typecheck/TcMType.hs

    {-
    (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
    -}

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

    module TcMType (
      TcTyVar, TcKind, TcType, TcTauType, TcThetaType, TcTyVarSet,

      --------------------------------
      -- Creating new mutable type variables
      newFlexiTyVar,
      newFlexiTyVarTy,              -- Kind -> TcM TcType
      newFlexiTyVarTys,             -- Int -> Kind -> TcM [TcType]
      newOpenFlexiTyVarTy,
      newMetaKindVar, newMetaKindVars,
      cloneMetaTyVar,
      newFmvTyVar, newFskTyVar,

      readMetaTyVar, writeMetaTyVar, writeMetaTyVarRef,
      newMetaDetails, isFilledMetaTyVar, isUnfilledMetaTyVar,

      --------------------------------
      -- Expected types
      ExpType(..), ExpSigmaType, ExpRhoType,
      mkCheckExpType, newOpenInferExpType, readExpType, readExpType_maybe,
      writeExpType, expTypeToType, checkingExpType_maybe, checkingExpType,
      tauifyExpType,

      --------------------------------
      -- Creating fresh type variables for pm checking
      genInstSkolTyVarsX,

      --------------------------------
      -- Creating new evidence variables
      newEvVar, newEvVars, newDict,
      newWanted, newWanteds,
      emitWanted, emitWantedEq, emitWantedEvVar, emitWantedEvVars,
      newTcEvBinds, addTcEvBind,

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

      --------------------------------
      -- Instantiation
      newMetaTyVars, newMetaTyVarX, newMetaSigTyVars,
      newSigTyVar,
      tcInstType,
      tcInstSkolTyVars, tcInstSkolTyVarsLoc, tcInstSuperSkolTyVarsX,
      tcInstSigTyVarsLoc, tcInstSigTyVars,
      tcInstSkolType,
      tcSkolDFunType, tcSuperSkolTyVars,

      instSkolTyCoVars, freshenTyVarBndrs, freshenCoVarBndrsX,

      --------------------------------
      -- Zonking and tidying
      zonkTidyTcType, zonkTidyOrigin,
      mkTypeErrorThing, mkTypeErrorThingArgs,
      tidyEvVar, tidyCt, tidySkolemInfo,
      skolemiseUnboundMetaTyVar,
      zonkTcTyVar, zonkTcTyVars, zonkTyCoVarsAndFV, zonkTcTypeAndFV,
      zonkTcTypeAndSplitDepVars, zonkTcTypesAndSplitDepVars,
      zonkQuantifiedTyVar, zonkQuantifiedTyVarOrType,
      quantifyTyVars, quantifyZonkedTyVars,
      defaultKindVar,
      zonkTcTyCoVarBndr, zonkTcTyBinder, zonkTcType, zonkTcTypes, zonkCo,
      zonkTyCoVarKind, zonkTcTypeMapper,

      zonkEvVar, zonkWC, zonkSimples, zonkId, zonkCt, zonkSkolemInfo,

      tcGetGlobalTyCoVars
      ) where

    #include "HsVersions.h"

    -- friends:
    import TyCoRep
    import TcType
    import Type
    import Kind
    import Coercion
    import Class
    import Var

    -- others:
    import TcRnMonad        -- TcType, amongst others
    import TcEvidence
    import Id
    import Name
    import VarSet
    import TysWiredIn
    import TysPrim
    import VarEnv
    import PrelNames
    import Util
    import Outputable
    import FastString
    import SrcLoc
    import Bag
    import Pair
    import qualified GHC.LanguageExtensions as LangExt

    import Control.Monad
    import Maybes
    import Data.List        ( mapAccumL, partition )
    import Control.Arrow    ( second )

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

    mkKindName :: Unique -> Name
    mkKindName unique = mkSystemName unique kind_var_occ

    kind_var_occ :: OccName -- Just one for all MetaKindVars
                            -- They may be jiggled by tidying
    kind_var_occ = mkOccName tvName "k"

    newMetaKindVar :: TcM TcKind
    newMetaKindVar = do { uniq <- newUnique
                        ; details <- newMetaDetails TauTv
                        ; let kv = mkTcTyVar (mkKindName uniq) liftedTypeKind details
                        ; return (mkTyVarTy kv) }

    newMetaKindVars :: Int -> TcM [TcKind]
    newMetaKindVars n = mapM (\ _ -> newMetaKindVar) (nOfThem n ())

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

    newEvVars :: TcThetaType -> TcM [EvVar]
    newEvVars theta = mapM newEvVar theta

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

    newEvVar :: TcPredType -> TcRnIf gbl lcl EvVar
    -- Creates new *rigid* variables for predicates
    newEvVar ty = do { name <- newSysName (predTypeOccName ty)
                     ; return (mkLocalIdOrCoVar name ty) }

    newWanted :: CtOrigin -> Maybe TypeOrKind -> PredType -> TcM CtEvidence
    -- Deals with both equality and non-equality predicates
    newWanted orig t_or_k pty
      = do loc <- getCtLocM orig t_or_k
           d <- if isEqPred pty then HoleDest  <$> newCoercionHole
                                else EvVarDest <$> newEvVar pty
           return $ CtWanted { ctev_dest = d
                             , ctev_pred = pty
                             , ctev_loc = loc }

    newWanteds :: CtOrigin -> ThetaType -> TcM [CtEvidence]
    newWanteds orig = mapM (newWanted orig Nothing)

    -- | Emits a new Wanted. Deals with both equalities and non-equalities.
    emitWanted :: CtOrigin -> TcPredType -> TcM EvTerm
    emitWanted origin pty
      = do { ev <- newWanted origin Nothing pty
           ; emitSimple $ mkNonCanonical ev
           ; return $ ctEvTerm ev }

    -- | Emits a new equality constraint
    emitWantedEq :: CtOrigin -> TypeOrKind -> Role -> TcType -> TcType -> TcM Coercion
    emitWantedEq origin t_or_k role ty1 ty2
      = do { hole <- newCoercionHole
           ; loc <- getCtLocM origin (Just t_or_k)
           ; emitSimple $ mkNonCanonical $
               CtWanted { ctev_pred = pty, ctev_dest = HoleDest hole, ctev_loc = loc }
           ; return (mkHoleCo hole role ty1 ty2) }
      where
        pty = mkPrimEqPredRole role ty1 ty2

    -- | Creates a new EvVar and immediately emits it as a Wanted.
    -- No equality predicates here.
    emitWantedEvVar :: CtOrigin -> TcPredType -> TcM EvVar
    emitWantedEvVar origin ty
      = do { new_cv <- newEvVar ty
           ; loc <- getCtLocM origin Nothing
           ; let ctev = CtWanted { ctev_dest = EvVarDest new_cv
                                 , ctev_pred = ty
                                 , ctev_loc  = loc }
           ; emitSimple $ mkNonCanonical ctev
           ; return new_cv }

    emitWantedEvVars :: CtOrigin -> [TcPredType] -> TcM [EvVar]
    emitWantedEvVars orig = mapM (emitWantedEvVar orig)

    newDict :: Class -> [TcType] -> TcM DictId
    newDict cls tys
      = do { name <- newSysName (mkDictOcc (getOccName cls))
           ; return (mkLocalId name (mkClassPred cls tys)) }

    predTypeOccName :: PredType -> OccName
    predTypeOccName ty = case classifyPredType ty of
        ClassPred cls _ -> mkDictOcc (getOccName cls)
        EqPred _ _ _    -> mkVarOccFS (fsLit "cobox")
        IrredPred _     -> mkVarOccFS (fsLit "irred")

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

    newCoercionHole :: TcM CoercionHole
    newCoercionHole
      = do { u <- newUnique
           ; traceTc "New coercion hole:" (ppr u)
           ; ref <- newMutVar Nothing
           ; return $ CoercionHole u ref }

    -- | Put a value in a coercion hole
    fillCoercionHole :: CoercionHole -> Coercion -> TcM ()
    fillCoercionHole (CoercionHole u ref) co
      = do {
    #ifdef DEBUG
           ; cts <- readTcRef ref
           ; whenIsJust cts $ \old_co ->
             pprPanic "Filling a filled coercion hole" (ppr u $$ ppr co $$ ppr old_co)
    #endif
           ; traceTc "Filling coercion hole" (ppr u <+> text ":=" <+> ppr co)
           ; writeTcRef ref (Just co) }

    -- | Is a coercion hole filled in?
    isFilledCoercionHole :: CoercionHole -> TcM Bool
    isFilledCoercionHole (CoercionHole _ ref) = isJust <$> readTcRef ref

    -- | Retrieve the contents of a coercion hole. Panics if the hole
    -- is unfilled
    unpackCoercionHole :: CoercionHole -> TcM Coercion
    unpackCoercionHole hole
      = do { contents <- unpackCoercionHole_maybe hole
           ; case contents of
               Just co -> return co
               Nothing -> pprPanic "Unfilled coercion hole" (ppr hole) }

    -- | Retrieve the contents of a coercion hole, if it is filled
    unpackCoercionHole_maybe :: CoercionHole -> TcM (Maybe Coercion)
    unpackCoercionHole_maybe (CoercionHole _ ref) = readTcRef ref

    -- | Check that a coercion is appropriate for filling a hole. (The hole
    -- itself is needed only for printing. NB: This must be /lazy/ in the coercion,
    -- as it's used in TcHsSyn in the presence of knots.
    -- 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 :: Coercion -> CoercionHole -> Role -> Type -> Type -> TcM Coercion
    checkCoercionHole co h r t1 t2
    -- co is already zonked, but t1 and t2 might not be
      | debugIsOn
      = do { t1 <- zonkTcType t1
           ; t2 <- zonkTcType t2
           ; let (Pair _t1 _t2, _role) = coercionKindRole co
           ; return $
             ASSERT2( t1 `eqType` _t1 && t2 `eqType` _t2 && r == _role
                    , (text "Bad coercion hole" <+>
                       ppr h <> colon <+> vcat [ ppr _t1, ppr _t2, ppr _role
                                               , ppr co, ppr t1, ppr t2
                                               , ppr r ]) )
             co }
      | otherwise
      = return co

    {-
    ************************************************************************
    *
        Expected types
    *
    ************************************************************************

    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.
    And, because there is only one branch of the case, we won't trigger
    Note [Case branches must never infer a non-tau type] of TcMatches.
    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. This is triggered in
    test gadt/gadt-escape1.

    -}

    -- actual data definition is in TcType

    -- | Make an 'ExpType' suitable for inferring a type of kind * or #.
    newOpenInferExpType :: TcM ExpType
    newOpenInferExpType
      = do { rr <- newFlexiTyVarTy runtimeRepTy
           ; u <- newUnique
           ; tclvl <- getTcLevel
           ; let ki = tYPE rr
           ; traceTc "newOpenInferExpType" (ppr u <+> dcolon <+> ppr ki)
           ; ref <- newMutVar Nothing
           ; return (Infer u tclvl ki 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 (Check ty)        = return (Just ty)
    readExpType_maybe (Infer _ _ _ ref) = readMutVar ref

    -- | Extract a type out of an ExpType. Otherwise, panics.
    readExpType :: ExpType -> TcM TcType
    readExpType exp_ty
      = do { mb_ty <- readExpType_maybe exp_ty
           ; case mb_ty of
               Just ty -> return ty
               Nothing -> pprPanic "Unknown expected type" (ppr exp_ty) }

    -- | Write into an 'ExpType'. It must be an 'Infer'.
    writeExpType :: ExpType -> TcType -> TcM ()
    writeExpType (Infer u tc_lvl ki ref) ty
      | debugIsOn
      = do { ki1 <- zonkTcType (typeKind ty)
           ; ki2 <- zonkTcType ki
           ; MASSERT2( ki1 `eqType` ki2, ppr ki1 $$ ppr ki2 $$ ppr u )
           ; lvl_now <- getTcLevel
           ; MASSERT2( tc_lvl == lvl_now, ppr u $$ ppr tc_lvl $$ ppr lvl_now )
           ; cts <- readTcRef ref
           ; case cts of
               Just already_there -> pprPanic "writeExpType"
                                       (vcat [ ppr u
                                             , ppr ty
                                             , ppr already_there ])
               Nothing -> write }
      | otherwise
      = write
      where
        write = do { traceTc "Filling ExpType" $
                       ppr u <+> text ":=" <+> ppr ty
                   ; writeTcRef ref (Just ty) }
    writeExpType (Check ty1) ty2 = pprPanic "writeExpType" (ppr ty1 $$ ppr ty2)

    -- | Returns the expected type when in checking mode.
    checkingExpType_maybe :: ExpType -> Maybe TcType
    checkingExpType_maybe (Check ty) = Just ty
    checkingExpType_maybe _          = Nothing

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

    tauifyExpType :: ExpType -> TcM ExpType
    -- ^ Turn a (Infer hole) type into a (Check alpha),
    -- where alpha is a fresh unificaiton variable
    tauifyExpType exp_ty = do { ty <- expTypeToType exp_ty
                              ; return (Check ty) }

    -- | Extracts the expected type if there is one, or generates a new
    -- TauTv if there isn't.
    expTypeToType :: ExpType -> TcM TcType
    expTypeToType (Check ty) = return ty
    expTypeToType (Infer u tc_lvl ki ref)
      = do { uniq <- newUnique
           ; tv_ref <- newMutVar Flexi
           ; let details = MetaTv { mtv_info = TauTv
                                  , mtv_ref  = tv_ref
                                  , mtv_tclvl = tc_lvl }
                 name   = mkMetaTyVarName uniq (fsLit "t")
                 tau_tv = mkTcTyVar name ki details
                 tau    = mkTyVarTy tau_tv
                 -- can't use newFlexiTyVarTy because we need to set the tc_lvl
                 -- See also Note [TcLevel of ExpType]

           ; writeMutVar ref (Just tau)
           ; traceTc "Forcing ExpType to be monomorphic:"
                     (ppr u <+> dcolon <+> ppr ki <+> text ":=" <+> ppr tau)
           ; return tau }

    {-
    ************************************************************************
    *                                                                      *
            SkolemTvs (immutable)
    *                                                                      *
    ************************************************************************
    -}

    tcInstType :: ([TyVar] -> TcM (TCvSubst, [TcTyVar]))
                       -- ^ How to instantiate the type variables
               -> TcType                                  -- ^ Type to instantiate
               -> TcM ([TcTyVar], TcThetaType, TcType)  -- ^ Result
                    -- (type vars, preds (incl equalities), rho)
    tcInstType inst_tyvars ty
      = case tcSplitForAllTys ty of
            ([],    rho) -> let     -- There may be overloading despite no type variables;
                                    --      (?x :: Int) => Int -> Int
                                    (theta, tau) = tcSplitPhiTy rho
                                in
                                return ([], theta, tau)

            (tyvars, rho) -> do { (subst, tyvars') <- inst_tyvars tyvars
                                ; let (theta, tau) = tcSplitPhiTy (substTyAddInScope subst rho)
                                ; return (tyvars', theta, tau) }

    tcSkolDFunType :: Type -> TcM ([TcTyVar], TcThetaType, TcType)
    -- Instantiate a type signature with skolem constants.
    -- We could give them fresh names, but no need to do so
    tcSkolDFunType ty = tcInstType tcInstSuperSkolTyVars ty

    tcSuperSkolTyVars :: [TyVar] -> (TCvSubst, [TcTyVar])
    -- Make skolem constants, but do *not* give them new names, as above
    -- Moreover, make them "super skolems"; see comments with superSkolemTv
    -- see Note [Kind substitution when instantiating]
    -- Precondition: tyvars should be ordered by scoping
    tcSuperSkolTyVars = mapAccumL tcSuperSkolTyVar emptyTCvSubst

    tcSuperSkolTyVar :: TCvSubst -> TyVar -> (TCvSubst, TcTyVar)
    tcSuperSkolTyVar subst tv
      = (extendTvSubstWithClone subst tv new_tv, new_tv)
      where
        kind   = substTyUnchecked subst (tyVarKind tv)
        new_tv = mkTcTyVar (tyVarName tv) kind superSkolemTv

    -- Wrappers
    -- we need to be able to do this from outside the TcM monad:
    tcInstSkolTyVarsLoc :: SrcSpan -> [TyVar] -> TcRnIf gbl lcl (TCvSubst, [TcTyVar])
    tcInstSkolTyVarsLoc loc = instSkolTyCoVars (mkTcSkolTyVar loc False)

    tcInstSkolTyVars :: [TyVar] -> TcM (TCvSubst, [TcTyVar])
    tcInstSkolTyVars = tcInstSkolTyVars' False emptyTCvSubst

    tcInstSuperSkolTyVars :: [TyVar] -> TcM (TCvSubst, [TcTyVar])
    tcInstSuperSkolTyVars = tcInstSuperSkolTyVarsX emptyTCvSubst

    tcInstSuperSkolTyVarsX :: TCvSubst -> [TyVar] -> TcM (TCvSubst, [TcTyVar])
    tcInstSuperSkolTyVarsX subst = tcInstSkolTyVars' True subst

    tcInstSkolTyVars' :: Bool -> TCvSubst -> [TyVar] -> TcM (TCvSubst, [TcTyVar])
    -- Precondition: tyvars should be ordered (kind vars first)
    -- see Note [Kind substitution when instantiating]
    -- Get the location from the monad; this is a complete freshening operation
    tcInstSkolTyVars' overlappable subst tvs
      = do { loc <- getSrcSpanM
           ; instSkolTyCoVarsX (mkTcSkolTyVar loc overlappable) subst tvs }

    mkTcSkolTyVar :: SrcSpan -> Bool -> Unique -> Name -> Kind -> TcTyVar
    mkTcSkolTyVar loc overlappable uniq old_name kind
      = mkTcTyVar (mkInternalName uniq (getOccName old_name) loc)
                  kind
                  (SkolemTv overlappable)

    tcInstSigTyVarsLoc :: SrcSpan -> [TyVar]
                       -> TcRnIf gbl lcl (TCvSubst, [TcTyVar])
    -- We specify the location
    tcInstSigTyVarsLoc loc = instSkolTyCoVars (mkTcSkolTyVar loc False)

    tcInstSigTyVars :: [TyVar] -> TcRnIf gbl lcl (TCvSubst, [TcTyVar])
    -- Get the location from the TyVar itself, not the monad
    tcInstSigTyVars
      = instSkolTyCoVars mk_tv
      where
        mk_tv uniq old_name kind
           = mkTcTyVar (setNameUnique old_name uniq) kind (SkolemTv False)

    tcInstSkolType :: TcType -> TcM ([TcTyVar], TcThetaType, TcType)
    -- Instantiate a type with fresh skolem constants
    -- Binding location comes from the monad
    tcInstSkolType ty = tcInstType tcInstSkolTyVars ty

    ------------------
    freshenTyVarBndrs :: [TyVar] -> TcRnIf gbl lcl (TCvSubst, [TyVar])
    -- ^ Give fresh uniques to a bunch of TyVars, but they stay
    --   as TyVars, rather than becoming TcTyVars
    -- Used in FamInst.newFamInst, and Inst.newClsInst
    freshenTyVarBndrs = instSkolTyCoVars mk_tv
      where
        mk_tv uniq old_name kind = mkTyVar (setNameUnique old_name uniq) kind

    freshenCoVarBndrsX :: TCvSubst -> [CoVar] -> TcRnIf gbl lcl (TCvSubst, [CoVar])
    -- ^ Give fresh uniques to a bunch of CoVars
    -- Used in FamInst.newFamInst
    freshenCoVarBndrsX subst = instSkolTyCoVarsX mk_cv subst
      where
        mk_cv uniq old_name kind = mkCoVar (setNameUnique old_name uniq) kind

    ------------------
    instSkolTyCoVars :: (Unique -> Name -> Kind -> TyCoVar)
                     -> [TyVar] -> TcRnIf gbl lcl (TCvSubst, [TyCoVar])
    instSkolTyCoVars mk_tcv = instSkolTyCoVarsX mk_tcv emptyTCvSubst

    instSkolTyCoVarsX :: (Unique -> Name -> Kind -> TyCoVar)
                      -> TCvSubst -> [TyCoVar] -> TcRnIf gbl lcl (TCvSubst, [TyCoVar])
    instSkolTyCoVarsX mk_tcv = mapAccumLM (instSkolTyCoVarX mk_tcv)

    instSkolTyCoVarX :: (Unique -> Name -> Kind -> TyCoVar)
                     -> TCvSubst -> TyCoVar -> TcRnIf gbl lcl (TCvSubst, TyCoVar)
    instSkolTyCoVarX mk_tcv subst tycovar
      = do  { uniq <- newUnique  -- using a new unique is critical. See
                                 -- Note [Skolems in zonkSyntaxExpr] in TcHsSyn
            ; let new_tcv = mk_tcv uniq old_name kind
                  subst1 | isTyVar new_tcv
                         = extendTvSubstWithClone subst tycovar new_tcv
                         | otherwise
                         = extendCvSubstWithClone subst tycovar new_tcv
            ; return (subst1, new_tcv) }
      where
        old_name = tyVarName tycovar
        kind     = substTyUnchecked subst (tyVarKind tycovar)

    newFskTyVar :: TcType -> TcM TcTyVar
    newFskTyVar fam_ty
      = do { uniq <- newUnique
           ; let name = mkSysTvName uniq (fsLit "fsk")
           ; return (mkTcTyVar name (typeKind fam_ty) (FlatSkol fam_ty)) }
    {-
    Note [Kind substitution when instantiating]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    When we instantiate a bunch of kind and type variables, first we
    expect them to be topologically sorted.
    Then we have to instantiate the kind variables, build a substitution
    from old variables to the new variables, then instantiate the type
    variables substituting the original kind.

    Exemple: If we want to instantiate
      [(k1 :: *), (k2 :: *), (a :: k1 -> k2), (b :: k1)]
    we want
      [(?k1 :: *), (?k2 :: *), (?a :: ?k1 -> ?k2), (?b :: ?k1)]
    instead of the buggous
      [(?k1 :: *), (?k2 :: *), (?a :: k1 -> k2), (?b :: k1)]

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

    mkMetaTyVarName :: Unique -> FastString -> Name
    -- Makes a /System/ Name, which is eagerly eliminated by
    -- the unifier; see TcUnify.nicer_to_update_tv1, and
    -- TcCanonical.canEqTyVarTyVar (nicer_to_update_tv2)
    mkMetaTyVarName uniq str = mkSysTvName uniq str

    newSigTyVar :: Name -> Kind -> TcM TcTyVar
    newSigTyVar name kind
      = do { details <- newMetaDetails SigTv
           ; return (mkTcTyVar name kind details) }

    newFmvTyVar :: TcType -> TcM TcTyVar
    -- Very like newMetaTyVar, except sets mtv_tclvl to one less
    -- so that the fmv is untouchable.
    newFmvTyVar fam_ty
      = do { uniq <- newUnique
           ; ref  <- newMutVar Flexi
           ; cur_lvl <- getTcLevel
           ; let details = MetaTv { mtv_info  = FlatMetaTv
                                  , mtv_ref   = ref
                                  , mtv_tclvl = fmvTcLevel cur_lvl }
                 name = mkMetaTyVarName uniq (fsLit "s")
           ; return (mkTcTyVar name (typeKind fam_ty) details) }

    newMetaDetails :: MetaInfo -> TcM TcTyVarDetails
    newMetaDetails info
      = do { ref <- newMutVar Flexi
           ; tclvl <- getTcLevel
           ; return (MetaTv { mtv_info = info
                            , mtv_ref = ref
                            , mtv_tclvl = tclvl }) }

    cloneMetaTyVar :: TcTyVar -> TcM TcTyVar
    cloneMetaTyVar tv
      = ASSERT( isTcTyVar tv )
        do  { uniq <- newUnique
            ; ref  <- newMutVar Flexi
            ; let name'    = setNameUnique (tyVarName tv) uniq
                  details' = case tcTyVarDetails tv of
                               details@(MetaTv {}) -> details { mtv_ref = ref }
                               _ -> pprPanic "cloneMetaTyVar" (ppr tv)
            ; return (mkTcTyVar name' (tyVarKind tv) details') }

    -- Works for both type and kind variables
    readMetaTyVar :: TyVar -> TcM MetaDetails
    readMetaTyVar tyvar = ASSERT2( isMetaTyVar tyvar, ppr tyvar )
                          readMutVar (metaTvRef tyvar)

    isFilledMetaTyVar :: TyVar -> TcM Bool
    -- True of a filled-in (Indirect) meta type variable
    isFilledMetaTyVar tv
      | MetaTv { mtv_ref = ref } <- tcTyVarDetails tv
      = do  { details <- readMutVar ref
            ; return (isIndirect details) }
      | otherwise = return False

    isUnfilledMetaTyVar :: TyVar -> TcM Bool
    -- True of a un-filled-in (Flexi) meta type variable
    isUnfilledMetaTyVar tv
      | MetaTv { mtv_ref = ref } <- tcTyVarDetails tv
      = do  { details <- readMutVar ref
            ; return (isFlexi details) }
      | otherwise = return False

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

    writeMetaTyVar tyvar ty
      | not debugIsOn
      = writeMetaTyVarRef tyvar (metaTvRef tyvar) ty

    -- Everything from here on only happens if DEBUG is on
      | not (isTcTyVar tyvar)
      = WARN( True, text "Writing to non-tc tyvar" <+> ppr tyvar )
        return ()

      | MetaTv { mtv_ref = ref } <- tcTyVarDetails tyvar
      = writeMetaTyVarRef tyvar ref ty

      | otherwise
      = WARN( True, text "Writing to non-meta tyvar" <+> ppr tyvar )
        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 ref ty
      | not debugIsOn
      = do { traceTc "writeMetaTyVar" (ppr tyvar <+> dcolon <+> ppr (tyVarKind tyvar)
                                       <+> text ":=" <+> ppr ty)
           ; writeTcRef ref (Indirect ty) }

    -- Everything from here on only happens if DEBUG is on
      | otherwise
      = do { meta_details <- readMutVar ref;
           -- Zonk kinds to allow the error check to work
           ; zonked_tv_kind <- zonkTcType tv_kind
           ; zonked_ty_kind <- zonkTcType ty_kind

           -- Check for double updates
           ; ASSERT2( isFlexi meta_details,
                      hang (text "Double update of meta tyvar")
                       2 (ppr tyvar $$ ppr meta_details) )

             traceTc "writeMetaTyVar" (ppr tyvar <+> text ":=" <+> ppr ty)
           ; writeMutVar ref (Indirect ty)
           ; when (   not (isPredTy tv_kind)
                        -- Don't check kinds for updates to coercion variables
                   && not (zonked_ty_kind `tcEqKind` zonked_tv_kind))
           $ WARN( True, hang (text "Ill-kinded update to meta tyvar")
                            2 (    ppr tyvar <+> text "::" <+> (ppr tv_kind $$ ppr zonked_tv_kind)
                               <+> text ":="
                               <+> ppr ty    <+> text "::" <+> (ppr ty_kind $$ ppr zonked_ty_kind) ) )
             (return ()) }
      where
        tv_kind = tyVarKind tyvar
        ty_kind = typeKind ty

    {-
    % Generating fresh variables for pattern match check
    -}

    -- UNINSTANTIATED VERSION OF tcInstSkolTyCoVars
    genInstSkolTyVarsX :: SrcSpan -> TCvSubst -> [TyVar]
                       -> TcRnIf gbl lcl (TCvSubst, [TcTyVar])
    -- Precondition: tyvars should be scoping-ordered
    -- see Note [Kind substitution when instantiating]
    -- Get the location from the monad; this is a complete freshening operation
    genInstSkolTyVarsX loc subst tvs = instSkolTyCoVarsX (mkTcSkolTyVar loc False) subst tvs

    {-
    ************************************************************************
    *                                                                      *
            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!
    -}

    newAnonMetaTyVar :: MetaInfo -> Kind -> TcM TcTyVar
    -- Make a new meta tyvar out of thin air
    newAnonMetaTyVar meta_info kind
      = do  { uniq <- newUnique
            ; let name = mkMetaTyVarName uniq s
                  s = case meta_info of
                            TauTv       -> fsLit "t"
                            FlatMetaTv  -> fsLit "fmv"
                            SigTv       -> fsLit "a"
            ; details <- newMetaDetails meta_info
            ; return (mkTcTyVar name kind details) }

    newFlexiTyVar :: Kind -> TcM TcTyVar
    newFlexiTyVar kind = newAnonMetaTyVar TauTv kind

    newFlexiTyVarTy  :: Kind -> TcM TcType
    newFlexiTyVarTy kind = do
        tc_tyvar <- newFlexiTyVar kind
        return (mkTyVarTy tc_tyvar)

    newFlexiTyVarTys :: Int -> Kind -> TcM [TcType]
    newFlexiTyVarTys n kind = mapM newFlexiTyVarTy (nOfThem n kind)

    -- | Create a tyvar that can be a lifted or unlifted type.
    newOpenFlexiTyVarTy :: TcM TcType
    newOpenFlexiTyVarTy
      = do { rr <- newFlexiTyVarTy runtimeRepTy
           ; newFlexiTyVarTy (tYPE rr) }

    newMetaSigTyVars :: [TyVar] -> TcM (TCvSubst, [TcTyVar])
    newMetaSigTyVars = mapAccumLM newMetaSigTyVarX emptyTCvSubst

    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 = mapAccumLM newMetaTyVarX 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.

    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 subst tyvar = new_meta_tv_x TauTv subst tyvar

    newMetaSigTyVarX :: TCvSubst -> TyVar -> TcM (TCvSubst, TcTyVar)
    -- Just like newMetaTyVarX, but make a SigTv
    newMetaSigTyVarX subst tyvar = new_meta_tv_x SigTv subst tyvar

    new_meta_tv_x :: MetaInfo -> TCvSubst -> TyVar -> TcM (TCvSubst, TcTyVar)
    new_meta_tv_x info subst tyvar
      = do  { uniq <- newUnique
            ; details <- newMetaDetails info
            ; let name   = mkSystemName uniq (getOccName tyvar)
                           -- See Note [Name of an instantiated type variable]
                  kind   = substTyUnchecked subst (tyVarKind tyvar)
                  new_tv = mkTcTyVar name kind details
                  subst1 = extendTvSubstWithClone subst tyvar new_tv
            ; return (subst1, new_tv) }

    {- 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.

    ************************************************************************
    *                                                                      *
                 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) and
      1. Zonks them and remove globals and covars
      2. Extends kvs1 with free kind vars in the kinds of tvs (removing globals)
      3. Calls zonkQuantifiedTyVar on each

    Step (2) is often unimportant, because the kind variable is often
    also free in the type.  Eg
         Typeable k (a::k)
    has free vars {k,a}.  But the type (see Trac #7916)
        (f::k->*) (a::k)
    has free vars {f,a}, but we must add 'k' as well! Hence step (3).

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

    Note that this function can accept covars, but will never return them.
    This is because we never want to infer a quantified covar!
    -}

    quantifyTyVars, quantifyZonkedTyVars
      :: TcTyCoVarSet   -- global tvs
      -> Pair TcTyCoVarSet    -- dependent tvs       We only distinguish
                              -- nondependent tvs    between these for
                              --                     -XNoPolyKinds
      -> 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.

    -- The zonked variant assumes everything is already zonked.

    quantifyTyVars gbl_tvs (Pair dep_tkvs nondep_tkvs)
      = do { dep_tkvs    <- zonkTyCoVarsAndFV dep_tkvs
           ; nondep_tkvs <- (`minusVarSet` dep_tkvs) <$>
                            zonkTyCoVarsAndFV nondep_tkvs
           ; gbl_tvs     <- zonkTyCoVarsAndFV gbl_tvs
           ; quantifyZonkedTyVars gbl_tvs (Pair dep_tkvs nondep_tkvs) }

    quantifyZonkedTyVars gbl_tvs (Pair dep_tkvs nondep_tkvs)
      = do { let all_cvs    = filterVarSet isCoVar $
                              dep_tkvs `unionVarSet` nondep_tkvs `minusVarSet` gbl_tvs
                 dep_kvs    = varSetElemsWellScoped $
                              dep_tkvs `minusVarSet` gbl_tvs
                                       `minusVarSet` (closeOverKinds all_cvs)
                                 -- remove any tvs that a covar depends on

                 nondep_tvs = varSetElemsWellScoped $
                              nondep_tkvs `minusVarSet` gbl_tvs
                               -- no worry about dependent cvs here, as these vars
                               -- are non-dependent

                              -- NB kinds of tvs are zonked by zonkTyCoVarsAndFV

                 -- In the non-PolyKinds case, default the kind variables
                 -- to *, and zonk the tyvars as usual.  Notice that this
                 -- may make quantifyTyVars return a shorter list
                 -- than it was passed, but that's ok
           ; poly_kinds <- xoptM LangExt.PolyKinds
           ; dep_vars2 <- if poly_kinds
                          then return dep_kvs
                          else do { let (meta_kvs, skolem_kvs) = partition is_meta dep_kvs
                                        is_meta kv = isTcTyVar kv && isMetaTyVar kv

                                  ; mapM_ defaultKindVar meta_kvs
                                  ; return skolem_kvs }  -- should be empty

           ; let quant_vars = dep_vars2 ++ nondep_tvs

           ; traceTc "quantifyTyVars"
               (vcat [ text "globals:" <+> ppr gbl_tvs
                     , text "nondep:" <+> ppr nondep_tvs
                     , text "dep:" <+> ppr dep_kvs
                     , text "dep2:" <+> ppr dep_vars2
                     , text "quant_vars:" <+> ppr quant_vars ])

           ; mapMaybeM zonk_quant quant_vars }
               -- Because of the order, any kind variables
               -- mentioned in the kinds of the type variables refer to
               -- the now-quantified versions
      where
        zonk_quant tkv
          | isTcTyVar tkv = zonkQuantifiedTyVar tkv
          | otherwise     = return $ Just tkv
          -- For associated types, we have the class variables
          -- in scope, and they are TyVars not TcTyVars

    zonkQuantifiedTyVar :: TcTyVar -> TcM (Maybe TcTyVar)
    -- The quantified type variables often include meta type variables
    -- we want to freeze them into ordinary type variables, and
    -- default their kind (e.g. from TYPE v to TYPE Lifted)
    -- 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.
    --
    -- This returns a tyvar if it should be quantified over; otherwise,
    -- it returns Nothing. Nothing is
    -- returned only if zonkQuantifiedTyVar is passed a RuntimeRep meta-tyvar,
    -- in order to default to PtrRepLifted.
    zonkQuantifiedTyVar tv = left_only `liftM` zonkQuantifiedTyVarOrType tv
      where left_only :: Either a b -> Maybe a
            left_only (Left x) =  Just x
            left_only (Right _) = Nothing

    -- | Like zonkQuantifiedTyVar, but if zonking reveals that the tyvar
    -- should become a type (when defaulting a RuntimeRep var to PtrRepLifted), it
    -- returns the type instead.
    zonkQuantifiedTyVarOrType :: TcTyVar -> TcM (Either TcTyVar TcType)
    zonkQuantifiedTyVarOrType tv
      = case tcTyVarDetails tv of
          SkolemTv {} -> do { kind <- zonkTcType (tyVarKind tv)
                            ; return $ Left $ setTyVarKind tv kind }
            -- It might be a skolem type variable,
            -- for example from a user type signature

          MetaTv { mtv_ref = ref } ->
              do when debugIsOn $ do
                     -- [Sept 04] Check for non-empty.
                     -- See note [Silly Type Synonym]
                     cts <- readMutVar ref
                     case cts of
                         Flexi -> return ()
                         Indirect ty -> WARN( True, ppr tv $$ ppr ty )
                                        return ()
                 if isRuntimeRepVar tv
                 then do { writeMetaTyVar tv ptrRepLiftedTy
                         ; return (Right ptrRepLiftedTy) }
                 else Left `liftM` skolemiseUnboundMetaTyVar tv vanillaSkolemTv
          _other -> pprPanic "zonkQuantifiedTyVar" (ppr tv) -- FlatSkol, RuntimeUnk

    -- | Take an (unconstrained) meta tyvar and default it. Works only on
    -- vars of type RuntimeRep and of type *. For other kinds, it issues
    -- an error. See Note [Defaulting with -XNoPolyKinds]
    defaultKindVar :: TcTyVar -> TcM Kind
    defaultKindVar kv
      | ASSERT( isMetaTyVar kv )
        isRuntimeRepVar kv
      = writeMetaTyVar kv ptrRepLiftedTy >> return ptrRepLiftedTy
      | isStarKind (tyVarKind kv)
      = writeMetaTyVar kv liftedTypeKind >> return liftedTypeKind
      | otherwise
      = do { addErr (vcat [ text "Cannot default kind variable" <+> quotes (ppr kv')
                          , text "of kind:" <+> ppr (tyVarKind kv')
                          , text "Perhaps enable PolyKinds or add a kind signature" ])
           ; return (mkTyVarTy kv) }
      where
        (_, kv') = tidyOpenTyCoVar emptyTidyEnv kv

    skolemiseUnboundMetaTyVar :: TcTyVar -> TcTyVarDetails -> 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 TyVar, not a regular TyVar
    --   See Note [Zonking to Skolem]
    skolemiseUnboundMetaTyVar tv details
      = ASSERT2( isMetaTyVar tv, ppr tv )
        do  { span <- getSrcSpanM    -- Get the location from "here"
                                     -- ie where we are generalising
            ; kind <- zonkTcType (tyVarKind tv)
            ; let uniq        = getUnique tv
                    -- NB: Use same Unique as original tyvar. This is
                    -- important for TcHsType.splitTelescopeTvs to work properly

                  tv_name     = getOccName tv
                  final_name  = mkInternalName uniq tv_name span
                  final_tv    = mkTcTyVar final_name kind details

            ; traceTc "Skolemising" (ppr tv <+> text ":=" <+> ppr final_tv)
            ; writeMetaTyVar tv (mkTyVarTy final_tv)
            ; return final_tv }

    {-
    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 instantiate 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 TcType. 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 TcSimplify.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 TcRnDriver.tcRnSrcDecls calling TcSimplify.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.  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 [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 ()

    ************************************************************************
    *                                                                      *
                  Zonking types
    *                                                                      *
    ************************************************************************

    -}

    -- | @tcGetGlobalTyCoVars@ returns a fully-zonked set of *scoped* tyvars free in
    -- the environment. To improve subsequent calls to the same function it writes
    -- the zonked set back into the environment. Note that this returns all
    -- variables free in anything (term-level or type-level) in scope. We thus
    -- don't have to worry about clashes with things that are not in scope, because
    -- if they are reachable, then they'll be returned here.
    tcGetGlobalTyCoVars :: TcM TcTyVarSet
    tcGetGlobalTyCoVars
      = do { (TcLclEnv {tcl_tyvars = gtv_var}) <- getLclEnv
           ; gbl_tvs  <- readMutVar gtv_var
           ; gbl_tvs' <- zonkTyCoVarsAndFV gbl_tvs
           ; writeMutVar gtv_var gbl_tvs'
           ; return gbl_tvs' }

    -- | Zonk a type without using the smart constructors; the result type
    -- is available for inspection within the type-checking knot.
    zonkTcTypeInKnot :: TcType -> TcM TcType
    zonkTcTypeInKnot = mapType (zonkTcTypeMapper { tcm_smart = False }) ()

    zonkTcTypeAndFV :: TcType -> TcM TyCoVarSet
    -- 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!
    -- NB: This might be called from within the knot, so don't use
    -- smart constructors. See Note [Zonking within the knot] in TcHsType
    zonkTcTypeAndFV ty
      = tyCoVarsOfType <$> zonkTcTypeInKnot ty

    -- | Zonk a type and call 'splitDepVarsOfType' on it.
    -- Works within the knot.
    zonkTcTypeAndSplitDepVars :: TcType -> TcM (Pair TyCoVarSet)
    zonkTcTypeAndSplitDepVars ty
      = splitDepVarsOfType <$> zonkTcTypeInKnot ty

    zonkTcTypesAndSplitDepVars :: [TcType] -> TcM (Pair TyCoVarSet)
    zonkTcTypesAndSplitDepVars tys
      = splitDepVarsOfTypes <$> mapM zonkTcTypeInKnot tys

    zonkTyCoVar :: TyCoVar -> TcM TcType
    -- Works on TyVars and TcTyVars
    zonkTyCoVar tv | isTcTyVar tv = zonkTcTyVar tv
                   | isTyVar   tv = mkTyVarTy <$> zonkTyCoVarKind tv
                   | otherwise    = ASSERT2( isCoVar tv, ppr tv )
                                    mkCoercionTy . mkCoVarCo <$> zonkTyCoVarKind tv
       -- Hackily, when typechecking type and class decls
       -- we have TyVars in scopeadded (only) in
       -- TcHsType.tcTyClTyVars, but it seems
       -- painful to make them into TcTyVars there

    zonkTyCoVarsAndFV :: TyCoVarSet -> TcM TyCoVarSet
    zonkTyCoVarsAndFV tycovars = tyCoVarsOfTypes <$> mapM zonkTyCoVar (varSetElems tycovars)

    zonkTcTyVars :: [TcTyVar] -> TcM [TcType]
    zonkTcTyVars tyvars = mapM zonkTcTyVar tyvars

    -----------------  Types
    zonkTyCoVarKind :: TyCoVar -> TcM TyCoVar
    zonkTyCoVarKind tv = do { kind' <- zonkTcType (tyVarKind tv)
                            ; return (setTyVarKind tv kind') }

    zonkTcTypes :: [TcType] -> TcM [TcType]
    zonkTcTypes tys = mapM zonkTcType tys

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

    zonkImplication :: Implication -> TcM Implication
    zonkImplication implic@(Implic { ic_skols  = skols
                                   , ic_given  = given
                                   , ic_wanted = wanted
                                   , ic_info   = info })
      = do { skols'  <- mapM zonkTcTyCoVarBndr skols  -- Need to zonk their kinds!
                                                      -- as Trac #7230 showed
           ; given'  <- mapM zonkEvVar given
           ; info'   <- zonkSkolemInfo info
           ; wanted' <- zonkWCRec wanted
           ; return (implic { ic_skols  = skols'
                            , ic_given  = given'
                            , ic_wanted = wanted'
                            , ic_info   = info' }) }

    zonkEvVar :: EvVar -> TcM EvVar
    zonkEvVar var = do { ty' <- zonkTcType (varType var)
                       ; return (setVarType var ty') }

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

    zonkWCRec :: WantedConstraints -> TcM WantedConstraints
    zonkWCRec (WC { wc_simple = simple, wc_impl = implic, wc_insol = insol })
      = do { simple' <- zonkSimples simple
           ; implic' <- mapBagM zonkImplication implic
           ; insol'  <- zonkSimples insol
           ; return (WC { wc_simple = simple', wc_impl = implic', wc_insol = insol' }) }

    zonkSimples :: Cts -> TcM Cts
    zonkSimples cts = do { cts' <- mapBagM zonkCt' cts
                         ; traceTc "zonkSimples done:" (ppr cts')
                         ; return cts' }

    zonkCt' :: Ct -> TcM Ct
    zonkCt' ct = zonkCt ct

    zonkCt :: Ct -> TcM Ct
    zonkCt ct@(CHoleCan { cc_ev = ev })
      = do { ev' <- zonkCtEvidence ev
           ; return $ ct { cc_ev = ev' } }
    zonkCt ct
      = do { fl' <- zonkCtEvidence (cc_ev ct)
           ; return (mkNonCanonical fl') }

    zonkCtEvidence :: CtEvidence -> TcM CtEvidence
    zonkCtEvidence ctev@(CtGiven { ctev_pred = pred })
      = do { pred' <- zonkTcType pred
           ; return (ctev { ctev_pred = pred'}) }
    zonkCtEvidence ctev@(CtWanted { ctev_pred = pred, ctev_dest = dest })
      = do { pred' <- zonkTcType pred
           ; let dest' = case dest of
                           EvVarDest ev -> EvVarDest $ setVarType ev pred'
                             -- necessary in simplifyInfer
                           HoleDest h   -> HoleDest h
           ; return (ctev { ctev_pred = pred', ctev_dest = dest' }) }
    zonkCtEvidence ctev@(CtDerived { ctev_pred = pred })
      = do { pred' <- zonkTcType pred
           ; return (ctev { ctev_pred = pred' }) }

    zonkSkolemInfo :: SkolemInfo -> TcM SkolemInfo
    zonkSkolemInfo (SigSkol cx ty)  = do { ty  <- readExpType ty
                                         ; ty' <- zonkTcType ty
                                         ; return (SigSkol cx (mkCheckExpType ty')) }
    zonkSkolemInfo (InferSkol ntys) = do { ntys' <- mapM do_one ntys
                                         ; return (InferSkol ntys') }
      where
        do_one (n, ty) = do { ty' <- zonkTcType ty; return (n, ty') }
    zonkSkolemInfo skol_info = return skol_info

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

    -}

    -- zonkId is used *during* typechecking just to zonk the Id's type
    zonkId :: TcId -> TcM TcId
    zonkId id
      = do { ty' <- zonkTcType (idType id)
           ; return (Id.setIdType id ty') }

    -- | A suitable TyCoMapper for zonking a type inside the knot, and
    -- before all metavars are filled in.
    zonkTcTypeMapper :: TyCoMapper () TcM
    zonkTcTypeMapper = TyCoMapper
      { tcm_smart = True
      , tcm_tyvar = const zonkTcTyVar
      , tcm_covar = const (\cv -> mkCoVarCo <$> zonkTyCoVarKind cv)
      , tcm_hole  = hole
      , tcm_tybinder = \_env tv _vis -> ((), ) <$> zonkTcTyCoVarBndr tv }
      where
        hole :: () -> CoercionHole -> Role -> Type -> Type
             -> TcM Coercion
        hole _ h r t1 t2
          = do { contents <- unpackCoercionHole_maybe h
               ; case contents of
                   Just co -> do { co <- zonkCo co
                                 ; checkCoercionHole co h r t1 t2 }
                   Nothing -> do { t1 <- zonkTcType t1
                                 ; t2 <- zonkTcType t2
                                 ; return $ mkHoleCo h r t1 t2 } }

    -- 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
    zonkTcType = mapType zonkTcTypeMapper ()

    -- | "Zonk" a coercion -- really, just zonk any types in the coercion
    zonkCo :: Coercion -> TcM Coercion
    zonkCo = mapCoercion zonkTcTypeMapper ()

    zonkTcTyCoVarBndr :: TcTyCoVar -> TcM TcTyCoVar
    -- A tyvar binder is never a unification variable (MetaTv),
    -- rather it is always a skolems.  BUT it may have a kind
    -- that has not yet been zonked, and may include kind
    -- unification variables.
    zonkTcTyCoVarBndr tyvar
        -- can't use isCoVar, because it looks at a TyCon. Argh.
      = ASSERT2( isImmutableTyVar tyvar || (not $ isTyVar tyvar), pprTvBndr tyvar )
        updateTyVarKindM zonkTcType tyvar

    -- | Zonk a TyBinder
    zonkTcTyBinder :: TcTyBinder -> TcM TcTyBinder
    zonkTcTyBinder (Anon ty)      = Anon <$> zonkTcType ty
    zonkTcTyBinder (Named tv vis) = Named <$> zonkTcTyCoVarBndr tv <*> pure vis

    zonkTcTyVar :: TcTyVar -> TcM TcType
    -- Simply look through all Flexis
    zonkTcTyVar tv
      | isTcTyVar tv
      = case tcTyVarDetails tv of
          SkolemTv {}   -> zonk_kind_and_return
          RuntimeUnk {} -> zonk_kind_and_return
          FlatSkol ty   -> zonkTcType ty
          MetaTv { mtv_ref = ref }
             -> do { cts <- readMutVar ref
                   ; case cts of
                        Flexi       -> zonk_kind_and_return
                        Indirect ty -> zonkTcType ty }

      | otherwise -- coercion variable
      = zonk_kind_and_return
      where
        zonk_kind_and_return = do { z_tv <- zonkTyCoVarKind tv
                                  ; return (mkTyVarTy z_tv) }

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

    zonkTidyTcType :: TidyEnv -> TcType -> TcM (TidyEnv, TcType)
    zonkTidyTcType env ty = do { ty' <- zonkTcType ty
                               ; return (tidyOpenType env ty') }

    -- | Make an 'ErrorThing' storing a type.
    mkTypeErrorThing :: TcType -> ErrorThing
    mkTypeErrorThing ty = ErrorThing ty (Just $ length $ snd $ repSplitAppTys ty)
                                     zonkTidyTcType
       -- NB: Use *rep*splitAppTys, else we get #11313

    -- | Make an 'ErrorThing' storing a type, with some extra args known about
    mkTypeErrorThingArgs :: TcType -> Int -> ErrorThing
    mkTypeErrorThingArgs ty num_args
      = ErrorThing ty (Just $ (length $ snd $ repSplitAppTys ty) + num_args)
                   zonkTidyTcType

    zonkTidyOrigin :: TidyEnv -> CtOrigin -> TcM (TidyEnv, CtOrigin)
    zonkTidyOrigin env (GivenOrigin skol_info)
      = do { skol_info1 <- zonkSkolemInfo skol_info
           ; let skol_info2 = tidySkolemInfo env skol_info1
           ; return (env, GivenOrigin skol_info2) }
    zonkTidyOrigin env orig@(TypeEqOrigin { uo_actual   = act
                                          , uo_expected = exp
                                          , uo_thing    = m_thing })
      = do { (env1, act') <- zonkTidyTcType env  act
           ; mb_exp <- readExpType_maybe exp  -- it really should be filled in.
                                              -- unless we're debugging.
           ; (env2, exp') <- case mb_exp of
               Just ty -> second mkCheckExpType <$> zonkTidyTcType env1 ty
               Nothing -> return (env1, exp)
           ; (env3, m_thing') <- zonkTidyErrorThing env2 m_thing
           ; return ( env3, orig { uo_actual   = act'
                                 , uo_expected = exp'
                                 , uo_thing    = m_thing' }) }
    zonkTidyOrigin env (KindEqOrigin ty1 m_ty2 orig t_or_k)
      = do { (env1, ty1')   <- zonkTidyTcType env  ty1
           ; (env2, m_ty2') <- case m_ty2 of
                                 Just ty2 -> second Just <$> zonkTidyTcType env1 ty2
                                 Nothing  -> return (env1, Nothing)
           ; (env3, orig')  <- zonkTidyOrigin env2 orig
           ; return (env3, KindEqOrigin ty1' m_ty2' orig' t_or_k) }
    zonkTidyOrigin env (FunDepOrigin1 p1 l1 p2 l2)
      = do { (env1, p1') <- zonkTidyTcType env  p1
           ; (env2, p2') <- zonkTidyTcType env1 p2
           ; return (env2, FunDepOrigin1 p1' l1 p2' l2) }
    zonkTidyOrigin env (FunDepOrigin2 p1 o1 p2 l2)
      = do { (env1, p1') <- zonkTidyTcType env  p1
           ; (env2, p2') <- zonkTidyTcType env1 p2
           ; (env3, o1') <- zonkTidyOrigin env2 o1
           ; return (env3, FunDepOrigin2 p1' o1' p2' l2) }
    zonkTidyOrigin env orig = return (env, orig)

    zonkTidyErrorThing :: TidyEnv -> Maybe ErrorThing
                       -> TcM (TidyEnv, Maybe ErrorThing)
    zonkTidyErrorThing env (Just (ErrorThing thing n_args zonker))
      = do { (env', thing') <- zonker env thing
           ; return (env', Just $ ErrorThing thing' n_args zonker) }
    zonkTidyErrorThing env Nothing
      = return (env, Nothing)

    ----------------
    tidyCt :: TidyEnv -> Ct -> Ct
    -- Used only in error reporting
    -- Also converts it to non-canonical
    tidyCt env ct
      = case ct of
         CHoleCan { cc_ev = ev }
           -> ct { cc_ev = tidy_ev env ev }
         _ -> mkNonCanonical (tidy_ev env (ctEvidence ct))
      where
        tidy_ev :: TidyEnv -> CtEvidence -> CtEvidence
         -- NB: we do not tidy the ctev_evar field because we don't
         --     show it in error messages
        tidy_ev env ctev@(CtGiven { ctev_pred = pred })
          = ctev { ctev_pred = tidyType env pred }
        tidy_ev env ctev@(CtWanted { ctev_pred = pred })
          = ctev { ctev_pred = tidyType env pred }
        tidy_ev env ctev@(CtDerived { ctev_pred = pred })
          = ctev { ctev_pred = tidyType env pred }

    ----------------
    tidyEvVar :: TidyEnv -> EvVar -> EvVar
    tidyEvVar env var = setVarType var (tidyType env (varType var))

    ----------------
    tidySkolemInfo :: TidyEnv -> SkolemInfo -> SkolemInfo
    tidySkolemInfo env (DerivSkol ty)       = DerivSkol (tidyType env ty)
    tidySkolemInfo env (SigSkol cx ty)      = SigSkol cx (mkCheckExpType $
                                                          tidyType env $
                                                          checkingExpType "tidySkolemInfo" ty)
    tidySkolemInfo env (InferSkol ids)      = InferSkol (mapSnd (tidyType env) ids)
    tidySkolemInfo env (UnifyForAllSkol ty) = UnifyForAllSkol (tidyType env ty)
    tidySkolemInfo _   info                 = info