyVar -> Kind
mkTyVarTy TyVar
tc_tyvar)

newFlexiTyVarTys :: Int -> Kind -> TcM [TcType]
newFlexiTyVarTys :: Int -> Kind -> TcM [Kind]
newFlexiTyVarTys Int
n Kind
kind = Int -> TcM Kind -> TcM [Kind]
forall (m :: * -> *) a. Applicative m => Int -> m a -> m [a]
replicateM Int
n (Kind -> TcM Kind
newFlexiTyVarTy Kind
kind)

newOpenTypeKind :: TcM TcKind
newOpenTypeKind :: TcM Kind
newOpenTypeKind
  = do { Kind
rr <- Kind -> TcM Kind
newFlexiTyVarTy Kind
runtimeRepTy
       ; Kind -> TcM Kind
forall (m :: * -> *) a. Monad m => a -> m a
return (Kind -> Kind
tYPE Kind
rr) }

-- | Create a tyvar that can be a lifted or unlifted type.
-- Returns alpha :: TYPE kappa, where both alpha and kappa are fresh
newOpenFlexiTyVarTy :: TcM TcType
newOpenFlexiTyVarTy :: TcM Kind
newOpenFlexiTyVarTy
  = do { TyVar
tv <- IOEnv (Env TcGblEnv TcLclEnv) TyVar
newOpenFlexiTyVar
       ; Kind -> TcM Kind
forall (m :: * -> *) a. Monad m => a -> m a
return (TyVar -> Kind
mkTyVarTy TyVar
tv) }

newOpenFlexiTyVar :: TcM TcTyVar
newOpenFlexiTyVar :: IOEnv (Env TcGblEnv TcLclEnv) TyVar
newOpenFlexiTyVar
  = do { Kind
kind <- TcM Kind
newOpenTypeKind
       ; Kind -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
newFlexiTyVar Kind
kind }

newOpenBoxedTypeKind :: TcM TcKind
newOpenBoxedTypeKind :: TcM Kind
newOpenBoxedTypeKind
  = do { Kind
lev <- Kind -> TcM Kind
newFlexiTyVarTy (TyCon -> Kind
mkTyConTy TyCon
levityTyCon)
       ; let rr :: Kind
rr = TyCon -> [Kind] -> Kind
mkTyConApp TyCon
boxedRepDataConTyCon [Kind
lev]
       ; Kind -> TcM Kind
forall (m :: * -> *) a. Monad m => a -> m a
return (Kind -> Kind
tYPE Kind
rr) }

newMetaTyVars :: [TyVar] -> TcM (TCvSubst, [TcTyVar])
-- Instantiate with META type variables
-- Note that this works for a sequence of kind, type, and coercion variables
-- variables.  Eg    [ (k:*), (a:k->k) ]
--             Gives [ (k7:*), (a8:k7->k7) ]
newMetaTyVars :: [TyVar] -> TcM (TCvSubst, [TyVar])
newMetaTyVars = TCvSubst -> [TyVar] -> TcM (TCvSubst, [TyVar])
newMetaTyVarsX TCvSubst
emptyTCvSubst
    -- emptyTCvSubst has an empty in-scope set, but that's fine here
    -- Since the tyvars are freshly made, they cannot possibly be
    -- captured by any existing for-alls.

newMetaTyVarsX :: TCvSubst -> [TyVar] -> TcM (TCvSubst, [TcTyVar])
-- Just like newMetaTyVars, but start with an existing substitution.
newMetaTyVarsX :: TCvSubst -> [TyVar] -> TcM (TCvSubst, [TyVar])
newMetaTyVarsX TCvSubst
subst = (TCvSubst
 -> TyVar -> IOEnv (Env TcGblEnv TcLclEnv) (TCvSubst, TyVar))
-> TCvSubst -> [TyVar] -> TcM (TCvSubst, [TyVar])
forall (m :: * -> *) acc x y.
Monad m =>
(acc -> x -> m (acc, y)) -> acc -> [x] -> m (acc, [y])
mapAccumLM TCvSubst
-> TyVar -> IOEnv (Env TcGblEnv TcLclEnv) (TCvSubst, TyVar)
newMetaTyVarX TCvSubst
subst

newMetaTyVarX :: TCvSubst -> TyVar -> TcM (TCvSubst, TcTyVar)
-- Make a new unification variable tyvar whose Name and Kind come from
-- an existing TyVar. We substitute kind variables in the kind.
newMetaTyVarX :: TCvSubst
-> TyVar -> IOEnv (Env TcGblEnv TcLclEnv) (TCvSubst, TyVar)
newMetaTyVarX = MetaInfo
-> TCvSubst
-> TyVar
-> IOEnv (Env TcGblEnv TcLclEnv) (TCvSubst, TyVar)
new_meta_tv_x MetaInfo
TauTv

newMetaTyVarTyVarX :: TCvSubst -> TyVar -> TcM (TCvSubst, TcTyVar)
-- Just like newMetaTyVarX, but make a TyVarTv
newMetaTyVarTyVarX :: TCvSubst
-> TyVar -> IOEnv (Env TcGblEnv TcLclEnv) (TCvSubst, TyVar)
newMetaTyVarTyVarX = MetaInfo
-> TCvSubst
-> TyVar
-> IOEnv (Env TcGblEnv TcLclEnv) (TCvSubst, TyVar)
new_meta_tv_x MetaInfo
TyVarTv

newWildCardX :: TCvSubst -> TyVar -> TcM (TCvSubst, TcTyVar)
newWildCardX :: TCvSubst
-> TyVar -> IOEnv (Env TcGblEnv TcLclEnv) (TCvSubst, TyVar)
newWildCardX TCvSubst
subst TyVar
tv
  = do { TyVar
new_tv <- MetaInfo -> Kind -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
newAnonMetaTyVar MetaInfo
TauTv (HasCallStack => TCvSubst -> Kind -> Kind
TCvSubst -> Kind -> Kind
substTy TCvSubst
subst (TyVar -> Kind
tyVarKind TyVar
tv))
       ; (TCvSubst, TyVar)
-> IOEnv (Env TcGblEnv TcLclEnv) (TCvSubst, TyVar)
forall (m :: * -> *) a. Monad m => a -> m a
return (TCvSubst -> TyVar -> TyVar -> TCvSubst
extendTvSubstWithClone TCvSubst
subst TyVar
tv TyVar
new_tv, TyVar
new_tv) }

new_meta_tv_x :: MetaInfo -> TCvSubst -> TyVar -> TcM (TCvSubst, TcTyVar)
new_meta_tv_x :: MetaInfo
-> TCvSubst
-> TyVar
-> IOEnv (Env TcGblEnv TcLclEnv) (TCvSubst, TyVar)
new_meta_tv_x MetaInfo
info TCvSubst
subst TyVar
tv
  = do  { TyVar
new_tv <- MetaInfo -> TyVar -> Kind -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
cloneAnonMetaTyVar MetaInfo
info TyVar
tv Kind
substd_kind
        ; let subst1 :: TCvSubst
subst1 = TCvSubst -> TyVar -> TyVar -> TCvSubst
extendTvSubstWithClone TCvSubst
subst TyVar
tv TyVar
new_tv
        ; (TCvSubst, TyVar)
-> IOEnv (Env TcGblEnv TcLclEnv) (TCvSubst, TyVar)
forall (m :: * -> *) a. Monad m => a -> m a
return (TCvSubst
subst1, TyVar
new_tv) }
  where
    substd_kind :: Kind
substd_kind = TCvSubst -> Kind -> Kind
substTyUnchecked TCvSubst
subst (TyVar -> Kind
tyVarKind TyVar
tv)
      -- NOTE: #12549 is fixed so we could use
      -- substTy here, but the tc_infer_args problem
      -- is not yet fixed so leaving as unchecked for now.
      -- OLD NOTE:
      -- Unchecked because we call newMetaTyVarX from
      -- tcInstTyBinder, which is called from tcInferTyApps
      -- which does not yet take enough trouble to ensure
      -- the in-scope set is right; e.g. #12785 trips
      -- if we use substTy here

newMetaTyVarTyAtLevel :: TcLevel -> TcKind -> TcM TcType
newMetaTyVarTyAtLevel :: TcLevel -> Kind -> TcM Kind
newMetaTyVarTyAtLevel TcLevel
tc_lvl Kind
kind
  = do  { TcTyVarDetails
details <- TcLevel -> TcM TcTyVarDetails
newTauTvDetailsAtLevel TcLevel
tc_lvl
        ; Name
name    <- FastString -> TcM Name
newMetaTyVarName (String -> FastString
fsLit String
"p")
        ; Kind -> TcM Kind
forall (m :: * -> *) a. Monad m => a -> m a
return (TyVar -> Kind
mkTyVarTy (Name -> Kind -> TcTyVarDetails -> TyVar
mkTcTyVar Name
name Kind
kind TcTyVarDetails
details)) }

{- *********************************************************************
*                                                                      *
          Finding variables to quantify over
*                                                                      *
********************************************************************* -}

{- Note [Dependent type variables]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In Haskell type inference we quantify over type variables; but we only
quantify over /kind/ variables when -XPolyKinds is on.  Without -XPolyKinds
we default the kind variables to *.

So, to support this defaulting, and only for that reason, when
collecting the free vars of a type (in candidateQTyVarsOfType and friends),
prior to quantifying, we must keep the type and kind variables separate.

But what does that mean in a system where kind variables /are/ type
variables? It's a fairly arbitrary distinction based on how the
variables appear:

  - "Kind variables" appear in the kind of some other free variable
    or in the kind of a locally quantified type variable
    (forall (a :: kappa). ...) or in the kind of a coercion
    (a |> (co :: kappa1 ~ kappa2)).

     These are the ones we default to * if -XPolyKinds is off

  - "Type variables" are all free vars that are not kind variables

E.g.  In the type    T k (a::k)
      'k' is a kind variable, because it occurs in the kind of 'a',
          even though it also appears at "top level" of the type
      'a' is a type variable, because it doesn't

We gather these variables using a CandidatesQTvs record:
  DV { dv_kvs: Variables free in the kind of a free type variable
               or of a forall-bound type variable
     , dv_tvs: Variables syntactically free in the type }

So:  dv_kvs            are the kind variables of the type
     (dv_tvs - dv_kvs) are the type variable of the type

Note that

* A variable can occur in both.
      T k (x::k)    The first occurrence of k makes it
                    show up in dv_tvs, the second in dv_kvs

* We include any coercion variables in the "dependent",
  "kind-variable" set because we never quantify over them.

* The "kind variables" might depend on each other; e.g
     (k1 :: k2), (k2 :: *)
  The "type variables" do not depend on each other; if
  one did, it'd be classified as a kind variable!

Note [CandidatesQTvs determinism and order]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
* Determinism: when we quantify over type variables we decide the
  order in which they appear in the final type. Because the order of
  type variables in the type can end up in the interface file and
  affects some optimizations like worker-wrapper, we want this order to
  be deterministic.

  To achieve that we use deterministic sets of variables that can be
  converted to lists in a deterministic order. For more information
  about deterministic sets see Note [Deterministic UniqFM] in GHC.Types.Unique.DFM.

* Order: as well as being deterministic, we use an
  accumulating-parameter style for candidateQTyVarsOfType so that we
  add variables one at a time, left to right.  That means we tend to
  produce the variables in left-to-right order.  This is just to make
  it bit more predictable for the programmer.

Note [Naughty quantification candidates]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider (#14880, dependent/should_compile/T14880-2), suppose
we are trying to generalise this type:

  forall arg. ... (alpha[tau]:arg) ...

We have a metavariable alpha whose kind mentions a skolem variable
bound inside the very type we are generalising.
This can arise while type-checking a user-written type signature
(see the test case for the full code).

We cannot generalise over alpha!  That would produce a type like
  forall {a :: arg}. forall arg. ...blah...
The fact that alpha's kind mentions arg renders it completely
ineligible for generalisation.

However, we are not going to learn any new constraints on alpha,
because its kind isn't even in scope in the outer context (but see Wrinkle).
So alpha is entirely unconstrained.

What then should we do with alpha?  During generalization, every
metavariable is either (A) promoted, (B) generalized, or (C) zapped
(according to Note [Recipe for checking a signature] in GHC.Tc.Gen.HsType).

 * We can't generalise it.
 * We can't promote it, because its kind prevents that
 * We can't simply leave it be, because this type is about to
   go into the typing environment (as the type of some let-bound
   variable, say), and then chaos erupts when we try to instantiate.

Previously, we zapped it to Any. This worked, but it had the unfortunate
effect of causing Any sometimes to appear in error messages. If this
kind of signature happens, the user probably has made a mistake -- no
one really wants Any in their types. So we now error. This must be
a hard error (failure in the monad) to avoid other messages from mentioning
Any.

We do this eager erroring in candidateQTyVars, which always precedes
generalisation, because at that moment we have a clear picture of what
skolems are in scope within the type itself (e.g. that 'forall arg').

This change is inspired by and described in Section 7.2 of "Kind Inference
for Datatypes", POPL'20.

Wrinkle:

We must make absolutely sure that alpha indeed is not
from an outer context. (Otherwise, we might indeed learn more information
about it.) This can be done easily: we just check alpha's TcLevel.
That level must be strictly greater than the ambient TcLevel in order
to treat it as naughty. We say "strictly greater than" because the call to
candidateQTyVars is made outside the bumped TcLevel, as stated in the
comment to candidateQTyVarsOfType. The level check is done in go_tv
in collect_cand_qtvs. Skipping this check caused #16517.

-}

data CandidatesQTvs
  -- See Note [Dependent type variables]
  -- See Note [CandidatesQTvs determinism and order]
  --
  -- Invariants:
  --   * All variables are fully zonked, including their kinds
  --   * All variables are at a level greater than the ambient level
  --     See Note [Use level numbers for quantification]
  --
  -- This *can* contain skolems. For example, in `data X k :: k -> Type`
  -- we need to know that the k is a dependent variable. This is done
  -- by collecting the candidates in the kind after skolemising. It also
  -- comes up when generalizing a associated type instance, where instance
  -- variables are skolems. (Recall that associated type instances are generalized
  -- independently from their enclosing class instance, and the associated
  -- type instance may be generalized by more, fewer, or different variables
  -- than the class instance.)
  --
  = DV { CandidatesQTvs -> DTyVarSet
dv_kvs :: DTyVarSet    -- "kind" metavariables (dependent)
       , CandidatesQTvs -> DTyVarSet
dv_tvs :: DTyVarSet    -- "type" metavariables (non-dependent)
         -- A variable may appear in both sets
         -- E.g.   T k (x::k)    The first occurrence of k makes it
         --                      show up in dv_tvs, the second in dv_kvs
         -- See Note [Dependent type variables]

       , CandidatesQTvs -> CoVarSet
dv_cvs :: CoVarSet
         -- These are covars. Included only so that we don't repeatedly
         -- look at covars' kinds in accumulator. Not used by quantifyTyVars.
    }

instance Semi.Semigroup CandidatesQTvs where
   (DV { dv_kvs :: CandidatesQTvs -> DTyVarSet
dv_kvs = DTyVarSet
kv1, dv_tvs :: CandidatesQTvs -> DTyVarSet
dv_tvs = DTyVarSet
tv1, dv_cvs :: CandidatesQTvs -> CoVarSet
dv_cvs = CoVarSet
cv1 })
     <> :: CandidatesQTvs -> CandidatesQTvs -> CandidatesQTvs
<> (DV { dv_kvs :: CandidatesQTvs -> DTyVarSet
dv_kvs = DTyVarSet
kv2, dv_tvs :: CandidatesQTvs -> DTyVarSet
dv_tvs = DTyVarSet
tv2, dv_cvs :: CandidatesQTvs -> CoVarSet
dv_cvs = CoVarSet
cv2 })
          = DV { dv_kvs :: DTyVarSet
dv_kvs = DTyVarSet
kv1 DTyVarSet -> DTyVarSet -> DTyVarSet
`unionDVarSet` DTyVarSet
kv2
               , dv_tvs :: DTyVarSet
dv_tvs = DTyVarSet
tv1 DTyVarSet -> DTyVarSet -> DTyVarSet
`unionDVarSet` DTyVarSet
tv2
               , dv_cvs :: CoVarSet
dv_cvs = CoVarSet
cv1 CoVarSet -> CoVarSet -> CoVarSet
`unionVarSet` CoVarSet
cv2 }

instance Monoid CandidatesQTvs where
   mempty :: CandidatesQTvs
mempty = DV { dv_kvs :: DTyVarSet
dv_kvs = DTyVarSet
emptyDVarSet, dv_tvs :: DTyVarSet
dv_tvs = DTyVarSet
emptyDVarSet, dv_cvs :: CoVarSet
dv_cvs = CoVarSet
emptyVarSet }
   mappend :: CandidatesQTvs -> CandidatesQTvs -> CandidatesQTvs
mappend = CandidatesQTvs -> CandidatesQTvs -> CandidatesQTvs
forall a. Semigroup a => a -> a -> a
(Semi.<>)

instance Outputable CandidatesQTvs where
  ppr :: CandidatesQTvs -> SDoc
ppr (DV {dv_kvs :: CandidatesQTvs -> DTyVarSet
dv_kvs = DTyVarSet
kvs, dv_tvs :: CandidatesQTvs -> DTyVarSet
dv_tvs = DTyVarSet
tvs, dv_cvs :: CandidatesQTvs -> CoVarSet
dv_cvs = CoVarSet
cvs })
    = String -> SDoc
text String
"DV" SDoc -> SDoc -> SDoc
<+> SDoc -> SDoc
braces ((SDoc -> SDoc) -> [SDoc] -> SDoc
forall a. (a -> SDoc) -> [a] -> SDoc
pprWithCommas SDoc -> SDoc
forall a. a -> a
id [ String -> SDoc
text String
"dv_kvs =" SDoc -> SDoc -> SDoc
<+> DTyVarSet -> SDoc
forall a. Outputable a => a -> SDoc
ppr DTyVarSet
kvs
                                             , String -> SDoc
text String
"dv_tvs =" SDoc -> SDoc -> SDoc
<+> DTyVarSet -> SDoc
forall a. Outputable a => a -> SDoc
ppr DTyVarSet
tvs
                                             , String -> SDoc
text String
"dv_cvs =" SDoc -> SDoc -> SDoc
<+> CoVarSet -> SDoc
forall a. Outputable a => a -> SDoc
ppr CoVarSet
cvs ])

isEmptyCandidates :: CandidatesQTvs -> Bool
isEmptyCandidates :: CandidatesQTvs -> Bool
isEmptyCandidates (DV { dv_kvs :: CandidatesQTvs -> DTyVarSet
dv_kvs = DTyVarSet
kvs, dv_tvs :: CandidatesQTvs -> DTyVarSet
dv_tvs = DTyVarSet
tvs })
  = DTyVarSet -> Bool
isEmptyDVarSet DTyVarSet
kvs Bool -> Bool -> Bool
&& DTyVarSet -> Bool
isEmptyDVarSet DTyVarSet
tvs

-- | Extract out the kind vars (in order) and type vars (in order) from
-- a 'CandidatesQTvs'. The lists are guaranteed to be distinct. Keeping
-- the lists separate is important only in the -XNoPolyKinds case.
candidateVars :: CandidatesQTvs -> ([TcTyVar], [TcTyVar])
candidateVars :: CandidatesQTvs -> ([TyVar], [TyVar])
candidateVars (DV { dv_kvs :: CandidatesQTvs -> DTyVarSet
dv_kvs = DTyVarSet
dep_kv_set, dv_tvs :: CandidatesQTvs -> DTyVarSet
dv_tvs = DTyVarSet
nondep_tkv_set })
  = ([TyVar]
dep_kvs, [TyVar]
nondep_tvs)
  where
    dep_kvs :: [TyVar]
dep_kvs = [TyVar] -> [TyVar]
scopedSort ([TyVar] -> [TyVar]) -> [TyVar] -> [TyVar]
forall a b. (a -> b) -> a -> b
$ DTyVarSet -> [TyVar]
dVarSetElems DTyVarSet
dep_kv_set
      -- scopedSort: put the kind variables into
      --    well-scoped order.
      --    E.g.  [k, (a::k)] not the other way round

    nondep_tvs :: [TyVar]
nondep_tvs = DTyVarSet -> [TyVar]
dVarSetElems (DTyVarSet
nondep_tkv_set DTyVarSet -> DTyVarSet -> DTyVarSet
`minusDVarSet` DTyVarSet
dep_kv_set)
      -- See Note [Dependent type variables]
      -- The `minus` dep_tkvs removes any kind-level vars
      --    e.g. T k (a::k)   Since k appear in a kind it'll
      --    be in dv_kvs, and is dependent. So remove it from
      --    dv_tvs which will also contain k
      -- NB kinds of tvs are already zonked

candidateKindVars :: CandidatesQTvs -> TyVarSet
candidateKindVars :: CandidatesQTvs -> CoVarSet
candidateKindVars CandidatesQTvs
dvs = DTyVarSet -> CoVarSet
dVarSetToVarSet (CandidatesQTvs -> DTyVarSet
dv_kvs CandidatesQTvs
dvs)

partitionCandidates :: CandidatesQTvs -> (TyVar -> Bool) -> (TyVarSet, CandidatesQTvs)
-- The selected TyVars are returned as a non-deterministic TyVarSet
partitionCandidates :: CandidatesQTvs -> (TyVar -> Bool) -> (CoVarSet, CandidatesQTvs)
partitionCandidates dvs :: CandidatesQTvs
dvs@(DV { dv_kvs :: CandidatesQTvs -> DTyVarSet
dv_kvs = DTyVarSet
kvs, dv_tvs :: CandidatesQTvs -> DTyVarSet
dv_tvs = DTyVarSet
tvs }) TyVar -> Bool
pred
  = (CoVarSet
extracted, CandidatesQTvs
dvs { dv_kvs :: DTyVarSet
dv_kvs = DTyVarSet
rest_kvs, dv_tvs :: DTyVarSet
dv_tvs = DTyVarSet
rest_tvs })
  where
    (DTyVarSet
extracted_kvs, DTyVarSet
rest_kvs) = (TyVar -> Bool) -> DTyVarSet -> (DTyVarSet, DTyVarSet)
partitionDVarSet TyVar -> Bool
pred DTyVarSet
kvs
    (DTyVarSet
extracted_tvs, DTyVarSet
rest_tvs) = (TyVar -> Bool) -> DTyVarSet -> (DTyVarSet, DTyVarSet)
partitionDVarSet TyVar -> Bool
pred DTyVarSet
tvs
    extracted :: CoVarSet
extracted = DTyVarSet -> CoVarSet
dVarSetToVarSet DTyVarSet
extracted_kvs CoVarSet -> CoVarSet -> CoVarSet
`unionVarSet` DTyVarSet -> CoVarSet
dVarSetToVarSet DTyVarSet
extracted_tvs

-- | Gathers free variables to use as quantification candidates (in
-- 'quantifyTyVars'). This might output the same var
-- in both sets, if it's used in both a type and a kind.
-- The variables to quantify must have a TcLevel strictly greater than
-- the ambient level. (See Wrinkle in Note [Naughty quantification candidates])
-- See Note [CandidatesQTvs determinism and order]
-- See Note [Dependent type variables]
candidateQTyVarsOfType :: TcType       -- not necessarily zonked
                       -> TcM CandidatesQTvs
candidateQTyVarsOfType :: Kind -> TcM CandidatesQTvs
candidateQTyVarsOfType Kind
ty = Kind
-> Bool -> CoVarSet -> CandidatesQTvs -> Kind -> TcM CandidatesQTvs
collect_cand_qtvs Kind
ty Bool
False CoVarSet
emptyVarSet CandidatesQTvs
forall a. Monoid a => a
mempty Kind
ty

-- | Like 'candidateQTyVarsOfType', but over a list of types
-- The variables to quantify must have a TcLevel strictly greater than
-- the ambient level. (See Wrinkle in Note [Naughty quantification candidates])
candidateQTyVarsOfTypes :: [Type] -> TcM CandidatesQTvs
candidateQTyVarsOfTypes :: [Kind] -> TcM CandidatesQTvs
candidateQTyVarsOfTypes [Kind]
tys = (CandidatesQTvs -> Kind -> TcM CandidatesQTvs)
-> CandidatesQTvs -> [Kind] -> TcM CandidatesQTvs
forall (t :: * -> *) (m :: * -> *) b a.
(Foldable t, Monad m) =>
(b -> a -> m b) -> b -> t a -> m b
foldlM (\CandidatesQTvs
acc Kind
ty -> Kind
-> Bool -> CoVarSet -> CandidatesQTvs -> Kind -> TcM CandidatesQTvs
collect_cand_qtvs Kind
ty Bool
False CoVarSet
emptyVarSet CandidatesQTvs
acc Kind
ty)
                                     CandidatesQTvs
forall a. Monoid a => a
mempty [Kind]
tys

-- | Like 'candidateQTyVarsOfType', but consider every free variable
-- to be dependent. This is appropriate when generalizing a *kind*,
-- instead of a type. (That way, -XNoPolyKinds will default the variables
-- to Type.)
candidateQTyVarsOfKind :: TcKind       -- Not necessarily zonked
                       -> TcM CandidatesQTvs
candidateQTyVarsOfKind :: Kind -> TcM CandidatesQTvs
candidateQTyVarsOfKind Kind
ty = Kind
-> Bool -> CoVarSet -> CandidatesQTvs -> Kind -> TcM CandidatesQTvs
collect_cand_qtvs Kind
ty Bool
True CoVarSet
emptyVarSet CandidatesQTvs
forall a. Monoid a => a
mempty Kind
ty

candidateQTyVarsOfKinds :: [TcKind]    -- Not necessarily zonked
                       -> TcM CandidatesQTvs
candidateQTyVarsOfKinds :: [Kind] -> TcM CandidatesQTvs
candidateQTyVarsOfKinds [Kind]
tys = (CandidatesQTvs -> Kind -> TcM CandidatesQTvs)
-> CandidatesQTvs -> [Kind] -> TcM CandidatesQTvs
forall (t :: * -> *) (m :: * -> *) b a.
(Foldable t, Monad m) =>
(b -> a -> m b) -> b -> t a -> m b
foldM (\CandidatesQTvs
acc Kind
ty -> Kind
-> Bool -> CoVarSet -> CandidatesQTvs -> Kind -> TcM CandidatesQTvs
collect_cand_qtvs Kind
ty Bool
True CoVarSet
emptyVarSet CandidatesQTvs
acc Kind
ty)
                                    CandidatesQTvs
forall a. Monoid a => a
mempty [Kind]
tys

delCandidates :: CandidatesQTvs -> [Var] -> CandidatesQTvs
delCandidates :: CandidatesQTvs -> [TyVar] -> CandidatesQTvs
delCandidates (DV { dv_kvs :: CandidatesQTvs -> DTyVarSet
dv_kvs = DTyVarSet
kvs, dv_tvs :: CandidatesQTvs -> DTyVarSet
dv_tvs = DTyVarSet
tvs, dv_cvs :: CandidatesQTvs -> CoVarSet
dv_cvs = CoVarSet
cvs }) [TyVar]
vars
  = DV { dv_kvs :: DTyVarSet
dv_kvs = DTyVarSet
kvs DTyVarSet -> [TyVar] -> DTyVarSet
`delDVarSetList` [TyVar]
vars
       , dv_tvs :: DTyVarSet
dv_tvs = DTyVarSet
tvs DTyVarSet -> [TyVar] -> DTyVarSet
`delDVarSetList` [TyVar]
vars
       , dv_cvs :: CoVarSet
dv_cvs = CoVarSet
cvs CoVarSet -> [TyVar] -> CoVarSet
`delVarSetList`  [TyVar]
vars }

collect_cand_qtvs
  :: TcType          -- original type that we started recurring into; for errors
  -> Bool            -- True <=> consider every fv in Type to be dependent
  -> VarSet          -- Bound variables (locals only)
  -> CandidatesQTvs  -- Accumulating parameter
  -> Type            -- Not necessarily zonked
  -> TcM CandidatesQTvs

-- Key points:
--   * Looks through meta-tyvars as it goes;
--     no need to zonk in advance
--
--   * Needs to be monadic anyway, because it handles naughty
--     quantification; see Note [Naughty quantification candidates]
--
--   * Returns fully-zonked CandidateQTvs, including their kinds
--     so that subsequent dependency analysis (to build a well
--     scoped telescope) works correctly

collect_cand_qtvs :: Kind
-> Bool -> CoVarSet -> CandidatesQTvs -> Kind -> TcM CandidatesQTvs
collect_cand_qtvs Kind
orig_ty Bool
is_dep CoVarSet
bound CandidatesQTvs
dvs Kind
ty
  = CandidatesQTvs -> Kind -> TcM CandidatesQTvs
go CandidatesQTvs
dvs Kind
ty
  where
    is_bound :: TyVar -> Bool
is_bound TyVar
tv = TyVar
tv TyVar -> CoVarSet -> Bool
`elemVarSet` CoVarSet
bound

    -----------------
    go :: CandidatesQTvs -> TcType -> TcM CandidatesQTvs
    -- Uses accumulating-parameter style
    go :: CandidatesQTvs -> Kind -> TcM CandidatesQTvs
go CandidatesQTvs
dv (AppTy Kind
t1 Kind
t2)    = (CandidatesQTvs -> Kind -> TcM CandidatesQTvs)
-> CandidatesQTvs -> [Kind] -> TcM CandidatesQTvs
forall (t :: * -> *) (m :: * -> *) b a.
(Foldable t, Monad m) =>
(b -> a -> m b) -> b -> t a -> m b
foldlM CandidatesQTvs -> Kind -> TcM CandidatesQTvs
go CandidatesQTvs
dv [Kind
t1, Kind
t2]
    go CandidatesQTvs
dv (TyConApp TyCon
tc [Kind]
tys) = CandidatesQTvs -> [TyConBinder] -> [Kind] -> TcM CandidatesQTvs
go_tc_args CandidatesQTvs
dv (TyCon -> [TyConBinder]
tyConBinders TyCon
tc) [Kind]
tys
    go CandidatesQTvs
dv (FunTy AnonArgFlag
_ Kind
w Kind
arg Kind
res) = (CandidatesQTvs -> Kind -> TcM CandidatesQTvs)
-> CandidatesQTvs -> [Kind] -> TcM CandidatesQTvs
forall (t :: * -> *) (m :: * -> *) b a.
(Foldable t, Monad m) =>
(b -> a -> m b) -> b -> t a -> m b
foldlM CandidatesQTvs -> Kind -> TcM CandidatesQTvs
go CandidatesQTvs
dv [Kind
w, Kind
arg, Kind
res]
    go CandidatesQTvs
dv (LitTy {})        = CandidatesQTvs -> TcM CandidatesQTvs
forall (m :: * -> *) a. Monad m => a -> m a
return CandidatesQTvs
dv
    go CandidatesQTvs
dv (CastTy Kind
ty Coercion
co)    = do CandidatesQTvs
dv1 <- CandidatesQTvs -> Kind -> TcM CandidatesQTvs
go CandidatesQTvs
dv Kind
ty
                                 Kind
-> CoVarSet -> CandidatesQTvs -> Coercion -> TcM CandidatesQTvs
collect_cand_qtvs_co Kind
orig_ty CoVarSet
bound CandidatesQTvs
dv1 Coercion
co
    go CandidatesQTvs
dv (CoercionTy Coercion
co)   = Kind
-> CoVarSet -> CandidatesQTvs -> Coercion -> TcM CandidatesQTvs
collect_cand_qtvs_co Kind
orig_ty CoVarSet
bound CandidatesQTvs
dv Coercion
co

    go CandidatesQTvs
dv (TyVarTy TyVar
tv)
      | TyVar -> Bool
is_bound TyVar
tv = CandidatesQTvs -> TcM CandidatesQTvs
forall (m :: * -> *) a. Monad m => a -> m a
return CandidatesQTvs
dv
      | Bool
otherwise   = do { Maybe Kind
m_contents <- TyVar -> TcM (Maybe Kind)
isFilledMetaTyVar_maybe TyVar
tv
                         ; case Maybe Kind
m_contents of
                             Just Kind
ind_ty -> CandidatesQTvs -> Kind -> TcM CandidatesQTvs
go CandidatesQTvs
dv Kind
ind_ty
                             Maybe Kind
Nothing     -> CandidatesQTvs -> TyVar -> TcM CandidatesQTvs
go_tv CandidatesQTvs
dv TyVar
tv }

    go CandidatesQTvs
dv (ForAllTy (Bndr TyVar
tv ArgFlag
_) Kind
ty)
      = do { CandidatesQTvs
dv1 <- Kind
-> Bool -> CoVarSet -> CandidatesQTvs -> Kind -> TcM CandidatesQTvs
collect_cand_qtvs Kind
orig_ty Bool
True CoVarSet
bound CandidatesQTvs
dv (TyVar -> Kind
tyVarKind TyVar
tv)
           ; Kind
-> Bool -> CoVarSet -> CandidatesQTvs -> Kind -> TcM CandidatesQTvs
collect_cand_qtvs Kind
orig_ty Bool
is_dep (CoVarSet
bound CoVarSet -> TyVar -> CoVarSet
`extendVarSet` TyVar
tv) CandidatesQTvs
dv1 Kind
ty }

      -- This makes sure that we default e.g. the alpha in Proxy alpha (Any alpha).
      -- Tested in polykinds/NestedProxies.
      -- We just might get this wrong in AppTy, but I don't think that's possible
      -- with -XNoPolyKinds. And fixing it would be non-performant, as we'd need
      -- to look at kinds.
    go_tc_args :: CandidatesQTvs -> [TyConBinder] -> [Kind] -> TcM CandidatesQTvs
go_tc_args CandidatesQTvs
dv (TyConBinder
tc_bndr:[TyConBinder]
tc_bndrs) (Kind
ty:[Kind]
tys)
      = do { CandidatesQTvs
dv1 <- Kind
-> Bool -> CoVarSet -> CandidatesQTvs -> Kind -> TcM CandidatesQTvs
collect_cand_qtvs Kind
orig_ty (Bool
is_dep Bool -> Bool -> Bool
|| TyConBinder -> Bool
isNamedTyConBinder TyConBinder
tc_bndr)
                                      CoVarSet
bound CandidatesQTvs
dv Kind
ty
           ; CandidatesQTvs -> [TyConBinder] -> [Kind] -> TcM CandidatesQTvs
go_tc_args CandidatesQTvs
dv1 [TyConBinder]
tc_bndrs [Kind]
tys }
    go_tc_args CandidatesQTvs
dv [TyConBinder]
_bndrs [Kind]
tys  -- _bndrs might be non-empty: undersaturation
                              -- tys might be non-empty: oversaturation
                              -- either way, the foldlM is correct
      = (CandidatesQTvs -> Kind -> TcM CandidatesQTvs)
-> CandidatesQTvs -> [Kind] -> TcM CandidatesQTvs
forall (t :: * -> *) (m :: * -> *) b a.
(Foldable t, Monad m) =>
(b -> a -> m b) -> b -> t a -> m b
foldlM CandidatesQTvs -> Kind -> TcM CandidatesQTvs
go CandidatesQTvs
dv [Kind]
tys

    -----------------
    go_tv :: CandidatesQTvs -> TyVar -> TcM CandidatesQTvs
go_tv dv :: CandidatesQTvs
dv@(DV { dv_kvs :: CandidatesQTvs -> DTyVarSet
dv_kvs = DTyVarSet
kvs, dv_tvs :: CandidatesQTvs -> DTyVarSet
dv_tvs = DTyVarSet
tvs }) TyVar
tv
      | TyVar
tv TyVar -> DTyVarSet -> Bool
`elemDVarSet` DTyVarSet
kvs
      = CandidatesQTvs -> TcM CandidatesQTvs
forall (m :: * -> *) a. Monad m => a -> m a
return CandidatesQTvs
dv  -- We have met this tyvar already

      | Bool -> Bool
not Bool
is_dep
      , TyVar
tv TyVar -> DTyVarSet -> Bool
`elemDVarSet` DTyVarSet
tvs
      = CandidatesQTvs -> TcM CandidatesQTvs
forall (m :: * -> *) a. Monad m => a -> m a
return CandidatesQTvs
dv  -- We have met this tyvar already

      | Bool
otherwise
      = do { Kind
tv_kind <- Kind -> TcM Kind
zonkTcType (TyVar -> Kind
tyVarKind TyVar
tv)
                 -- This zonk is annoying, but it is necessary, both to
                 -- ensure that the collected candidates have zonked kinds
                 -- (#15795) and to make the naughty check
                 -- (which comes next) works correctly

           ; let tv_kind_vars :: CoVarSet
tv_kind_vars = Kind -> CoVarSet
tyCoVarsOfType Kind
tv_kind
           ; TcLevel
cur_lvl <- TcM TcLevel
getTcLevel
           ; if |  TyVar -> TcLevel
tcTyVarLevel TyVar
tv TcLevel -> TcLevel -> Bool
forall a. Ord a => a -> a -> Bool
<= TcLevel
cur_lvl
                -> CandidatesQTvs -> TcM CandidatesQTvs
forall (m :: * -> *) a. Monad m => a -> m a
return CandidatesQTvs
dv   -- this variable is from an outer context; skip
                               -- See Note [Use level numbers for quantification]

                |  CoVarSet -> CoVarSet -> Bool
intersectsVarSet CoVarSet
bound CoVarSet
tv_kind_vars
                   -- the tyvar must not be from an outer context, but we have
                   -- already checked for this.
                   -- See Note [Naughty quantification candidates]
                -> do { String -> SDoc -> TcRn ()
traceTc String
"Naughty quantifier" (SDoc -> TcRn ()) -> SDoc -> TcRn ()
forall a b. (a -> b) -> a -> b
$
                          [SDoc] -> SDoc
vcat [ TyVar -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyVar
tv SDoc -> SDoc -> SDoc
<+> SDoc
dcolon SDoc -> SDoc -> SDoc
<+> Kind -> SDoc
forall a. Outputable a => a -> SDoc
ppr Kind
tv_kind
                               , String -> SDoc
text String
"bound:" SDoc -> SDoc -> SDoc
<+> [TyVar] -> SDoc
pprTyVars (CoVarSet -> [TyVar]
forall elt. UniqSet elt -> [elt]
nonDetEltsUniqSet CoVarSet
bound)
                               , String -> SDoc
text String
"fvs:" SDoc -> SDoc -> SDoc
<+> [TyVar] -> SDoc
pprTyVars (CoVarSet -> [TyVar]
forall elt. UniqSet elt -> [elt]
nonDetEltsUniqSet CoVarSet
tv_kind_vars) ]

                      ; let escapees :: CoVarSet
escapees = CoVarSet -> CoVarSet -> CoVarSet
intersectVarSet CoVarSet
bound CoVarSet
tv_kind_vars
                      ; Kind -> TyVar -> CoVarSet -> TcM CandidatesQTvs
forall a. Kind -> TyVar -> CoVarSet -> TcM a
naughtyQuantification Kind
orig_ty TyVar
tv CoVarSet
escapees }

                |  Bool
otherwise
                -> do { let tv' :: TyVar
tv' = TyVar
tv TyVar -> Kind -> TyVar
`setTyVarKind` Kind
tv_kind
                            dv' :: CandidatesQTvs
dv' | Bool
is_dep    = CandidatesQTvs
dv { dv_kvs :: DTyVarSet
dv_kvs = DTyVarSet
kvs DTyVarSet -> TyVar -> DTyVarSet
`extendDVarSet` TyVar
tv' }
                                | Bool
otherwise = CandidatesQTvs
dv { dv_tvs :: DTyVarSet
dv_tvs = DTyVarSet
tvs DTyVarSet -> TyVar -> DTyVarSet
`extendDVarSet` TyVar
tv' }
                                -- See Note [Order of accumulation]

                        -- See Note [Recurring into kinds for candidateQTyVars]
                      ; Kind
-> Bool -> CoVarSet -> CandidatesQTvs -> Kind -> TcM CandidatesQTvs
collect_cand_qtvs Kind
orig_ty Bool
True CoVarSet
bound CandidatesQTvs
dv' Kind
tv_kind } }

collect_cand_qtvs_co :: TcType -- original type at top of recursion; for errors
                     -> VarSet -- bound variables
                     -> CandidatesQTvs -> Coercion
                     -> TcM CandidatesQTvs
collect_cand_qtvs_co :: Kind
-> CoVarSet -> CandidatesQTvs -> Coercion -> TcM CandidatesQTvs
collect_cand_qtvs_co Kind
orig_ty CoVarSet
bound = CandidatesQTvs -> Coercion -> TcM CandidatesQTvs
go_co
  where
    go_co :: CandidatesQTvs -> Coercion -> TcM CandidatesQTvs
go_co CandidatesQTvs
dv (Refl Kind
ty)             = Kind
-> Bool -> CoVarSet -> CandidatesQTvs -> Kind -> TcM CandidatesQTvs
collect_cand_qtvs Kind
orig_ty Bool
True CoVarSet
bound CandidatesQTvs
dv Kind
ty
    go_co CandidatesQTvs
dv (GRefl Role
_ Kind
ty MCoercionN
mco)      = do CandidatesQTvs
dv1 <- Kind
-> Bool -> CoVarSet -> CandidatesQTvs -> Kind -> TcM CandidatesQTvs
collect_cand_qtvs Kind
orig_ty Bool
True CoVarSet
bound CandidatesQTvs
dv Kind
ty
                                        CandidatesQTvs -> MCoercionN -> TcM CandidatesQTvs
go_mco CandidatesQTvs
dv1 MCoercionN
mco
    go_co CandidatesQTvs
dv (TyConAppCo Role
_ TyCon
_ [Coercion]
cos)  = (CandidatesQTvs -> Coercion -> TcM CandidatesQTvs)
-> CandidatesQTvs -> [Coercion] -> TcM CandidatesQTvs
forall (t :: * -> *) (m :: * -> *) b a.
(Foldable t, Monad m) =>
(b -> a -> m b) -> b -> t a -> m b
foldlM CandidatesQTvs -> Coercion -> TcM CandidatesQTvs
go_co CandidatesQTvs
dv [Coercion]
cos
    go_co CandidatesQTvs
dv (AppCo Coercion
co1 Coercion
co2)       = (CandidatesQTvs -> Coercion -> TcM CandidatesQTvs)
-> CandidatesQTvs -> [Coercion] -> TcM CandidatesQTvs
forall (t :: * -> *) (m :: * -> *) b a.
(Foldable t, Monad m) =>
(b -> a -> m b) -> b -> t a -> m b
foldlM CandidatesQTvs -> Coercion -> TcM CandidatesQTvs
go_co CandidatesQTvs
dv [Coercion
co1, Coercion
co2]
    go_co CandidatesQTvs
dv (FunCo Role
_ Coercion
w Coercion
co1 Coercion
co2)   = (CandidatesQTvs -> Coercion -> TcM CandidatesQTvs)
-> CandidatesQTvs -> [Coercion] -> TcM CandidatesQTvs
forall (t :: * -> *) (m :: * -> *) b a.
(Foldable t, Monad m) =>
(b -> a -> m b) -> b -> t a -> m b
foldlM CandidatesQTvs -> Coercion -> TcM CandidatesQTvs
go_co CandidatesQTvs
dv [Coercion
w, Coercion
co1, Coercion
co2]
    go_co CandidatesQTvs
dv (AxiomInstCo CoAxiom Branched
_ Int
_ [Coercion]
cos) = (CandidatesQTvs -> Coercion -> TcM CandidatesQTvs)
-> CandidatesQTvs -> [Coercion] -> TcM CandidatesQTvs
forall (t :: * -> *) (m :: * -> *) b a.
(Foldable t, Monad m) =>
(b -> a -> m b) -> b -> t a -> m b
foldlM CandidatesQTvs -> Coercion -> TcM CandidatesQTvs
go_co CandidatesQTvs
dv [Coercion]
cos
    go_co CandidatesQTvs
dv (AxiomRuleCo CoAxiomRule
_ [Coercion]
cos)   = (CandidatesQTvs -> Coercion -> TcM CandidatesQTvs)
-> CandidatesQTvs -> [Coercion] -> TcM CandidatesQTvs
forall (t :: * -> *) (m :: * -> *) b a.
(Foldable t, Monad m) =>
(b -> a -> m b) -> b -> t a -> m b
foldlM CandidatesQTvs -> Coercion -> TcM CandidatesQTvs
go_co CandidatesQTvs
dv [Coercion]
cos
    go_co CandidatesQTvs
dv (UnivCo UnivCoProvenance
prov Role
_ Kind
t1 Kind
t2) = do CandidatesQTvs
dv1 <- CandidatesQTvs -> UnivCoProvenance -> TcM CandidatesQTvs
go_prov CandidatesQTvs
dv UnivCoProvenance
prov
                                        CandidatesQTvs
dv2 <- Kind
-> Bool -> CoVarSet -> CandidatesQTvs -> Kind -> TcM CandidatesQTvs
collect_cand_qtvs Kind
orig_ty Bool
True CoVarSet
bound CandidatesQTvs
dv1 Kind
t1
                                        Kind
-> Bool -> CoVarSet -> CandidatesQTvs -> Kind -> TcM CandidatesQTvs
collect_cand_qtvs Kind
orig_ty Bool
True CoVarSet
bound CandidatesQTvs
dv2 Kind
t2
    go_co CandidatesQTvs
dv (SymCo Coercion
co)            = CandidatesQTvs -> Coercion -> TcM CandidatesQTvs
go_co CandidatesQTvs
dv Coercion
co
    go_co CandidatesQTvs
dv (TransCo Coercion
co1 Coercion
co2)     = (CandidatesQTvs -> Coercion -> TcM CandidatesQTvs)
-> CandidatesQTvs -> [Coercion] -> TcM CandidatesQTvs
forall (t :: * -> *) (m :: * -> *) b a.
(Foldable t, Monad m) =>
(b -> a -> m b) -> b -> t a -> m b
foldlM CandidatesQTvs -> Coercion -> TcM CandidatesQTvs
go_co CandidatesQTvs
dv [Coercion
co1, Coercion
co2]
    go_co CandidatesQTvs
dv (NthCo Role
_ Int
_ Coercion
co)        = CandidatesQTvs -> Coercion -> TcM CandidatesQTvs
go_co CandidatesQTvs
dv Coercion
co
    go_co CandidatesQTvs
dv (LRCo LeftOrRight
_ Coercion
co)           = CandidatesQTvs -> Coercion -> TcM CandidatesQTvs
go_co CandidatesQTvs
dv Coercion
co
    go_co CandidatesQTvs
dv (InstCo Coercion
co1 Coercion
co2)      = (CandidatesQTvs -> Coercion -> TcM CandidatesQTvs)
-> CandidatesQTvs -> [Coercion] -> TcM CandidatesQTvs
forall (t :: * -> *) (m :: * -> *) b a.
(Foldable t, Monad m) =>
(b -> a -> m b) -> b -> t a -> m b
foldlM CandidatesQTvs -> Coercion -> TcM CandidatesQTvs
go_co CandidatesQTvs
dv [Coercion
co1, Coercion
co2]
    go_co CandidatesQTvs
dv (KindCo Coercion
co)           = CandidatesQTvs -> Coercion -> TcM CandidatesQTvs
go_co CandidatesQTvs
dv Coercion
co
    go_co CandidatesQTvs
dv (SubCo Coercion
co)            = CandidatesQTvs -> Coercion -> TcM CandidatesQTvs
go_co CandidatesQTvs
dv Coercion
co

    go_co CandidatesQTvs
dv (HoleCo CoercionHole
hole)
      = do Maybe Coercion
m_co <- CoercionHole -> IOEnv (Env TcGblEnv TcLclEnv) (Maybe Coercion)
unpackCoercionHole_maybe CoercionHole
hole
           case Maybe Coercion
m_co of
             Just Coercion
co -> CandidatesQTvs -> Coercion -> TcM CandidatesQTvs
go_co CandidatesQTvs
dv Coercion
co
             Maybe Coercion
Nothing -> CandidatesQTvs -> TyVar -> TcM CandidatesQTvs
go_cv CandidatesQTvs
dv (CoercionHole -> TyVar
coHoleCoVar CoercionHole
hole)

    go_co CandidatesQTvs
dv (CoVarCo TyVar
cv) = CandidatesQTvs -> TyVar -> TcM CandidatesQTvs
go_cv CandidatesQTvs
dv TyVar
cv

    go_co CandidatesQTvs
dv (ForAllCo TyVar
tcv Coercion
kind_co Coercion
co)
      = do { CandidatesQTvs
dv1 <- CandidatesQTvs -> Coercion -> TcM CandidatesQTvs
go_co CandidatesQTvs
dv Coercion
kind_co
           ; Kind
-> CoVarSet -> CandidatesQTvs -> Coercion -> TcM CandidatesQTvs
collect_cand_qtvs_co Kind
orig_ty (CoVarSet
bound CoVarSet -> TyVar -> CoVarSet
`extendVarSet` TyVar
tcv) CandidatesQTvs
dv1 Coercion
co }

    go_mco :: CandidatesQTvs -> MCoercionN -> TcM CandidatesQTvs
go_mco CandidatesQTvs
dv MCoercionN
MRefl    = CandidatesQTvs -> TcM CandidatesQTvs
forall (m :: * -> *) a. Monad m => a -> m a
return CandidatesQTvs
dv
    go_mco CandidatesQTvs
dv (MCo Coercion
co) = CandidatesQTvs -> Coercion -> TcM CandidatesQTvs
go_co CandidatesQTvs
dv Coercion
co

    go_prov :: CandidatesQTvs -> UnivCoProvenance -> TcM CandidatesQTvs
go_prov CandidatesQTvs
dv (PhantomProv Coercion
co)    = CandidatesQTvs -> Coercion -> TcM CandidatesQTvs
go_co CandidatesQTvs
dv Coercion
co
    go_prov CandidatesQTvs
dv (ProofIrrelProv Coercion
co) = CandidatesQTvs -> Coercion -> TcM CandidatesQTvs
go_co CandidatesQTvs
dv Coercion
co
    go_prov CandidatesQTvs
dv (PluginProv String
_)      = CandidatesQTvs -> TcM CandidatesQTvs
forall (m :: * -> *) a. Monad m => a -> m a
return CandidatesQTvs
dv
    go_prov CandidatesQTvs
dv UnivCoProvenance
CorePrepProv        = CandidatesQTvs -> TcM CandidatesQTvs
forall (m :: * -> *) a. Monad m => a -> m a
return CandidatesQTvs
dv

    go_cv :: CandidatesQTvs -> CoVar -> TcM CandidatesQTvs
    go_cv :: CandidatesQTvs -> TyVar -> TcM CandidatesQTvs
go_cv dv :: CandidatesQTvs
dv@(DV { dv_cvs :: CandidatesQTvs -> CoVarSet
dv_cvs = CoVarSet
cvs }) TyVar
cv
      | TyVar -> Bool
is_bound TyVar
cv         = CandidatesQTvs -> TcM CandidatesQTvs
forall (m :: * -> *) a. Monad m => a -> m a
return CandidatesQTvs
dv
      | TyVar
cv TyVar -> CoVarSet -> Bool
`elemVarSet` CoVarSet
cvs = CandidatesQTvs -> TcM CandidatesQTvs
forall (m :: * -> *) a. Monad m => a -> m a
return CandidatesQTvs
dv

        -- See Note [Recurring into kinds for candidateQTyVars]
      | Bool
otherwise           = Kind
-> Bool -> CoVarSet -> CandidatesQTvs -> Kind -> TcM CandidatesQTvs
collect_cand_qtvs Kind
orig_ty Bool
True CoVarSet
bound
                                    (CandidatesQTvs
dv { dv_cvs :: CoVarSet
dv_cvs = CoVarSet
cvs CoVarSet -> TyVar -> CoVarSet
`extendVarSet` TyVar
cv })
                                    (TyVar -> Kind
idType TyVar
cv)

    is_bound :: TyVar -> Bool
is_bound TyVar
tv = TyVar
tv TyVar -> CoVarSet -> Bool
`elemVarSet` CoVarSet
bound

{- Note [Order of accumulation]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
You might be tempted (like I was) to use unitDVarSet and mappend
rather than extendDVarSet.  However, the union algorithm for
deterministic sets depends on (roughly) the size of the sets. The
elements from the smaller set end up to the right of the elements from
the larger one. When sets are equal, the left-hand argument to
`mappend` goes to the right of the right-hand argument.

In our case, if we use unitDVarSet and mappend, we learn that the free
variables of (a -> b -> c -> d) are [b, a, c, d], and we then quantify
over them in that order. (The a comes after the b because we union the
singleton sets as ({a} `mappend` {b}), producing {b, a}. Thereafter,
the size criterion works to our advantage.) This is just annoying to
users, so I use `extendDVarSet`, which unambiguously puts the new
element to the right.

Note that the unitDVarSet/mappend implementation would not be wrong
against any specification -- just suboptimal and confounding to users.

Note [Recurring into kinds for candidateQTyVars]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
First, read Note [Closing over free variable kinds] in GHC.Core.TyCo.FVs, paying
attention to the end of the Note about using an empty bound set when
traversing a variable's kind.

That Note concludes with the recommendation that we empty out the bound
set when recurring into the kind of a type variable. Yet, we do not do
this here. I have two tasks in order to convince you that this code is
right. First, I must show why it is safe to ignore the reasoning in that
Note. Then, I must show why is is necessary to contradict the reasoning in
that Note.

Why it is safe: There can be no
shadowing in the candidateQ... functions: they work on the output of
type inference, which is seeded by the renamer and its insistence to
use different Uniques for different variables. (In contrast, the Core
functions work on the output of optimizations, which may introduce
shadowing.) Without shadowing, the problem studied by
Note [Closing over free variable kinds] in GHC.Core.TyCo.FVs cannot happen.

Why it is necessary:
Wiping the bound set would be just plain wrong here. Consider

  forall k1 k2 (a :: k1). Proxy k2 (a |> (hole :: k1 ~# k2))

We really don't want to think k1 and k2 are free here. (It's true that we'll
never be able to fill in `hole`, but we don't want to go off the rails just
because we have an insoluble coercion hole.) So: why is it wrong to wipe
the bound variables here but right in Core? Because the final statement
in Note [Closing over free variable kinds] in GHC.Core.TyCo.FVs is wrong: not
every variable is either free or bound. A variable can be a hole, too!
The reasoning in that Note then breaks down.

And the reasoning applies just as well to free non-hole variables, so we
retain the bound set always.

-}

{- *********************************************************************
*                                                                      *
             Quantification
*                                                                      *
************************************************************************

Note [quantifyTyVars]
~~~~~~~~~~~~~~~~~~~~~
quantifyTyVars is given the free vars of a type that we
are about to wrap in a forall.

It takes these free type/kind variables (partitioned into dependent and
non-dependent variables) skolemises metavariables with a TcLevel greater
than the ambient level (see Note [Use level numbers for quantification]).

* This function distinguishes between dependent and non-dependent
  variables only to keep correct defaulting behavior with -XNoPolyKinds.
  With -XPolyKinds, it treats both classes of variables identically.

* quantifyTyVars never quantifies over
    - a coercion variable (or any tv mentioned in the kind of a covar)
    - a runtime-rep variable

Note [Use level numbers for quantification]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The level numbers assigned to metavariables are very useful. Not only
do they track touchability (Note [TcLevel invariants] in GHC.Tc.Utils.TcType),
but they also allow us to determine which variables to
generalise. The rule is this:

  When generalising, quantify only metavariables with a TcLevel greater
  than the ambient level.

This works because we bump the level every time we go inside a new
source-level construct. In a traditional generalisation algorithm, we
would gather all free variables that aren't free in an environment.
However, if a variable is in that environment, it will always have a lower
TcLevel: it came from an outer scope. So we can replace the "free in
environment" check with a level-number check.

Here is an example:

  f x = x + (z True)
    where
      z y = x * x

We start by saying (x :: alpha[1]). When inferring the type of z, we'll
quickly discover that z :: alpha[1]. But it would be disastrous to
generalise over alpha in the type of z. So we need to know that alpha
comes from an outer environment. By contrast, the type of y is beta[2],
and we are free to generalise over it. What's the difference between
alpha[1] and beta[2]? Their levels. beta[2] has the right TcLevel for
generalisation, and so we generalise it. alpha[1] does not, and so
we leave it alone.

Note that not *every* variable with a higher level will get
generalised, either due to the monomorphism restriction or other
quirks. See, for example, the code in GHC.Tc.Solver.decideMonoTyVars
and in GHC.Tc.Gen.HsType.kindGeneralizeSome, both of which exclude
certain otherwise-eligible variables from being generalised.

Using level numbers for quantification is implemented in the candidateQTyVars...
functions, by adding only those variables with a level strictly higher than
the ambient level to the set of candidates.

Note [quantifyTyVars determinism]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The results of quantifyTyVars are wrapped in a forall and can end up in the
interface file. One such example is inferred type signatures. They also affect
the results of optimizations, for example worker-wrapper. This means that to
get deterministic builds quantifyTyVars needs to be deterministic.

To achieve this CandidatesQTvs is backed by deterministic sets which allows them
to be later converted to a list in a deterministic order.

For more information about deterministic sets see
Note [Deterministic UniqFM] in GHC.Types.Unique.DFM.
-}

quantifyTyVars :: CandidatesQTvs   -- See Note [Dependent type variables]
                                   -- Already zonked
               -> TcM [TcTyVar]
-- See Note [quantifyTyVars]
-- Can be given a mixture of TcTyVars and TyVars, in the case of
--   associated type declarations. Also accepts covars, but *never* returns any.
-- According to Note [Use level numbers for quantification] and the
-- invariants on CandidateQTvs, we do not have to filter out variables
-- free in the environment here. Just quantify unconditionally, subject
-- to the restrictions in Note [quantifyTyVars].
quantifyTyVars :: CandidatesQTvs -> TcM [TyVar]
quantifyTyVars CandidatesQTvs
dvs
       -- short-circuit common case
  | CandidatesQTvs -> Bool
isEmptyCandidates CandidatesQTvs
dvs
  = do { String -> SDoc -> TcRn ()
traceTc String
"quantifyTyVars has nothing to quantify" SDoc
empty
       ; [TyVar] -> TcM [TyVar]
forall (m :: * -> *) a. Monad m => a -> m a
return [] }

  | Bool
otherwise
  = do { String -> SDoc -> TcRn ()
traceTc String
"quantifyTyVars {" (CandidatesQTvs -> SDoc
forall a. Outputable a => a -> SDoc
ppr CandidatesQTvs
dvs)

       ; [TyVar]
undefaulted <- CandidatesQTvs -> TcM [TyVar]
defaultTyVars CandidatesQTvs
dvs
       ; [TyVar]
final_qtvs  <- (TyVar -> IOEnv (Env TcGblEnv TcLclEnv) (Maybe TyVar))
-> [TyVar] -> TcM [TyVar]
forall (m :: * -> *) a b.
Applicative m =>
(a -> m (Maybe b)) -> [a] -> m [b]
mapMaybeM TyVar -> IOEnv (Env TcGblEnv TcLclEnv) (Maybe TyVar)
zonk_quant [TyVar]
undefaulted

       ; String -> SDoc -> TcRn ()
traceTc String
"quantifyTyVars }"
           ([SDoc] -> SDoc
vcat [ String -> SDoc
text String
"undefaulted:" SDoc -> SDoc -> SDoc
<+> [TyVar] -> SDoc
pprTyVars [TyVar]
undefaulted
                 , String -> SDoc
text String
"final_qtvs:"  SDoc -> SDoc -> SDoc
<+> [TyVar] -> SDoc
pprTyVars [TyVar]
final_qtvs ])

       -- We should never quantify over coercion variables; check this
       ; let co_vars :: [TyVar]
co_vars = (TyVar -> Bool) -> [TyVar] -> [TyVar]
forall a. (a -> Bool) -> [a] -> [a]
filter TyVar -> Bool
isCoVar [TyVar]
final_qtvs
       ; MASSERT2( null co_vars, ppr co_vars )

       ; [TyVar] -> TcM [TyVar]
forall (m :: * -> *) a. Monad m => a -> m a
return [TyVar]
final_qtvs }
  where
    -- zonk_quant returns a tyvar if it should be quantified over;
    -- otherwise, it returns Nothing. The latter case happens for
    -- non-meta-tyvars
    zonk_quant :: TyVar -> IOEnv (Env TcGblEnv TcLclEnv) (Maybe TyVar)
zonk_quant TyVar
tkv
      | Bool -> Bool
not (TyVar -> Bool
isTyVar TyVar
tkv)
      = Maybe TyVar -> IOEnv (Env TcGblEnv TcLclEnv) (Maybe TyVar)
forall (m :: * -> *) a. Monad m => a -> m a
return Maybe TyVar
forall a. Maybe a
Nothing   -- this can happen for a covar that's associated with
                         -- a coercion hole. Test case: typecheck/should_compile/T2494

      | Bool -> Bool
not (TyVar -> Bool
isTcTyVar TyVar
tkv)
      = Maybe TyVar -> IOEnv (Env TcGblEnv TcLclEnv) (Maybe TyVar)
forall (m :: * -> *) a. Monad m => a -> m a
return (TyVar -> Maybe TyVar
forall a. a -> Maybe a
Just TyVar
tkv)  -- For associated types in a class with a standalone
                           -- kind signature, we have the class variables in
                           -- scope, and they are TyVars not TcTyVars
      | Bool
otherwise
      = TyVar -> Maybe TyVar
forall a. a -> Maybe a
Just (TyVar -> Maybe TyVar)
-> IOEnv (Env TcGblEnv TcLclEnv) TyVar
-> IOEnv (Env TcGblEnv TcLclEnv) (Maybe TyVar)
forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
<$> TyVar -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
skolemiseQuantifiedTyVar TyVar
tkv

isQuantifiableTv :: TcLevel   -- Level of the context, outside the quantification
                 -> TcTyVar
                 -> Bool
isQuantifiableTv :: TcLevel -> TyVar -> Bool
isQuantifiableTv TcLevel
outer_tclvl TyVar
tcv
  | TyVar -> Bool
isTcTyVar TyVar
tcv  -- Might be a CoVar; change this when gather covars separately
  = TyVar -> TcLevel
tcTyVarLevel TyVar
tcv TcLevel -> TcLevel -> Bool
forall a. Ord a => a -> a -> Bool
> TcLevel
outer_tclvl
  | Bool
otherwise
  = Bool
False

zonkAndSkolemise :: TcTyCoVar -> TcM TcTyCoVar
-- A tyvar binder is never a unification variable (TauTv),
-- rather it is always a skolem. It *might* be a TyVarTv.
-- (Because non-CUSK type declarations use TyVarTvs.)
-- Regardless, it may have a kind that has not yet been zonked,
-- and may include kind unification variables.
zonkAndSkolemise :: TyVar -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
zonkAndSkolemise TyVar
tyvar
  | TyVar -> Bool
isTyVarTyVar TyVar
tyvar
     -- We want to preserve the binding location of the original TyVarTv.
     -- This is important for error messages. If we don't do this, then
     -- we get bad locations in, e.g., typecheck/should_fail/T2688
  = do { TyVar
zonked_tyvar <- HasDebugCallStack => TyVar -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
TyVar -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
zonkTcTyVarToTyVar TyVar
tyvar
       ; TyVar -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
skolemiseQuantifiedTyVar TyVar
zonked_tyvar }

  | Bool
otherwise
  = ASSERT2( isImmutableTyVar tyvar || isCoVar tyvar, pprTyVar tyvar )
    TyVar -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
zonkTyCoVarKind TyVar
tyvar

skolemiseQuantifiedTyVar :: TcTyVar -> TcM TcTyVar
-- The quantified type variables often include meta type variables
-- we want to freeze them into ordinary type variables
-- The meta tyvar is updated to point to the new skolem TyVar.  Now any
-- bound occurrences of the original type variable will get zonked to
-- the immutable version.
--
-- We leave skolem TyVars alone; they are immutable.
--
-- This function is called on both kind and type variables,
-- but kind variables *only* if PolyKinds is on.

skolemiseQuantifiedTyVar :: TyVar -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
skolemiseQuantifiedTyVar TyVar
tv
  = case TyVar -> TcTyVarDetails
tcTyVarDetails TyVar
tv of
      SkolemTv {} -> do { Kind
kind <- Kind -> TcM Kind
zonkTcType (TyVar -> Kind
tyVarKind TyVar
tv)
                        ; TyVar -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
forall (m :: * -> *) a. Monad m => a -> m a
return (TyVar -> Kind -> TyVar
setTyVarKind TyVar
tv Kind
kind) }
        -- It might be a skolem type variable,
        -- for example from a user type signature

      MetaTv {} -> TyVar -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
skolemiseUnboundMetaTyVar TyVar
tv

      TcTyVarDetails
_other -> String -> SDoc -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
forall a. HasCallStack => String -> SDoc -> a
pprPanic String
"skolemiseQuantifiedTyVar" (TyVar -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyVar
tv) -- RuntimeUnk

defaultTyVar :: Bool      -- True <=> please default this kind variable to *
             -> TcTyVar   -- If it's a MetaTyVar then it is unbound
             -> TcM Bool  -- True <=> defaulted away altogether

defaultTyVar :: Bool -> TyVar -> TcRnIf TcGblEnv TcLclEnv Bool
defaultTyVar Bool
default_kind TyVar
tv
  | Bool -> Bool
not (TyVar -> Bool
isMetaTyVar TyVar
tv)
  = Bool -> TcRnIf TcGblEnv TcLclEnv Bool
forall (m :: * -> *) a. Monad m => a -> m a
return Bool
False

  | TyVar -> Bool
isTyVarTyVar TyVar
tv
    -- Do not default TyVarTvs. Doing so would violate the invariants
    -- on TyVarTvs; see Note [TyVarTv] in GHC.Tc.Utils.TcMType.
    -- #13343 is an example; #14555 is another
    -- See Note [Inferring kinds for type declarations] in GHC.Tc.TyCl
  = Bool -> TcRnIf TcGblEnv TcLclEnv Bool
forall (m :: * -> *) a. Monad m => a -> m a
return Bool
False


  | TyVar -> Bool
isRuntimeRepVar TyVar
tv  -- Do not quantify over a RuntimeRep var
                        -- unless it is a TyVarTv, handled earlier
  = do { String -> SDoc -> TcRn ()
traceTc String
"Defaulting a RuntimeRep var to LiftedRep" (TyVar -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyVar
tv)
       ; TyVar -> Kind -> TcRn ()
writeMetaTyVar TyVar
tv Kind
liftedRepTy
       ; Bool -> TcRnIf TcGblEnv TcLclEnv Bool
forall (m :: * -> *) a. Monad m => a -> m a
return Bool
True }
  | TyVar -> Bool
isLevityVar TyVar
tv
  = do { String -> SDoc -> TcRn ()
traceTc String
"Defaulting a Levity var to Lifted" (TyVar -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyVar
tv)
       ; TyVar -> Kind -> TcRn ()
writeMetaTyVar TyVar
tv Kind
liftedDataConTy
       ; Bool -> TcRnIf TcGblEnv TcLclEnv Bool
forall (m :: * -> *) a. Monad m => a -> m a
return Bool
True }
  | TyVar -> Bool
isMultiplicityVar TyVar
tv
  = do { String -> SDoc -> TcRn ()
traceTc String
"Defaulting a Multiplicty var to Many" (TyVar -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyVar
tv)
       ; TyVar -> Kind -> TcRn ()
writeMetaTyVar TyVar
tv Kind
manyDataConTy
       ; Bool -> TcRnIf TcGblEnv TcLclEnv Bool
forall (m :: * -> *) a. Monad m => a -> m a
return Bool
True }

  | Bool
default_kind            -- -XNoPolyKinds and this is a kind var
  = TyVar -> TcRnIf TcGblEnv TcLclEnv Bool
default_kind_var TyVar
tv     -- so default it to * if possible

  | Bool
otherwise
  = Bool -> TcRnIf TcGblEnv TcLclEnv Bool
forall (m :: * -> *) a. Monad m => a -> m a
return Bool
False

  where
    default_kind_var :: TyVar -> TcM Bool
       -- defaultKindVar is used exclusively with -XNoPolyKinds
       -- See Note [Defaulting with -XNoPolyKinds]
       -- It takes an (unconstrained) meta tyvar and defaults it.
       -- Works only on vars of type *; for other kinds, it issues an error.
    default_kind_var :: TyVar -> TcRnIf TcGblEnv TcLclEnv Bool
default_kind_var TyVar
kv
      | Kind -> Bool
isLiftedTypeKind (TyVar -> Kind
tyVarKind TyVar
kv)
      = do { String -> SDoc -> TcRn ()
traceTc String
"Defaulting a kind var to *" (TyVar -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyVar
kv)
           ; TyVar -> Kind -> TcRn ()
writeMetaTyVar TyVar
kv Kind
liftedTypeKind
           ; Bool -> TcRnIf TcGblEnv TcLclEnv Bool
forall (m :: * -> *) a. Monad m => a -> m a
return Bool
True }
      | Bool
otherwise
      = do { SDoc -> TcRn ()
addErr ([SDoc] -> SDoc
vcat [ String -> SDoc
text String
"Cannot default kind variable" SDoc -> SDoc -> SDoc
<+> SDoc -> SDoc
quotes (TyVar -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyVar
kv')
                          , String -> SDoc
text String
"of kind:" SDoc -> SDoc -> SDoc
<+> Kind -> SDoc
forall a. Outputable a => a -> SDoc
ppr (TyVar -> Kind
tyVarKind TyVar
kv')
                          , String -> SDoc
text String
"Perhaps enable PolyKinds or add a kind signature" ])
           -- We failed to default it, so return False to say so.
           -- Hence, it'll get skolemised.  That might seem odd, but we must either
           -- promote, skolemise, or zap-to-Any, to satisfy GHC.Tc.Gen.HsType
           --    Note [Recipe for checking a signature]
           -- Otherwise we get level-number assertion failures. It doesn't matter much
           -- because we are in an error situation anyway.
           ; Bool -> TcRnIf TcGblEnv TcLclEnv Bool
forall (m :: * -> *) a. Monad m => a -> m a
return Bool
False
        }
      where
        (TidyEnv
_, TyVar
kv') = TidyEnv -> TyVar -> (TidyEnv, TyVar)
tidyOpenTyCoVar TidyEnv
emptyTidyEnv TyVar
kv

-- | Default some unconstrained type variables:
--     RuntimeRep tyvars default to LiftedRep
--     Multiplicity tyvars default to Many
--     Type tyvars from dv_kvs default to Type, when -XNoPolyKinds
--     (under -XNoPolyKinds, non-defaulting vars in dv_kvs is an error)
defaultTyVars :: CandidatesQTvs  -- ^ all candidates for quantification
              -> TcM [TcTyVar]   -- ^ those variables not defaulted
defaultTyVars :: CandidatesQTvs -> TcM [TyVar]
defaultTyVars CandidatesQTvs
dvs
  = do { Bool
poly_kinds <- Extension -> TcRnIf TcGblEnv TcLclEnv Bool
forall gbl lcl. Extension -> TcRnIf gbl lcl Bool
xoptM Extension
LangExt.PolyKinds
       ; [Bool]
defaulted_kvs <- (TyVar -> TcRnIf TcGblEnv TcLclEnv Bool)
-> [TyVar] -> IOEnv (Env TcGblEnv TcLclEnv) [Bool]
forall (t :: * -> *) (m :: * -> *) a b.
(Traversable t, Monad m) =>
(a -> m b) -> t a -> m (t b)
mapM (Bool -> TyVar -> TcRnIf TcGblEnv TcLclEnv Bool
defaultTyVar (Bool -> Bool
not Bool
poly_kinds)) [TyVar]
dep_kvs
       ; [Bool]
defaulted_tvs <- (TyVar -> TcRnIf TcGblEnv TcLclEnv Bool)
-> [TyVar] -> IOEnv (Env TcGblEnv TcLclEnv) [Bool]
forall (t :: * -> *) (m :: * -> *) a b.
(Traversable t, Monad m) =>
(a -> m b) -> t a -> m (t b)
mapM (Bool -> TyVar -> TcRnIf TcGblEnv TcLclEnv Bool
defaultTyVar Bool
False)            [TyVar]
nondep_tvs
       ; let undefaulted_kvs :: [TyVar]
undefaulted_kvs = [ TyVar
kv | (TyVar
kv, Bool
False) <- [TyVar]
dep_kvs    [TyVar] -> [Bool] -> [(TyVar, Bool)]
forall a b. [a] -> [b] -> [(a, b)]
`zip` [Bool]
defaulted_kvs ]
             undefaulted_tvs :: [TyVar]
undefaulted_tvs = [ TyVar
tv | (TyVar
tv, Bool
False) <- [TyVar]
nondep_tvs [TyVar] -> [Bool] -> [(TyVar, Bool)]
forall a b. [a] -> [b] -> [(a, b)]
`zip` [Bool]
defaulted_tvs ]
       ; [TyVar] -> TcM [TyVar]
forall (m :: * -> *) a. Monad m => a -> m a
return ([TyVar]
undefaulted_kvs [TyVar] -> [TyVar] -> [TyVar]
forall a. [a] -> [a] -> [a]
++ [TyVar]
undefaulted_tvs) }
          -- NB: kvs before tvs because tvs may depend on kvs
  where
    ([TyVar]
dep_kvs, [TyVar]
nondep_tvs) = CandidatesQTvs -> ([TyVar], [TyVar])
candidateVars CandidatesQTvs
dvs

skolemiseUnboundMetaTyVar :: TcTyVar -> TcM TyVar
-- We have a Meta tyvar with a ref-cell inside it
-- Skolemise it, so that we are totally out of Meta-tyvar-land
-- We create a skolem TcTyVar, not a regular TyVar
--   See Note [Zonking to Skolem]
skolemiseUnboundMetaTyVar :: TyVar -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
skolemiseUnboundMetaTyVar TyVar
tv
  = ASSERT2( isMetaTyVar tv, ppr tv )
    do  { Bool -> TcRn () -> TcRn ()
forall (f :: * -> *). Applicative f => Bool -> f () -> f ()
when Bool
debugIsOn (TyVar -> TcRn ()
check_empty TyVar
tv)
        ; SrcSpan
here <- TcRn SrcSpan
getSrcSpanM    -- Get the location from "here"
                                 -- ie where we are generalising
        ; Kind
kind <- Kind -> TcM Kind
zonkTcType (TyVar -> Kind
tyVarKind TyVar
tv)
        ; let tv_name :: Name
tv_name     = TyVar -> Name
tyVarName TyVar
tv
              -- See Note [Skolemising and identity]
              final_name :: Name
final_name | Name -> Bool
isSystemName Name
tv_name
                         = Unique -> OccName -> SrcSpan -> Name
mkInternalName (Name -> Unique
nameUnique Name
tv_name)
                                          (Name -> OccName
nameOccName Name
tv_name) SrcSpan
here
                         | Bool
otherwise
                         = Name
tv_name
              final_tv :: TyVar
final_tv = Name -> Kind -> TcTyVarDetails -> TyVar
mkTcTyVar Name
final_name Kind
kind TcTyVarDetails
details

        ; String -> SDoc -> TcRn ()
traceTc String
"Skolemising" (TyVar -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyVar
tv SDoc -> SDoc -> SDoc
<+> String -> SDoc
text String
":=" SDoc -> SDoc -> SDoc
<+> TyVar -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyVar
final_tv)
        ; TyVar -> Kind -> TcRn ()
writeMetaTyVar TyVar
tv (TyVar -> Kind
mkTyVarTy TyVar
final_tv)
        ; TyVar -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
forall (m :: * -> *) a. Monad m => a -> m a
return TyVar
final_tv }

  where
    details :: TcTyVarDetails
details = TcLevel -> Bool -> TcTyVarDetails
SkolemTv (TyVar -> TcLevel
metaTyVarTcLevel TyVar
tv) Bool
False
    check_empty :: TyVar -> TcRn ()
check_empty TyVar
tv       -- [Sept 04] Check for non-empty.
      = Bool -> TcRn () -> TcRn ()
forall (f :: * -> *). Applicative f => Bool -> f () -> f ()
when Bool
debugIsOn (TcRn () -> TcRn ()) -> TcRn () -> TcRn ()
forall a b. (a -> b) -> a -> b
$  -- See note [Silly Type Synonym]
        do { MetaDetails
cts <- TyVar -> TcM MetaDetails
readMetaTyVar TyVar
tv
           ; case MetaDetails
cts of
               MetaDetails
Flexi       -> () -> TcRn ()
forall (m :: * -> *) a. Monad m => a -> m a
return ()
               Indirect Kind
ty -> WARN( True, ppr tv $$ ppr ty )
                              () -> TcRn ()
forall (m :: * -> *) a. Monad m => a -> m a
return () }

{- Note [Error on unconstrained meta-variables]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider

  type C :: Type -> Type -> Constraint
  class (forall a. a b ~ a c) => C b c

or

  type T = forall a. Proxy a

or

  data (forall a. a b ~ a c) => T b c

We will infer a :: Type -> kappa... but then we get no further information
on kappa. What to do?

 A. We could choose kappa := Type. But this only works when the kind of kappa
    is Type (true in this example, but not always).
 B. We could default to Any.
 C. We could quantify.
 D. We could error.

We choose (D), as described in #17567. Discussion of alternatives is below.

(One last example: type instance F Int = Proxy Any, where the unconstrained
kind variable is the inferred kind of Any. The four examples here illustrate
all cases in which this Note applies.)

To do this, we must take an extra step before doing the final zonk to create
e.g. a TyCon. (There is no problem in the final term-level zonk. See the
section on alternative (B) below.) This extra step is needed only for
constructs that do not quantify their free meta-variables, such as a class
constraint or right-hand side of a type synonym.

Specifically: before the final zonk, every construct must either call
quantifyTyVars or doNotQuantifyTyVars. The latter issues an error
if it is passed any free variables. (Exception: we still default
RuntimeRep and Multiplicity variables.)

Because no meta-variables remain after quantifying or erroring, we perform
the zonk with NoFlexi, which panics upon seeing a meta-variable.

Alternatives not implemented:

A. As stated above, this works only sometimes. We might have a free
   meta-variable of kind Nat, for example.

B. This is what we used to do, but it caused Any to appear in error
   messages sometimes. See #17567 for several examples. Defaulting to
   Any during the final, whole-program zonk is OK, though, because
   we are completely done type-checking at that point. No chance to
   leak into an error message.

C. Examine the class declaration at the top of this Note again.
   Where should we quantify? We might imagine quantifying and
   putting the kind variable in the forall of the quantified constraint.
   But what if there are nested foralls? Which one should get the
   variable? Other constructs have other problems. (For example,
   the right-hand side of a type family instance equation may not
   be a poly-type.)

   More broadly, the GHC AST defines a set of places where it performs
   implicit lexical generalization. For example, in a type
   signature

     f :: Proxy a -> Bool

   the otherwise-unbound a is lexically quantified, giving us

     f :: forall a. Proxy a -> Bool

   The places that allow lexical quantification are marked in the AST with
   HsImplicitBndrs. HsImplicitBndrs offers a binding site for otherwise-unbound
   variables.

   Later, during type-checking, we discover that a's kind is unconstrained.
   We thus quantify *again*, to

     f :: forall {k} (a :: k). Proxy @k a -> Bool

   It is this second quantification that this Note is really about --
   let's call it *inferred quantification*.
   So there are two sorts of implicit quantification in types:
     1. Lexical quantification: signalled by HsImplicitBndrs, occurs over
        variables mentioned by the user but with no explicit binding site,
        suppressed by a user-written forall (by the forall-or-nothing rule,
        in Note [forall-or-nothing rule] in GHC.Hs.Type).
     2. Inferred quantification: no signal in HsSyn, occurs over unconstrained
        variables invented by the type-checker, possible only with -XPolyKinds,
        unaffected by forall-or-nothing rule
   These two quantifications are performed in different compiler phases, and are
   essentially unrelated. However, it is convenient
   for programmers to remember only one set of implicit quantification
   sites. So, we choose to use the same places (those with HsImplicitBndrs)
   for lexical quantification as for inferred quantification of unconstrained
   meta-variables. Accordingly, there is no quantification in a class
   constraint, or the other constructs that call doNotQuantifyTyVars.
-}

doNotQuantifyTyVars :: CandidatesQTvs
                    -> (TidyEnv -> TcM (TidyEnv, SDoc))
                            -- ^ like "the class context (D a b, E foogle)"
                    -> TcM ()
doNotQuantifyTyVars :: CandidatesQTvs -> (TidyEnv -> TcM (TidyEnv, SDoc)) -> TcRn ()
doNotQuantifyTyVars CandidatesQTvs
dvs TidyEnv -> TcM (TidyEnv, SDoc)
where_found
  | CandidatesQTvs -> Bool
isEmptyCandidates CandidatesQTvs
dvs
  = String -> SDoc -> TcRn ()
traceTc String
"doNotQuantifyTyVars has nothing to error on" SDoc
empty

  | Bool
otherwise
  = do { String -> SDoc -> TcRn ()
traceTc String
"doNotQuantifyTyVars" (CandidatesQTvs -> SDoc
forall a. Outputable a => a -> SDoc
ppr CandidatesQTvs
dvs)
       ; [TyVar]
undefaulted <- CandidatesQTvs -> TcM [TyVar]
defaultTyVars CandidatesQTvs
dvs
          -- could have regular TyVars here, in an associated type RHS, or
          -- bound by a type declaration head. So filter looking only for
          -- metavars. e.g. b and c in `class (forall a. a b ~ a c) => C b c`
          -- are OK
       ; let leftover_metas :: [TyVar]
leftover_metas = (TyVar -> Bool) -> [TyVar] -> [TyVar]
forall a. (a -> Bool) -> [a] -> [a]
filter TyVar -> Bool
isMetaTyVar [TyVar]
undefaulted
       ; Bool -> TcRn () -> TcRn ()
forall (f :: * -> *). Applicative f => Bool -> f () -> f ()
unless ([TyVar] -> Bool
forall (t :: * -> *) a. Foldable t => t a -> Bool
null [TyVar]
leftover_metas) (TcRn () -> TcRn ()) -> TcRn () -> TcRn ()
forall a b. (a -> b) -> a -> b
$
         do { let (TidyEnv
tidy_env1, [TyVar]
tidied_tvs) = TidyEnv -> [TyVar] -> (TidyEnv, [TyVar])
tidyOpenTyCoVars TidyEnv
emptyTidyEnv [TyVar]
leftover_metas
            ; (TidyEnv
tidy_env2, SDoc
where_doc) <- TidyEnv -> TcM (TidyEnv, SDoc)
where_found TidyEnv
tidy_env1
            ; let doc :: SDoc
doc = [SDoc] -> SDoc
vcat [ String -> SDoc
text String
"Uninferrable type variable"
                               SDoc -> SDoc -> SDoc
<> [TyVar] -> SDoc
forall a. [a] -> SDoc
plural [TyVar]
tidied_tvs
                               SDoc -> SDoc -> SDoc
<+> (TyVar -> SDoc) -> [TyVar] -> SDoc
forall a. (a -> SDoc) -> [a] -> SDoc
pprWithCommas TyVar -> SDoc
pprTyVar [TyVar]
tidied_tvs
                               SDoc -> SDoc -> SDoc
<+> String -> SDoc
text String
"in"
                             , SDoc
where_doc ]
            ; (TidyEnv, SDoc) -> TcRn ()
forall a. (TidyEnv, SDoc) -> TcM a
failWithTcM (TidyEnv
tidy_env2, Bool -> SDoc -> SDoc
pprWithExplicitKindsWhen Bool
True SDoc
doc) }
       ; String -> SDoc -> TcRn ()
traceTc String
"doNotQuantifyTyVars success" SDoc
empty }

{- Note [Defaulting with -XNoPolyKinds]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider

  data Compose f g a = Mk (f (g a))

We infer

  Compose :: forall k1 k2. (k2 -> *) -> (k1 -> k2) -> k1 -> *
  Mk :: forall k1 k2 (f :: k2 -> *) (g :: k1 -> k2) (a :: k1).
        f (g a) -> Compose k1 k2 f g a

Now, in another module, we have -XNoPolyKinds -XDataKinds in effect.
What does 'Mk mean? Pre GHC-8.0 with -XNoPolyKinds,
we just defaulted all kind variables to *. But that's no good here,
because the kind variables in 'Mk aren't of kind *, so defaulting to *
is ill-kinded.

After some debate on #11334, we decided to issue an error in this case.
The code is in defaultKindVar.

Note [What is a meta variable?]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
A "meta type-variable", also know as a "unification variable" is a placeholder
introduced by the typechecker for an as-yet-unknown monotype.

For example, when we see a call `reverse (f xs)`, we know that we calling
    reverse :: forall a. [a] -> [a]
So we know that the argument `f xs` must be a "list of something". But what is
the "something"? We don't know until we explore the `f xs` a bit more. So we set
out what we do know at the call of `reverse` by instantiating its type with a fresh
meta tyvar, `alpha` say. So now the type of the argument `f xs`, and of the
result, is `[alpha]`. The unification variable `alpha` stands for the
as-yet-unknown type of the elements of the list.

As type inference progresses we may learn more about `alpha`. For example, suppose
`f` has the type
    f :: forall b. b -> [Maybe b]
Then we instantiate `f`'s type with another fresh unification variable, say
`beta`; and equate `f`'s result type with reverse's argument type, thus
`[alpha] ~ [Maybe beta]`.

Now we can solve this equality to learn that `alpha ~ Maybe beta`, so we've
refined our knowledge about `alpha`. And so on.

If you found this Note useful, you may also want to have a look at
Section 5 of "Practical type inference for higher rank types" (Peyton Jones,
Vytiniotis, Weirich and Shields. J. Functional Programming. 2011).

Note [What is zonking?]
~~~~~~~~~~~~~~~~~~~~~~~
GHC relies heavily on mutability in the typechecker for efficient operation.
For this reason, throughout much of the type checking process meta type
variables (the MetaTv constructor of TcTyVarDetails) are represented by mutable
variables (known as TcRefs).

Zonking is the process of ripping out these mutable variables and replacing them
with a real Type. This involves traversing the entire type expression, but the
interesting part of replacing the mutable variables occurs in zonkTyVarOcc.

There are two ways to zonk a Type:

 * zonkTcTypeToType, which is intended to be used at the end of type-checking
   for the final zonk. It has to deal with unfilled metavars, either by filling
   it with a value like Any or failing (determined by the UnboundTyVarZonker
   used).

 * zonkTcType, which will happily ignore unfilled metavars. This is the
   appropriate function to use while in the middle of type-checking.

Note [Zonking to Skolem]
~~~~~~~~~~~~~~~~~~~~~~~~
We used to zonk quantified type variables to regular TyVars.  However, this
leads to problems.  Consider this program from the regression test suite:

  eval :: Int -> String -> String -> String
  eval 0 root actual = evalRHS 0 root actual

  evalRHS :: Int -> a
  evalRHS 0 root actual = eval 0 root actual

It leads to the deferral of an equality (wrapped in an implication constraint)

  forall a. () => ((String -> String -> String) ~ a)

which is propagated up to the toplevel (see GHC.Tc.Solver.tcSimplifyInferCheck).
In the meantime `a' is zonked and quantified to form `evalRHS's signature.
This has the *side effect* of also zonking the `a' in the deferred equality
(which at this point is being handed around wrapped in an implication
constraint).

Finally, the equality (with the zonked `a') will be handed back to the
simplifier by GHC.Tc.Module.tcRnSrcDecls calling GHC.Tc.Solver.tcSimplifyTop.
If we zonk `a' with a regular type variable, we will have this regular type
variable now floating around in the simplifier, which in many places assumes to
only see proper TcTyVars.

We can avoid this problem by zonking with a skolem TcTyVar.  The
skolem is rigid (which we require for a quantified variable), but is
still a TcTyVar that the simplifier knows how to deal with.

Note [Skolemising and identity]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In some places, we make a TyVarTv for a binder. E.g.
    class C a where ...
As Note [Inferring kinds for type declarations] discusses,
we make a TyVarTv for 'a'.  Later we skolemise it, and we'd
like to retain its identity, location info etc.  (If we don't
retain its identity we'll have to do some pointless swizzling;
see GHC.Tc.TyCl.swizzleTcTyConBndrs.  If we retain its identity
but not its location we'll lose the detailed binding site info.

Conclusion: use the Name of the TyVarTv.  But we don't want
to do that when skolemising random unification variables;
there the location we want is the skolemisation site.

Fortunately we can tell the difference: random unification
variables have System Names.  That's why final_name is
set based on the isSystemName test.


Note [Silly Type Synonyms]
~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider this:
        type C u a = u  -- Note 'a' unused

        foo :: (forall a. C u a -> C u a) -> u
        foo x = ...

        bar :: Num u => u
        bar = foo (\t -> t + t)

* From the (\t -> t+t) we get type  {Num d} =>  d -> d
  where d is fresh.

* Now unify with type of foo's arg, and we get:
        {Num (C d a)} =>  C d a -> C d a
  where a is fresh.

* Now abstract over the 'a', but float out the Num (C d a) constraint
  because it does not 'really' mention a.  (see exactTyVarsOfType)
  The arg to foo becomes
        \/\a -> \t -> t+t

* So we get a dict binding for Num (C d a), which is zonked to give
        a = ()
  [Note Sept 04: now that we are zonking quantified type variables
  on construction, the 'a' will be frozen as a regular tyvar on
  quantification, so the floated dict will still have type (C d a).
  Which renders this whole note moot; happily!]

* Then the \/\a abstraction has a zonked 'a' in it.

All very silly.   I think its harmless to ignore the problem.  We'll end up with
a \/\a in the final result but all the occurrences of a will be zonked to ()
-}

{- *********************************************************************
*                                                                      *
              Promotion
*                                                                      *
********************************************************************* -}

promoteMetaTyVarTo :: TcLevel -> TcTyVar -> TcM Bool
-- When we float a constraint out of an implication we must restore
-- invariant (WantedInv) in Note [TcLevel invariants] in GHC.Tc.Utils.TcType
-- Return True <=> we did some promotion
-- Also returns either the original tyvar (no promotion) or the new one
-- See Note [Promoting unification variables]
promoteMetaTyVarTo :: TcLevel -> TyVar -> TcRnIf TcGblEnv TcLclEnv Bool
promoteMetaTyVarTo TcLevel
tclvl TyVar
tv
  | ASSERT2( isMetaTyVar tv, ppr tv )
    TyVar -> TcLevel
tcTyVarLevel TyVar
tv TcLevel -> TcLevel -> Bool
`strictlyDeeperThan` TcLevel
tclvl
  = do { TyVar
cloned_tv <- TyVar -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
cloneMetaTyVar TyVar
tv
       ; let rhs_tv :: TyVar
rhs_tv = TyVar -> TcLevel -> TyVar
setMetaTyVarTcLevel TyVar
cloned_tv TcLevel
tclvl
       ; TyVar -> Kind -> TcRn ()
writeMetaTyVar TyVar
tv (TyVar -> Kind
mkTyVarTy TyVar
rhs_tv)
       ; String -> SDoc -> TcRn ()
traceTc String
"promoteTyVar" (TyVar -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyVar
tv SDoc -> SDoc -> SDoc
<+> String -> SDoc
text String
"-->" SDoc -> SDoc -> SDoc
<+> TyVar -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyVar
rhs_tv)
       ; Bool -> TcRnIf TcGblEnv TcLclEnv Bool
forall (m :: * -> *) a. Monad m => a -> m a
return Bool
True }
   | Bool
otherwise
   = Bool -> TcRnIf TcGblEnv TcLclEnv Bool
forall (m :: * -> *) a. Monad m => a -> m a
return Bool
False

-- Returns whether or not *any* tyvar is defaulted
promoteTyVarSet :: TcTyVarSet -> TcM Bool
promoteTyVarSet :: CoVarSet -> TcRnIf TcGblEnv TcLclEnv Bool
promoteTyVarSet CoVarSet
tvs
  = do { TcLevel
tclvl <- TcM TcLevel
getTcLevel
       ; [Bool]
bools <- (TyVar -> TcRnIf TcGblEnv TcLclEnv Bool)
-> [TyVar] -> IOEnv (Env TcGblEnv TcLclEnv) [Bool]
forall (t :: * -> *) (m :: * -> *) a b.
(Traversable t, Monad m) =>
(a -> m b) -> t a -> m (t b)
mapM (TcLevel -> TyVar -> TcRnIf TcGblEnv TcLclEnv Bool
promoteMetaTyVarTo TcLevel
tclvl)  ([TyVar] -> IOEnv (Env TcGblEnv TcLclEnv) [Bool])
-> [TyVar] -> IOEnv (Env TcGblEnv TcLclEnv) [Bool]
forall a b. (a -> b) -> a -> b
$
                  (TyVar -> Bool) -> [TyVar] -> [TyVar]
forall a. (a -> Bool) -> [a] -> [a]
filter TyVar -> Bool
isPromotableMetaTyVar ([TyVar] -> [TyVar]) -> [TyVar] -> [TyVar]
forall a b. (a -> b) -> a -> b
$
                  CoVarSet -> [TyVar]
forall elt. UniqSet elt -> [elt]
nonDetEltsUniqSet CoVarSet
tvs
         -- Non-determinism is OK because order of promotion doesn't matter
       ; Bool -> TcRnIf TcGblEnv TcLclEnv Bool
forall (m :: * -> *) a. Monad m => a -> m a
return ([Bool] -> Bool
forall (t :: * -> *). Foldable t => t Bool -> Bool
or [Bool]
bools) }


{- *********************************************************************
*                                                                      *
              Zonking types
*                                                                      *
********************************************************************* -}

zonkTcTypeAndFV :: TcType -> TcM DTyCoVarSet
-- Zonk a type and take its free variables
-- With kind polymorphism it can be essential to zonk *first*
-- so that we find the right set of free variables.  Eg
--    forall k1. forall (a:k2). a
-- where k2:=k1 is in the substitution.  We don't want
-- k2 to look free in this type!
zonkTcTypeAndFV :: Kind -> TcM DTyVarSet
zonkTcTypeAndFV Kind
ty
  = Kind -> DTyVarSet
tyCoVarsOfTypeDSet (Kind -> DTyVarSet) -> TcM Kind -> TcM DTyVarSet
forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
<$> Kind -> TcM Kind
zonkTcType Kind
ty

zonkTyCoVar :: TyCoVar -> TcM TcType
-- Works on TyVars and TcTyVars
zonkTyCoVar :: TyVar -> TcM Kind
zonkTyCoVar TyVar
tv | TyVar -> Bool
isTcTyVar TyVar
tv = TyVar -> TcM Kind
zonkTcTyVar TyVar
tv
               | TyVar -> Bool
isTyVar   TyVar
tv = TyVar -> Kind
mkTyVarTy (TyVar -> Kind) -> IOEnv (Env TcGblEnv TcLclEnv) TyVar -> TcM Kind
forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
<$> TyVar -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
zonkTyCoVarKind TyVar
tv
               | Bool
otherwise    = ASSERT2( isCoVar tv, ppr tv )
                                Coercion -> Kind
mkCoercionTy (Coercion -> Kind) -> (TyVar -> Coercion) -> TyVar -> Kind
forall b c a. (b -> c) -> (a -> b) -> a -> c
. TyVar -> Coercion
mkCoVarCo (TyVar -> Kind) -> IOEnv (Env TcGblEnv TcLclEnv) TyVar -> TcM Kind
forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
<$> TyVar -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
zonkTyCoVarKind TyVar
tv
   -- Hackily, when typechecking type and class decls
   -- we have TyVars in scope added (only) in
   -- GHC.Tc.Gen.HsType.bindTyClTyVars, but it seems
   -- painful to make them into TcTyVars there

zonkTyCoVarsAndFV :: TyCoVarSet -> TcM TyCoVarSet
zonkTyCoVarsAndFV :: CoVarSet -> TcM CoVarSet
zonkTyCoVarsAndFV CoVarSet
tycovars
  = [Kind] -> CoVarSet
tyCoVarsOfTypes ([Kind] -> CoVarSet) -> TcM [Kind] -> TcM CoVarSet
forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
<$> (TyVar -> TcM Kind) -> [TyVar] -> TcM [Kind]
forall (t :: * -> *) (m :: * -> *) a b.
(Traversable t, Monad m) =>
(a -> m b) -> t a -> m (t b)
mapM TyVar -> TcM Kind
zonkTyCoVar (CoVarSet -> [TyVar]
forall elt. UniqSet elt -> [elt]
nonDetEltsUniqSet CoVarSet
tycovars)
  -- It's OK to use nonDetEltsUniqSet here because we immediately forget about
  -- the ordering by turning it into a nondeterministic set and the order
  -- of zonking doesn't matter for determinism.

zonkDTyCoVarSetAndFV :: DTyCoVarSet -> TcM DTyCoVarSet
zonkDTyCoVarSetAndFV :: DTyVarSet -> TcM DTyVarSet
zonkDTyCoVarSetAndFV DTyVarSet
tycovars
  = [TyVar] -> DTyVarSet
mkDVarSet ([TyVar] -> DTyVarSet) -> TcM [TyVar] -> TcM DTyVarSet
forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
<$> ([TyVar] -> TcM [TyVar]
zonkTyCoVarsAndFVList ([TyVar] -> TcM [TyVar]) -> [TyVar] -> TcM [TyVar]
forall a b. (a -> b) -> a -> b
$ DTyVarSet -> [TyVar]
dVarSetElems DTyVarSet
tycovars)

-- Takes a list of TyCoVars, zonks them and returns a
-- deterministically ordered list of their free variables.
zonkTyCoVarsAndFVList :: [TyCoVar] -> TcM [TyCoVar]
zonkTyCoVarsAndFVList :: [TyVar] -> TcM [TyVar]
zonkTyCoVarsAndFVList [TyVar]
tycovars
  = [Kind] -> [TyVar]
tyCoVarsOfTypesList ([Kind] -> [TyVar]) -> TcM [Kind] -> TcM [TyVar]
forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
<$> (TyVar -> TcM Kind) -> [TyVar] -> TcM [Kind]
forall (t :: * -> *) (m :: * -> *) a b.
(Traversable t, Monad m) =>
(a -> m b) -> t a -> m (t b)
mapM TyVar -> TcM Kind
zonkTyCoVar [TyVar]
tycovars

zonkTcTyVars :: [TcTyVar] -> TcM [TcType]
zonkTcTyVars :: [TyVar] -> TcM [Kind]
zonkTcTyVars [TyVar]
tyvars = (TyVar -> TcM Kind) -> [TyVar] -> TcM [Kind]
forall (t :: * -> *) (m :: * -> *) a b.
(Traversable t, Monad m) =>
(a -> m b) -> t a -> m (t b)
mapM TyVar -> TcM Kind
zonkTcTyVar [TyVar]
tyvars

-----------------  Types
zonkTyCoVarKind :: TyCoVar -> TcM TyCoVar
zonkTyCoVarKind :: TyVar -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
zonkTyCoVarKind TyVar
tv = do { Kind
kind' <- Kind -> TcM Kind
zonkTcType (TyVar -> Kind
tyVarKind TyVar
tv)
                        ; TyVar -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
forall (m :: * -> *) a. Monad m => a -> m a
return (TyVar -> Kind -> TyVar
setTyVarKind TyVar
tv Kind
kind') }

zonkTyCoVarKindBinder :: (VarBndr TyCoVar fl) -> TcM (VarBndr TyCoVar fl)
zonkTyCoVarKindBinder :: forall fl. VarBndr TyVar fl -> TcM (VarBndr TyVar fl)
zonkTyCoVarKindBinder (Bndr TyVar
tv fl
fl) = do { Kind
kind' <- Kind -> TcM Kind
zonkTcType (TyVar -> Kind
tyVarKind TyVar
tv)
                                        ; VarBndr TyVar fl -> TcM (VarBndr TyVar fl)
forall (m :: * -> *) a. Monad m => a -> m a
return (VarBndr TyVar fl -> TcM (VarBndr TyVar fl))
-> VarBndr TyVar fl -> TcM (VarBndr TyVar fl)
forall a b. (a -> b) -> a -> b
$ TyVar -> fl -> VarBndr TyVar fl
forall var argf. var -> argf -> VarBndr var argf
Bndr (TyVar -> Kind -> TyVar
setTyVarKind TyVar
tv Kind
kind') fl
fl }

{-
************************************************************************
*                                                                      *
              Zonking constraints
*                                                                      *
************************************************************************
-}

zonkImplication :: Implication -> TcM Implication
zonkImplication :: Implication -> IOEnv (Env TcGblEnv TcLclEnv) Implication
zonkImplication implic :: Implication
implic@(Implic { ic_skols :: Implication -> [TyVar]
ic_skols  = [TyVar]
skols
                               , ic_given :: Implication -> [TyVar]
ic_given  = [TyVar]
given
                               , ic_wanted :: Implication -> WantedConstraints
ic_wanted = WantedConstraints
wanted
                               , ic_info :: Implication -> SkolemInfo
ic_info   = SkolemInfo
info })
  = do { [TyVar]
skols'  <- (TyVar -> IOEnv (Env TcGblEnv TcLclEnv) TyVar)
-> [TyVar] -> TcM [TyVar]
forall (t :: * -> *) (m :: * -> *) a b.
(Traversable t, Monad m) =>
(a -> m b) -> t a -> m (t b)
mapM TyVar -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
zonkTyCoVarKind [TyVar]
skols  -- Need to zonk their kinds!
                                                -- as #7230 showed
       ; [TyVar]
given'  <- (TyVar -> IOEnv (Env TcGblEnv TcLclEnv) TyVar)
-> [TyVar] -> TcM [TyVar]
forall (t :: * -> *) (m :: * -> *) a b.
(Traversable t, Monad m) =>
(a -> m b) -> t a -> m (t b)
mapM TyVar -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
zonkEvVar [TyVar]
given
       ; SkolemInfo
info'   <- SkolemInfo -> TcM SkolemInfo
zonkSkolemInfo SkolemInfo
info
       ; WantedConstraints
wanted' <- WantedConstraints -> TcM WantedConstraints
zonkWCRec WantedConstraints
wanted
       ; Implication -> IOEnv (Env TcGblEnv TcLclEnv) Implication
forall (m :: * -> *) a. Monad m => a -> m a
return (Implication
implic { ic_skols :: [TyVar]
ic_skols  = [TyVar]
skols'
                        , ic_given :: [TyVar]
ic_given  = [TyVar]
given'
                        , ic_wanted :: WantedConstraints
ic_wanted = WantedConstraints
wanted'
                        , ic_info :: SkolemInfo
ic_info   = SkolemInfo
info' }) }

zonkEvVar :: EvVar -> TcM EvVar
zonkEvVar :: TyVar -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
zonkEvVar TyVar
var = (Kind -> TcM Kind) -> TyVar -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
forall (m :: * -> *).
Monad m =>
(Kind -> m Kind) -> TyVar -> m TyVar
updateIdTypeAndMultM Kind -> TcM Kind
zonkTcType TyVar
var


zonkWC :: WantedConstraints -> TcM WantedConstraints
zonkWC :: WantedConstraints -> TcM WantedConstraints
zonkWC WantedConstraints
wc = WantedConstraints -> TcM WantedConstraints
zonkWCRec WantedConstraints
wc

zonkWCRec :: WantedConstraints -> TcM WantedConstraints
zonkWCRec :: WantedConstraints -> TcM WantedConstraints
zonkWCRec (WC { wc_simple :: WantedConstraints -> Bag Ct
wc_simple = Bag Ct
simple, wc_impl :: WantedConstraints -> Bag Implication
wc_impl = Bag Implication
implic, wc_holes :: WantedConstraints -> Bag Hole
wc_holes = Bag Hole
holes })
  = do { Bag Ct
simple' <- Bag Ct -> IOEnv (Env TcGblEnv TcLclEnv) (Bag Ct)
zonkSimples Bag Ct
simple
       ; Bag Implication
implic' <- (Implication -> IOEnv (Env TcGblEnv TcLclEnv) Implication)
-> Bag Implication
-> IOEnv (Env TcGblEnv TcLclEnv) (Bag Implication)
forall (m :: * -> *) a b.
Monad m =>
(a -> m b) -> Bag a -> m (Bag b)
mapBagM Implication -> IOEnv (Env TcGblEnv TcLclEnv) Implication
zonkImplication Bag Implication
implic
       ; Bag Hole
holes'  <- (Hole -> IOEnv (Env TcGblEnv TcLclEnv) Hole)
-> Bag Hole -> IOEnv (Env TcGblEnv TcLclEnv) (Bag Hole)
forall (m :: * -> *) a b.
Monad m =>
(a -> m b) -> Bag a -> m (Bag b)
mapBagM Hole -> IOEnv (Env TcGblEnv TcLclEnv) Hole
zonkHole Bag Hole
holes
       ; WantedConstraints -> TcM WantedConstraints
forall (m :: * -> *) a. Monad m => a -> m a
return (WC { wc_simple :: Bag Ct
wc_simple = Bag Ct
simple', wc_impl :: Bag Implication
wc_impl = Bag Implication
implic', wc_holes :: Bag Hole
wc_holes = Bag Hole
holes' }) }

zonkSimples :: Cts -> TcM Cts
zonkSimples :: Bag Ct -> IOEnv (Env TcGblEnv TcLclEnv) (Bag Ct)
zonkSimples Bag Ct
cts = do { Bag Ct
cts' <- (Ct -> TcM Ct) -> Bag Ct -> IOEnv (Env TcGblEnv TcLclEnv) (Bag Ct)
forall (m :: * -> *) a b.
Monad m =>
(a -> m b) -> Bag a -> m (Bag b)
mapBagM Ct -> TcM Ct
zonkCt Bag Ct
cts
                     ; String -> SDoc -> TcRn ()
traceTc String
"zonkSimples done:" (Bag Ct -> SDoc
forall a. Outputable a => a -> SDoc
ppr Bag Ct
cts')
                     ; Bag Ct -> IOEnv (Env TcGblEnv TcLclEnv) (Bag Ct)
forall (m :: * -> *) a. Monad m => a -> m a
return Bag Ct
cts' }

zonkHole :: Hole -> TcM Hole
zonkHole :: Hole -> IOEnv (Env TcGblEnv TcLclEnv) Hole
zonkHole hole :: Hole
hole@(Hole { hole_ty :: Hole -> Kind
hole_ty = Kind
ty })
  = do { Kind
ty' <- Kind -> TcM Kind
zonkTcType Kind
ty
       ; Hole -> IOEnv (Env TcGblEnv TcLclEnv) Hole
forall (m :: * -> *) a. Monad m => a -> m a
return (Hole
hole { hole_ty :: Kind
hole_ty = Kind
ty' }) }

{- Note [zonkCt behaviour]
~~~~~~~~~~~~~~~~~~~~~~~~~~
zonkCt tries to maintain the canonical form of a Ct.  For example,
  - a CDictCan should stay a CDictCan;
  - a CIrredCan should stay a CIrredCan with its cc_reason flag intact

Why?, for example:
- For CDictCan, the @GHC.Tc.Solver.expandSuperClasses@ step, which runs after the
  simple wanted and plugin loop, looks for @CDictCan@s. If a plugin is in use,
  constraints are zonked before being passed to the plugin. This means if we
  don't preserve a canonical form, @expandSuperClasses@ fails to expand
  superclasses. This is what happened in #11525.

- For CIrredCan we want to see if a constraint is insoluble with insolubleWC

On the other hand, we change CEqCan to CNonCanonical, because of all of
CEqCan's invariants, which can break during zonking. (Example: a ~R alpha, where
we have alpha := N Int, where N is a newtype.) Besides, the constraint
will be canonicalised again, so there is little benefit in keeping the
CEqCan structure.

NB: Constraints are always rewritten etc by the canonicaliser in
@GHC.Tc.Solver.Canonical@ even if they come in as CDictCan. Only canonical constraints that
are actually in the inert set carry all the guarantees. So it is okay if zonkCt
creates e.g. a CDictCan where the cc_tyars are /not/ fully reduced.
-}

zonkCt :: Ct -> TcM Ct
-- See Note [zonkCt behaviour]
zonkCt :: Ct -> TcM Ct
zonkCt ct :: Ct
ct@(CDictCan { cc_ev :: Ct -> CtEvidence
cc_ev = CtEvidence
ev, cc_tyargs :: Ct -> [Kind]
cc_tyargs = [Kind]
args })
  = do { CtEvidence
ev'   <- CtEvidence -> TcM CtEvidence
zonkCtEvidence CtEvidence
ev
       ; [Kind]
args' <- (Kind -> TcM Kind) -> [Kind] -> TcM [Kind]
forall (t :: * -> *) (m :: * -> *) a b.
(Traversable t, Monad m) =>
(a -> m b) -> t a -> m (t b)
mapM Kind -> TcM Kind
zonkTcType [Kind]
args
       ; Ct -> TcM Ct
forall (m :: * -> *) a. Monad m => a -> m a
return (Ct -> TcM Ct) -> Ct -> TcM Ct
forall a b. (a -> b) -> a -> b
$ Ct
ct { cc_ev :: CtEvidence
cc_ev = CtEvidence
ev', cc_tyargs :: [Kind]
cc_tyargs = [Kind]
args' } }

zonkCt (CEqCan { cc_ev :: Ct -> CtEvidence
cc_ev = CtEvidence
ev })
  = CtEvidence -> Ct
mkNonCanonical (CtEvidence -> Ct) -> TcM CtEvidence -> TcM Ct
forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
<$> CtEvidence -> TcM CtEvidence
zonkCtEvidence CtEvidence
ev

zonkCt ct :: Ct
ct@(CIrredCan { cc_ev :: Ct -> CtEvidence
cc_ev = CtEvidence
ev }) -- Preserve the cc_reason flag
  = do { CtEvidence
ev' <- CtEvidence -> TcM CtEvidence
zonkCtEvidence CtEvidence
ev
       ; Ct -> TcM Ct
forall (m :: * -> *) a. Monad m => a -> m a
return (Ct
ct { cc_ev :: CtEvidence
cc_ev = CtEvidence
ev' }) }

zonkCt Ct
ct
  = do { CtEvidence
fl' <- CtEvidence -> TcM CtEvidence
zonkCtEvidence (Ct -> CtEvidence
ctEvidence Ct
ct)
       ; Ct -> TcM Ct
forall (m :: * -> *) a. Monad m => a -> m a
return (CtEvidence -> Ct
mkNonCanonical CtEvidence
fl') }

zonkCtEvidence :: CtEvidence -> TcM CtEvidence
zonkCtEvidence :: CtEvidence -> TcM CtEvidence
zonkCtEvidence ctev :: CtEvidence
ctev@(CtGiven { ctev_pred :: CtEvidence -> Kind
ctev_pred = Kind
pred })
  = do { Kind
pred' <- Kind -> TcM Kind
zonkTcType Kind
pred
       ; CtEvidence -> TcM CtEvidence
forall (m :: * -> *) a. Monad m => a -> m a
return (CtEvidence
ctev { ctev_pred :: Kind
ctev_pred = Kind
pred'}) }
zonkCtEvidence ctev :: CtEvidence
ctev@(CtWanted { ctev_pred :: CtEvidence -> Kind
ctev_pred = Kind
pred, ctev_dest :: CtEvidence -> TcEvDest
ctev_dest = TcEvDest
dest })
  = do { Kind
pred' <- Kind -> TcM Kind
zonkTcType Kind
pred
       ; let dest' :: TcEvDest
dest' = case TcEvDest
dest of
                       EvVarDest TyVar
ev -> TyVar -> TcEvDest
EvVarDest (TyVar -> TcEvDest) -> TyVar -> TcEvDest
forall a b. (a -> b) -> a -> b
$ TyVar -> Kind -> TyVar
setVarType TyVar
ev Kind
pred'
                         -- necessary in simplifyInfer
                       HoleDest CoercionHole
h   -> CoercionHole -> TcEvDest
HoleDest CoercionHole
h
       ; CtEvidence -> TcM CtEvidence
forall (m :: * -> *) a. Monad m => a -> m a
return (CtEvidence
ctev { ctev_pred :: Kind
ctev_pred = Kind
pred', ctev_dest :: TcEvDest
ctev_dest = TcEvDest
dest' }) }
zonkCtEvidence ctev :: CtEvidence
ctev@(CtDerived { ctev_pred :: CtEvidence -> Kind
ctev_pred = Kind
pred })
  = do { Kind
pred' <- Kind -> TcM Kind
zonkTcType Kind
pred
       ; CtEvidence -> TcM CtEvidence
forall (m :: * -> *) a. Monad m => a -> m a
return (CtEvidence
ctev { ctev_pred :: Kind
ctev_pred = Kind
pred' }) }

zonkSkolemInfo :: SkolemInfo -> TcM SkolemInfo
zonkSkolemInfo :: SkolemInfo -> TcM SkolemInfo
zonkSkolemInfo (SigSkol UserTypeCtxt
cx Kind
ty [(Name, TyVar)]
tv_prs)  = do { Kind
ty' <- Kind -> TcM Kind
zonkTcType Kind
ty
                                            ; SkolemInfo -> TcM SkolemInfo
forall (m :: * -> *) a. Monad m => a -> m a
return (UserTypeCtxt -> Kind -> [(Name, TyVar)] -> SkolemInfo
SigSkol UserTypeCtxt
cx Kind
ty' [(Name, TyVar)]
tv_prs) }
zonkSkolemInfo (InferSkol [(Name, Kind)]
ntys) = do { [(Name, Kind)]
ntys' <- ((Name, Kind) -> IOEnv (Env TcGblEnv TcLclEnv) (Name, Kind))
-> [(Name, Kind)] -> IOEnv (Env TcGblEnv TcLclEnv) [(Name, Kind)]
forall (t :: * -> *) (m :: * -> *) a b.
(Traversable t, Monad m) =>
(a -> m b) -> t a -> m (t b)
mapM (Name, Kind) -> IOEnv (Env TcGblEnv TcLclEnv) (Name, Kind)
forall {a}. (a, Kind) -> IOEnv (Env TcGblEnv TcLclEnv) (a, Kind)
do_one [(Name, Kind)]
ntys
                                     ; SkolemInfo -> TcM SkolemInfo
forall (m :: * -> *) a. Monad m => a -> m a
return ([(Name, Kind)] -> SkolemInfo
InferSkol [(Name, Kind)]
ntys') }
  where
    do_one :: (a, Kind) -> IOEnv (Env TcGblEnv TcLclEnv) (a, Kind)
do_one (a
n, Kind
ty) = do { Kind
ty' <- Kind -> TcM Kind
zonkTcType Kind
ty; (a, Kind) -> IOEnv (Env TcGblEnv TcLclEnv) (a, Kind)
forall (m :: * -> *) a. Monad m => a -> m a
return (a
n, Kind
ty') }
zonkSkolemInfo SkolemInfo
skol_info = SkolemInfo -> TcM SkolemInfo
forall (m :: * -> *) a. Monad m => a -> m a
return SkolemInfo
skol_info

{-
%************************************************************************
%*                                                                      *
\subsection{Zonking -- the main work-horses: zonkTcType, zonkTcTyVar}
*                                                                      *
*              For internal use only!                                  *
*                                                                      *
************************************************************************

-}

-- For unbound, mutable tyvars, zonkType uses the function given to it
-- For tyvars bound at a for-all, zonkType zonks them to an immutable
--      type variable and zonks the kind too
zonkTcType  :: TcType -> TcM TcType
zonkTcTypes :: [TcType] -> TcM [TcType]
zonkCo      :: Coercion -> TcM Coercion

(Kind -> TcM Kind
zonkTcType, [Kind] -> TcM [Kind]
zonkTcTypes, Coercion -> TcM Coercion
zonkCo, [Coercion] -> IOEnv (Env TcGblEnv TcLclEnv) [Coercion]
_)
  = TyCoMapper () (IOEnv (Env TcGblEnv TcLclEnv))
-> (Kind -> TcM Kind, [Kind] -> TcM [Kind],
    Coercion -> TcM Coercion,
    [Coercion] -> IOEnv (Env TcGblEnv TcLclEnv) [Coercion])
forall (m :: * -> *).
Monad m =>
TyCoMapper () m
-> (Kind -> m Kind, [Kind] -> m [Kind], Coercion -> m Coercion,
    [Coercion] -> m [Coercion])
mapTyCo TyCoMapper () (IOEnv (Env TcGblEnv TcLclEnv))
zonkTcTypeMapper

-- | A suitable TyCoMapper for zonking a type during type-checking,
-- before all metavars are filled in.
zonkTcTypeMapper :: TyCoMapper () TcM
zonkTcTypeMapper :: TyCoMapper () (IOEnv (Env TcGblEnv TcLclEnv))
zonkTcTypeMapper = TyCoMapper
  { tcm_tyvar :: () -> TyVar -> TcM Kind
tcm_tyvar = (TyVar -> TcM Kind) -> () -> TyVar -> TcM Kind
forall a b. a -> b -> a
const TyVar -> TcM Kind
zonkTcTyVar
  , tcm_covar :: () -> TyVar -> TcM Coercion
tcm_covar = (TyVar -> TcM Coercion) -> () -> TyVar -> TcM Coercion
forall a b. a -> b -> a
const (\TyVar
cv -> TyVar -> Coercion
mkCoVarCo (TyVar -> Coercion)
-> IOEnv (Env TcGblEnv TcLclEnv) TyVar -> TcM Coercion
forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
<$> TyVar -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
zonkTyCoVarKind TyVar
cv)
  , tcm_hole :: () -> CoercionHole -> TcM Coercion
tcm_hole  = () -> CoercionHole -> TcM Coercion
hole
  , tcm_tycobinder :: () -> TyVar -> ArgFlag -> IOEnv (Env TcGblEnv TcLclEnv) ((), TyVar)
tcm_tycobinder = \()
_env TyVar
tv ArgFlag
_vis -> ((), ) (TyVar -> ((), TyVar))
-> IOEnv (Env TcGblEnv TcLclEnv) TyVar
-> IOEnv (Env TcGblEnv TcLclEnv) ((), TyVar)
forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
<$> TyVar -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
zonkTyCoVarKind TyVar
tv
  , tcm_tycon :: TyCon -> IOEnv (Env TcGblEnv TcLclEnv) TyCon
tcm_tycon      = TyCon -> IOEnv (Env TcGblEnv TcLclEnv) TyCon
zonkTcTyCon }
  where
    hole :: () -> CoercionHole -> TcM Coercion
    hole :: () -> CoercionHole -> TcM Coercion
hole ()
_ hole :: CoercionHole
hole@(CoercionHole { ch_ref :: CoercionHole -> IORef (Maybe Coercion)
ch_ref = IORef (Maybe Coercion)
ref, ch_co_var :: CoercionHole -> TyVar
ch_co_var = TyVar
cv })
      = do { Maybe Coercion
contents <- IORef (Maybe Coercion)
-> IOEnv (Env TcGblEnv TcLclEnv) (Maybe Coercion)
forall a gbl lcl. TcRef a -> TcRnIf gbl lcl a
readTcRef IORef (Maybe Coercion)
ref
           ; case Maybe Coercion
contents of
               Just Coercion
co -> do { Coercion
co' <- Coercion -> TcM Coercion
zonkCo Coercion
co
                             ; TyVar -> Coercion -> TcM Coercion
checkCoercionHole TyVar
cv Coercion
co' }
               Maybe Coercion
Nothing -> do { TyVar
cv' <- TyVar -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
zonkCoVar TyVar
cv
                             ; Coercion -> TcM Coercion
forall (m :: * -> *) a. Monad m => a -> m a
return (Coercion -> TcM Coercion) -> Coercion -> TcM Coercion
forall a b. (a -> b) -> a -> b
$ CoercionHole -> Coercion
HoleCo (CoercionHole
hole { ch_co_var :: TyVar
ch_co_var = TyVar
cv' }) } }

zonkTcTyCon :: TcTyCon -> TcM TcTyCon
-- Only called on TcTyCons
-- A non-poly TcTyCon may have unification
-- variables that need zonking, but poly ones cannot
zonkTcTyCon :: TyCon -> IOEnv (Env TcGblEnv TcLclEnv) TyCon
zonkTcTyCon TyCon
tc
 | TyCon -> Bool
tcTyConIsPoly TyCon
tc = TyCon -> IOEnv (Env TcGblEnv TcLclEnv) TyCon
forall (m :: * -> *) a. Monad m => a -> m a
return TyCon
tc
 | Bool
otherwise        = do { Kind
tck' <- Kind -> TcM Kind
zonkTcType (TyCon -> Kind
tyConKind TyCon
tc)
                         ; TyCon -> IOEnv (Env TcGblEnv TcLclEnv) TyCon
forall (m :: * -> *) a. Monad m => a -> m a
return (TyCon -> Kind -> TyCon
setTcTyConKind TyCon
tc Kind
tck') }

zonkTcTyVar :: TcTyVar -> TcM TcType
-- Simply look through all Flexis
zonkTcTyVar :: TyVar -> TcM Kind
zonkTcTyVar TyVar
tv
  | TyVar -> Bool
isTcTyVar TyVar
tv
  = case TyVar -> TcTyVarDetails
tcTyVarDetails TyVar
tv of
      SkolemTv {}   -> TcM Kind
zonk_kind_and_return
      RuntimeUnk {} -> TcM Kind
zonk_kind_and_return
      MetaTv { mtv_ref :: TcTyVarDetails -> IORef MetaDetails
mtv_ref = IORef MetaDetails
ref }
         -> do { MetaDetails
cts <- IORef MetaDetails -> TcM MetaDetails
forall a env. IORef a -> IOEnv env a
readMutVar IORef MetaDetails
ref
               ; case MetaDetails
cts of
                    MetaDetails
Flexi       -> TcM Kind
zonk_kind_and_return
                    Indirect Kind
ty -> do { Kind
zty <- Kind -> TcM Kind
zonkTcType Kind
ty
                                      ; IORef MetaDetails -> MetaDetails -> TcRn ()
forall a gbl lcl. TcRef a -> a -> TcRnIf gbl lcl ()
writeTcRef IORef MetaDetails
ref (Kind -> MetaDetails
Indirect Kind
zty)
                                        -- See Note [Sharing in zonking]
                                      ; Kind -> TcM Kind
forall (m :: * -> *) a. Monad m => a -> m a
return Kind
zty } }

  | Bool
otherwise -- coercion variable
  = TcM Kind
zonk_kind_and_return
  where
    zonk_kind_and_return :: TcM Kind
zonk_kind_and_return = do { TyVar
z_tv <- TyVar -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
zonkTyCoVarKind TyVar
tv
                              ; Kind -> TcM Kind
forall (m :: * -> *) a. Monad m => a -> m a
return (TyVar -> Kind
mkTyVarTy TyVar
z_tv) }

-- Variant that assumes that any result of zonking is still a TyVar.
-- Should be used only on skolems and TyVarTvs
zonkTcTyVarToTyVar :: HasDebugCallStack => TcTyVar -> TcM TcTyVar
zonkTcTyVarToTyVar :: HasDebugCallStack => TyVar -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
zonkTcTyVarToTyVar TyVar
tv
  = do { Kind
ty <- TyVar -> TcM Kind
zonkTcTyVar TyVar
tv
       ; let tv' :: TyVar
tv' = case Kind -> Maybe TyVar
tcGetTyVar_maybe Kind
ty of
                     Just TyVar
tv' -> TyVar
tv'
                     Maybe TyVar
Nothing  -> String -> SDoc -> TyVar
forall a. HasCallStack => String -> SDoc -> a
pprPanic String
"zonkTcTyVarToTyVar"
                                          (TyVar -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyVar
tv SDoc -> SDoc -> SDoc
$$ Kind -> SDoc
forall a. Outputable a => a -> SDoc
ppr Kind
ty)
       ; TyVar -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
forall (m :: * -> *) a. Monad m => a -> m a
return TyVar
tv' }

zonkInvisTVBinder :: VarBndr TcTyVar spec -> TcM (VarBndr TyVar spec)
zonkInvisTVBinder :: forall fl. VarBndr TyVar fl -> TcM (VarBndr TyVar fl)
zonkInvisTVBinder (Bndr TyVar
tv spec
spec) = do { TyVar
tv' <- HasDebugCallStack => TyVar -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
TyVar -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
zonkTcTyVarToTyVar TyVar
tv
                                      ; VarBndr TyVar spec -> TcM (VarBndr TyVar spec)
forall (m :: * -> *) a. Monad m => a -> m a
return (TyVar -> spec -> VarBndr TyVar spec
forall var argf. var -> argf -> VarBndr var argf
Bndr TyVar
tv' spec
spec) }

-- zonkId is used *during* typechecking just to zonk the Id's type
zonkId :: TcId -> TcM TcId
zonkId :: TyVar -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
zonkId TyVar
id = (Kind -> TcM Kind) -> TyVar -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
forall (m :: * -> *).
Monad m =>
(Kind -> m Kind) -> TyVar -> m TyVar
Id.updateIdTypeAndMultM Kind -> TcM Kind
zonkTcType TyVar
id

zonkCoVar :: CoVar -> TcM CoVar
zonkCoVar :: TyVar -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
zonkCoVar = TyVar -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
zonkId

{- Note [Sharing in zonking]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Suppose we have
   alpha :-> beta :-> gamma :-> ty
where the ":->" means that the unification variable has been
filled in with Indirect. Then when zonking alpha, it'd be nice
to short-circuit beta too, so we end up with
   alpha :-> zty
   beta  :-> zty
   gamma :-> zty
where zty is the zonked version of ty.  That way, if we come across
beta later, we'll have less work to do.  (And indeed the same for
alpha.)

This is easily achieved: just overwrite (Indirect ty) with (Indirect
zty).  Non-systematic perf comparisons suggest that this is a modest
win.

But c.f Note [Sharing when zonking to Type] in GHC.Tc.Utils.Zonk.

%************************************************************************
%*                                                                      *
                 Tidying
*                                                                      *
************************************************************************
-}

zonkTidyTcType :: TidyEnv -> TcType -> TcM (TidyEnv, TcType)
zonkTidyTcType :: TidyEnv -> Kind -> TcM (TidyEnv, Kind)
zonkTidyTcType TidyEnv
env Kind
ty = do { Kind
ty' <- Kind -> TcM Kind
zonkTcType Kind
ty
                           ; (TidyEnv, Kind) -> TcM (TidyEnv, Kind)
forall (m :: * -> *) a. Monad m => a -> m a
return (TidyEnv -> Kind -> (TidyEnv, Kind)
tidyOpenType TidyEnv
env Kind
ty') }

zonkTidyTcTypes :: TidyEnv -> [TcType] -> TcM (TidyEnv, [TcType])
zonkTidyTcTypes :: TidyEnv -> [Kind] -> TcM (TidyEnv, [Kind])
zonkTidyTcTypes = [Kind] -> TidyEnv -> [Kind] -> TcM (TidyEnv, [Kind])
zonkTidyTcTypes' []
  where zonkTidyTcTypes' :: [Kind] -> TidyEnv -> [Kind] -> TcM (TidyEnv, [Kind])
zonkTidyTcTypes' [Kind]
zs TidyEnv
env [] = (TidyEnv, [Kind]) -> TcM (TidyEnv, [Kind])
forall (m :: * -> *) a. Monad m => a -> m a
return (TidyEnv
env, [Kind] -> [Kind]
forall a. [a] -> [a]
reverse [Kind]
zs)
        zonkTidyTcTypes' [Kind]
zs TidyEnv
env (Kind
ty:[Kind]
tys)
          = do { (TidyEnv
env', Kind
ty') <- TidyEnv -> Kind -> TcM (TidyEnv, Kind)
zonkTidyTcType TidyEnv
env Kind
ty
               ; [Kind] -> TidyEnv -> [Kind] -> TcM (TidyEnv, [Kind])
zonkTidyTcTypes' (Kind
ty'Kind -> [Kind] -> [Kind]
forall a. a -> [a] -> [a]
:[Kind]
zs) TidyEnv
env' [Kind]
tys }

zonkTidyOrigin :: TidyEnv -> CtOrigin -> TcM (TidyEnv, CtOrigin)
zonkTidyOrigin :: TidyEnv -> CtOrigin -> TcM (TidyEnv, CtOrigin)
zonkTidyOrigin TidyEnv
env (GivenOrigin SkolemInfo
skol_info)
  = do { SkolemInfo
skol_info1 <- SkolemInfo -> TcM SkolemInfo
zonkSkolemInfo SkolemInfo
skol_info
       ; let skol_info2 :: SkolemInfo
skol_info2 = TidyEnv -> SkolemInfo -> SkolemInfo
tidySkolemInfo TidyEnv
env SkolemInfo
skol_info1
       ; (TidyEnv, CtOrigin) -> TcM (TidyEnv, CtOrigin)
forall (m :: * -> *) a. Monad m => a -> m a
return (TidyEnv
env, SkolemInfo -> CtOrigin
GivenOrigin SkolemInfo
skol_info2) }
zonkTidyOrigin TidyEnv
env orig :: CtOrigin
orig@(TypeEqOrigin { uo_actual :: CtOrigin -> Kind
uo_actual   = Kind
act
                                      , uo_expected :: CtOrigin -> Kind
uo_expected = Kind
exp })
  = do { (TidyEnv
env1, Kind
act') <- TidyEnv -> Kind -> TcM (TidyEnv, Kind)
zonkTidyTcType TidyEnv
env  Kind
act
       ; (TidyEnv
env2, Kind
exp') <- TidyEnv -> Kind -> TcM (TidyEnv, Kind)
zonkTidyTcType TidyEnv
env1 Kind
exp
       ; (TidyEnv, CtOrigin) -> TcM (TidyEnv, CtOrigin)
forall (m :: * -> *) a. Monad m => a -> m a
return ( TidyEnv
env2, CtOrigin
orig { uo_actual :: Kind
uo_actual   = Kind
act'
                             , uo_expected :: Kind
uo_expected = Kind
exp' }) }
zonkTidyOrigin TidyEnv
env (KindEqOrigin Kind
ty1 Kind
ty2 CtOrigin
orig Maybe TypeOrKind
t_or_k)
  = do { (TidyEnv
env1, Kind
ty1')  <- TidyEnv -> Kind -> TcM (TidyEnv, Kind)
zonkTidyTcType TidyEnv
env  Kind
ty1
       ; (TidyEnv
env2, Kind
ty2')  <- TidyEnv -> Kind -> TcM (TidyEnv, Kind)
zonkTidyTcType TidyEnv
env1 Kind
ty2
       ; (TidyEnv
env3, CtOrigin
orig') <- TidyEnv -> CtOrigin -> TcM (TidyEnv, CtOrigin)
zonkTidyOrigin TidyEnv
env2 CtOrigin
orig
       ; (TidyEnv, CtOrigin) -> TcM (TidyEnv, CtOrigin)
forall (m :: * -> *) a. Monad m => a -> m a
return (TidyEnv
env3, Kind -> Kind -> CtOrigin -> Maybe TypeOrKind -> CtOrigin
KindEqOrigin Kind
ty1' Kind
ty2' CtOrigin
orig' Maybe TypeOrKind
t_or_k) }
zonkTidyOrigin TidyEnv
env (FunDepOrigin1 Kind
p1 CtOrigin
o1 RealSrcSpan
l1 Kind
p2 CtOrigin
o2 RealSrcSpan
l2)
  = do { (TidyEnv
env1, Kind
p1') <- TidyEnv -> Kind -> TcM (TidyEnv, Kind)
zonkTidyTcType TidyEnv
env  Kind
p1
       ; (TidyEnv
env2, Kind
p2') <- TidyEnv -> Kind -> TcM (TidyEnv, Kind)
zonkTidyTcType TidyEnv
env1 Kind
p2
       ; (TidyEnv, CtOrigin) -> TcM (TidyEnv, CtOrigin)
forall (m :: * -> *) a. Monad m => a -> m a
return (TidyEnv
env2, Kind
-> CtOrigin
-> RealSrcSpan
-> Kind
-> CtOrigin
-> RealSrcSpan
-> CtOrigin
FunDepOrigin1 Kind
p1' CtOrigin
o1 RealSrcSpan
l1 Kind
p2' CtOrigin
o2 RealSrcSpan
l2) }
zonkTidyOrigin TidyEnv
env (FunDepOrigin2 Kind
p1 CtOrigin
o1 Kind
p2 SrcSpan
l2)
  = do { (TidyEnv
env1, Kind
p1') <- TidyEnv -> Kind -> TcM (TidyEnv, Kind)
zonkTidyTcType TidyEnv
env  Kind
p1
       ; (TidyEnv
env2, Kind
p2') <- TidyEnv -> Kind -> TcM (TidyEnv, Kind)
zonkTidyTcType TidyEnv
env1 Kind
p2
       ; (TidyEnv
env3, CtOrigin
o1') <- TidyEnv -> CtOrigin -> TcM (TidyEnv, CtOrigin)
zonkTidyOrigin TidyEnv
env2 CtOrigin
o1
       ; (TidyEnv, CtOrigin) -> TcM (TidyEnv, CtOrigin)
forall (m :: * -> *) a. Monad m => a -> m a
return (TidyEnv
env3, Kind -> CtOrigin -> Kind -> SrcSpan -> CtOrigin
FunDepOrigin2 Kind
p1' CtOrigin
o1' Kind
p2' SrcSpan
l2) }
zonkTidyOrigin TidyEnv
env CtOrigin
orig = (TidyEnv, CtOrigin) -> TcM (TidyEnv, CtOrigin)
forall (m :: * -> *) a. Monad m => a -> m a
return (TidyEnv
env, CtOrigin
orig)

----------------
tidyCt :: TidyEnv -> Ct -> Ct
-- Used only in error reporting
tidyCt :: TidyEnv -> Ct -> Ct
tidyCt TidyEnv
env Ct
ct
  = Ct
ct { cc_ev :: CtEvidence
cc_ev = CtEvidence -> CtEvidence
tidy_ev (Ct -> CtEvidence
ctEvidence Ct
ct) }
  where
    tidy_ev :: CtEvidence -> CtEvidence
     -- NB: we do not tidy the ctev_evar field because we don't
     --     show it in error messages
    tidy_ev :: CtEvidence -> CtEvidence
tidy_ev CtEvidence
ctev = CtEvidence
ctev { ctev_pred :: Kind
ctev_pred = TidyEnv -> Kind -> Kind
tidyType TidyEnv
env (CtEvidence -> Kind
ctev_pred CtEvidence
ctev) }

tidyHole :: TidyEnv -> Hole -> Hole
tidyHole :: TidyEnv -> Hole -> Hole
tidyHole TidyEnv
env h :: Hole
h@(Hole { hole_ty :: Hole -> Kind
hole_ty = Kind
ty }) = Hole
h { hole_ty :: Kind
hole_ty = TidyEnv -> Kind -> Kind
tidyType TidyEnv
env Kind
ty }

----------------
tidyEvVar :: TidyEnv -> EvVar -> EvVar
tidyEvVar :: TidyEnv -> TyVar -> TyVar
tidyEvVar TidyEnv
env TyVar
var = (Kind -> Kind) -> TyVar -> TyVar
updateIdTypeAndMult (TidyEnv -> Kind -> Kind
tidyType TidyEnv
env) TyVar
var

----------------
tidySkolemInfo :: TidyEnv -> SkolemInfo -> SkolemInfo
tidySkolemInfo :: TidyEnv -> SkolemInfo -> SkolemInfo
tidySkolemInfo TidyEnv
env (DerivSkol Kind
ty)         = Kind -> SkolemInfo
DerivSkol (TidyEnv -> Kind -> Kind
tidyType TidyEnv
env Kind
ty)
tidySkolemInfo TidyEnv
env (SigSkol UserTypeCtxt
cx Kind
ty [(Name, TyVar)]
tv_prs) = TidyEnv -> UserTypeCtxt -> Kind -> [(Name, TyVar)] -> SkolemInfo
tidySigSkol TidyEnv
env UserTypeCtxt
cx Kind
ty [(Name, TyVar)]
tv_prs
tidySkolemInfo TidyEnv
env (InferSkol [(Name, Kind)]
ids)        = [(Name, Kind)] -> SkolemInfo
InferSkol ((Kind -> Kind) -> [(Name, Kind)] -> [(Name, Kind)]
forall b c a. (b -> c) -> [(a, b)] -> [(a, c)]
mapSnd (TidyEnv -> Kind -> Kind
tidyType TidyEnv
env) [(Name, Kind)]
ids)
tidySkolemInfo TidyEnv
env (UnifyForAllSkol Kind
ty)   = Kind -> SkolemInfo
UnifyForAllSkol (TidyEnv -> Kind -> Kind
tidyType TidyEnv
env Kind
ty)
tidySkolemInfo TidyEnv
_   SkolemInfo
info                   = SkolemInfo
info

tidySigSkol :: TidyEnv -> UserTypeCtxt
            -> TcType -> [(Name,TcTyVar)] -> SkolemInfo
-- We need to take special care when tidying SigSkol
-- See Note [SigSkol SkolemInfo] in "GHC.Tc.Types.Origin"
tidySigSkol :: TidyEnv -> UserTypeCtxt -> Kind -> [(Name, TyVar)] -> SkolemInfo
tidySigSkol TidyEnv
env UserTypeCtxt
cx Kind
ty [(Name, TyVar)]
tv_prs
  = UserTypeCtxt -> Kind -> [(Name, TyVar)] -> SkolemInfo
SigSkol UserTypeCtxt
cx (TidyEnv -> Kind -> Kind
tidy_ty TidyEnv
env Kind
ty) [(Name, TyVar)]
tv_prs'
  where
    tv_prs' :: [(Name, TyVar)]
tv_prs' = (TyVar -> TyVar) -> [(Name, TyVar)] -> [(Name, TyVar)]
forall b c a. (b -> c) -> [(a, b)] -> [(a, c)]
mapSnd (TidyEnv -> TyVar -> TyVar
tidyTyCoVarOcc TidyEnv
env) [(Name, TyVar)]
tv_prs
    inst_env :: NameEnv TyVar
inst_env = [(Name, TyVar)] -> NameEnv TyVar
forall a. [(Name, a)] -> NameEnv a
mkNameEnv [(Name, TyVar)]
tv_prs'

    tidy_ty :: TidyEnv -> Kind -> Kind
tidy_ty TidyEnv
env (ForAllTy (Bndr TyVar
tv ArgFlag
vis) Kind
ty)
      = VarBndr TyVar ArgFlag -> Kind -> Kind
ForAllTy (TyVar -> ArgFlag -> VarBndr TyVar ArgFlag
forall var argf. var -> argf -> VarBndr var argf
Bndr TyVar
tv' ArgFlag
vis) (TidyEnv -> Kind -> Kind
tidy_ty TidyEnv
env' Kind
ty)
      where
        (TidyEnv
env', TyVar
tv') = TidyEnv -> TyVar -> (TidyEnv, TyVar)
tidy_tv_bndr TidyEnv
env TyVar
tv

    tidy_ty TidyEnv
env ty :: Kind
ty@(FunTy AnonArgFlag
InvisArg Kind
w Kind
arg Kind
res) -- Look under  c => t
      = Kind
ty { ft_mult :: Kind
ft_mult = TidyEnv -> Kind -> Kind
tidy_ty TidyEnv
env Kind
w,
             ft_arg :: Kind
ft_arg = TidyEnv -> Kind -> Kind
tidyType TidyEnv
env Kind
arg,
             ft_res :: Kind
ft_res = TidyEnv -> Kind -> Kind
tidy_ty TidyEnv
env Kind
res }

    tidy_ty TidyEnv
env Kind
ty = TidyEnv -> Kind -> Kind
tidyType TidyEnv
env Kind
ty

    tidy_tv_bndr :: TidyEnv -> TyCoVar -> (TidyEnv, TyCoVar)
    tidy_tv_bndr :: TidyEnv -> TyVar -> (TidyEnv, TyVar)
tidy_tv_bndr env :: TidyEnv
env@(TidyOccEnv
occ_env, VarEnv TyVar
subst) TyVar
tv
      | Just TyVar
tv' <- NameEnv TyVar -> Name -> Maybe TyVar
forall a. NameEnv a -> Name -> Maybe a
lookupNameEnv NameEnv TyVar
inst_env (TyVar -> Name
tyVarName TyVar
tv)
      = ((TidyOccEnv
occ_env, VarEnv TyVar -> TyVar -> TyVar -> VarEnv TyVar
forall a. VarEnv a -> TyVar -> a -> VarEnv a
extendVarEnv VarEnv TyVar
subst TyVar
tv TyVar
tv'), TyVar
tv')

      | Bool
otherwise
      = TidyEnv -> TyVar -> (TidyEnv, TyVar)
tidyVarBndr TidyEnv
env TyVar
tv

-------------------------------------------------------------------------
{-
%************************************************************************
%*                                                                      *
             Levity polymorphism checks
*                                                                       *
*************************************************************************

See Note [Levity polymorphism checking] in GHC.HsToCore.Monad

-}

-- | According to the rules around representation polymorphism
-- (see https://gitlab.haskell.org/ghc/ghc/wikis/no-sub-kinds), no binder
-- can have a representation-polymorphic type. This check ensures
-- that we respect this rule. It is a bit regrettable that this error
-- occurs in zonking, after which we should have reported all errors.
-- But it's hard to see where else to do it, because this can be discovered
-- only after all solving is done. And, perhaps most importantly, this
-- isn't really a compositional property of a type system, so it's
-- not a terrible surprise that the check has to go in an awkward spot.
ensureNotLevPoly :: Type  -- its zonked type
                 -> SDoc  -- where this happened
                 -> TcM ()
ensureNotLevPoly :: Kind -> SDoc -> TcRn ()
ensureNotLevPoly Kind
ty SDoc
doc
  = TcRn () -> TcRn ()
whenNoErrs (TcRn () -> TcRn ()) -> TcRn () -> TcRn ()
forall a b. (a -> b) -> a -> b
$   -- sometimes we end up zonking bogus definitions of type
                   -- forall a. a. See, for example, test ghci/scripts/T9140
    SDoc -> Kind -> TcRn ()
checkForLevPoly SDoc
doc Kind
ty

  -- See Note [Levity polymorphism checking] in GHC.HsToCore.Monad
checkForLevPoly :: SDoc -> Type -> TcM ()
checkForLevPoly :: SDoc -> Kind -> TcRn ()
checkForLevPoly = (SDoc -> TcRn ()) -> SDoc -> Kind -> TcRn ()
forall (m :: * -> *).
Monad m =>
(SDoc -> m ()) -> SDoc -> Kind -> m ()
checkForLevPolyX SDoc -> TcRn ()
addErr

checkForLevPolyX :: Monad m
                 => (SDoc -> m ())  -- how to report an error
                 -> SDoc -> Type -> m ()
checkForLevPolyX :: forall (m :: * -> *).
Monad m =>
(SDoc -> m ()) -> SDoc -> Kind -> m ()
checkForLevPolyX SDoc -> m ()
add_err SDoc
extra Kind
ty
  | Kind -> Bool
isTypeLevPoly Kind
ty
  = SDoc -> m ()
add_err (Kind -> SDoc
formatLevPolyErr Kind
ty SDoc -> SDoc -> SDoc
$$ SDoc
extra)
  | Bool
otherwise
  = () -> m ()
forall (m :: * -> *) a. Monad m => a -> m a
return ()

formatLevPolyErr :: Type  -- levity-polymorphic type
                 -> SDoc
formatLevPolyErr :: Kind -> SDoc
formatLevPolyErr Kind
ty
  = SDoc -> Int -> SDoc -> SDoc
hang (String -> SDoc
text String
"A levity-polymorphic type is not allowed here:")
       Int
2 ([SDoc] -> SDoc
vcat [ String -> SDoc
text String
"Type:" SDoc -> SDoc -> SDoc
<+> Kind -> SDoc
pprWithTYPE Kind
tidy_ty
               , String -> SDoc
text String
"Kind:" SDoc -> SDoc -> SDoc
<+> Kind -> SDoc
pprWithTYPE Kind
tidy_ki ])
  where
    (TidyEnv
tidy_env, Kind
tidy_ty) = TidyEnv -> Kind -> (TidyEnv, Kind)
tidyOpenType TidyEnv
emptyTidyEnv Kind
ty
    tidy_ki :: Kind
tidy_ki             = TidyEnv -> Kind -> Kind
tidyType TidyEnv
tidy_env (HasDebugCallStack => Kind -> Kind
Kind -> Kind
tcTypeKind Kind
ty)

{-
%************************************************************************
%*                                                                      *
             Error messages
*                                                                       *
*************************************************************************

-}

-- See Note [Naughty quantification candidates]
naughtyQuantification :: TcType   -- original type user wanted to quantify
                      -> TcTyVar  -- naughty var
                      -> TyVarSet -- skolems that would escape
                      -> TcM a
naughtyQuantification :: forall a. Kind -> TyVar -> CoVarSet -> TcM a
naughtyQuantification Kind
orig_ty TyVar
tv CoVarSet
escapees
  = do { Kind
orig_ty1 <- Kind -> TcM Kind
zonkTcType Kind
orig_ty  -- in case it's not zonked

       ; [TyVar]
escapees' <- (TyVar -> IOEnv (Env TcGblEnv TcLclEnv) TyVar)
-> [TyVar] -> TcM [TyVar]
forall (t :: * -> *) (m :: * -> *) a b.
(Traversable t, Monad m) =>
(a -> m b) -> t a -> m (t b)
mapM HasDebugCallStack => TyVar -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
TyVar -> IOEnv (Env TcGblEnv TcLclEnv) TyVar
zonkTcTyVarToTyVar ([TyVar] -> TcM [TyVar]) -> [TyVar] -> TcM [TyVar]
forall a b. (a -> b) -> a -> b
$
                      CoVarSet -> [TyVar]
forall elt. UniqSet elt -> [elt]
nonDetEltsUniqSet CoVarSet
escapees
                     -- we'll just be printing, so no harmful non-determinism

       ; let fvs :: [TyVar]
fvs  = Kind -> [TyVar]
tyCoVarsOfTypeWellScoped Kind
orig_ty1
             env0 :: TidyEnv
env0 = TidyEnv -> [TyVar] -> TidyEnv
tidyFreeTyCoVars TidyEnv
emptyTidyEnv [TyVar]
fvs
             env :: TidyEnv
env  = TidyEnv
env0 TidyEnv -> [TyVar] -> TidyEnv
`delTidyEnvList` [TyVar]
escapees'
                    -- this avoids gratuitous renaming of the escaped
                    -- variables; very confusing to users!

             orig_ty' :: Kind
orig_ty'   = TidyEnv -> Kind -> Kind
tidyType TidyEnv
env Kind
orig_ty1
             ppr_tidied :: [TyVar] -> SDoc
ppr_tidied = [TyVar] -> SDoc
pprTyVars ([TyVar] -> SDoc) -> ([TyVar] -> [TyVar]) -> [TyVar] -> SDoc
forall b c a. (b -> c) -> (a -> b) -> a -> c
. (TyVar -> TyVar) -> [TyVar] -> [TyVar]
forall a b. (a -> b) -> [a] -> [b]
map (TidyEnv -> TyVar -> TyVar
tidyTyCoVarOcc TidyEnv
env)
             doc :: SDoc
doc = Bool -> SDoc -> SDoc
pprWithExplicitKindsWhen Bool
True (SDoc -> SDoc) -> SDoc -> SDoc
forall a b. (a -> b) -> a -> b
$
                   [SDoc] -> SDoc
vcat [ [SDoc] -> SDoc
sep [ String -> SDoc
text String
"Cannot generalise type; skolem" SDoc -> SDoc -> SDoc
<> [TyVar] -> SDoc
forall a. [a] -> SDoc
plural [TyVar]
escapees'
                              , SDoc -> SDoc
quotes (SDoc -> SDoc) -> SDoc -> SDoc
forall a b. (a -> b) -> a -> b
$ [TyVar] -> SDoc
ppr_tidied [TyVar]
escapees'
                              , String -> SDoc
text String
"would escape" SDoc -> SDoc -> SDoc
<+> [TyVar] -> SDoc
forall a. [a] -> SDoc
itsOrTheir [TyVar]
escapees' SDoc -> SDoc -> SDoc
<+> String -> SDoc
text String
"scope"
                              ]
                        , [SDoc] -> SDoc
sep [ String -> SDoc
text String
"if I tried to quantify"
                              , [TyVar] -> SDoc
ppr_tidied [TyVar
tv]
                              , String -> SDoc
text String
"in this type:"
                              ]
                        , Int -> SDoc -> SDoc
nest Int
2 (Kind -> SDoc
pprTidiedType Kind
orig_ty')
                        , String -> SDoc
text String
"(Indeed, I sometimes struggle even printing this correctly,"
                        , String -> SDoc
text String
" due to its ill-scoped nature.)"
                        ]

       ; (TidyEnv, SDoc) -> TcM a
forall a. (TidyEnv, SDoc) -> TcM a
failWithTcM (TidyEnv
env, SDoc
doc) }