typecheck/TcType.hs

    {-
    (c) The University of Glasgow 2006
    (c) The GRASP/AQUA Project, Glasgow University, 1992-1998

    \section[TcType]{Types used in the typechecker}

    This module provides the Type interface for front-end parts of the
    compiler.  These parts

            * treat "source types" as opaque:
                    newtypes, and predicates are meaningful.
            * look through usage types

    The "tc" prefix is for "TypeChecker", because the type checker
    is the principal client.
    -}

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

    module TcType (
      --------------------------------
      -- Types
      TcType, TcSigmaType, TcRhoType, TcTauType, TcPredType, TcThetaType,
      TcTyVar, TcTyVarSet, TcDTyVarSet, TcTyCoVarSet, TcDTyCoVarSet,
      TcKind, TcCoVar, TcTyCoVar, TcTyVarBinder, TcTyCon,

      ExpType(..), InferResult(..), ExpSigmaType, ExpRhoType, mkCheckExpType,

      SyntaxOpType(..), synKnownType, mkSynFunTys,

      -- TcLevel
      TcLevel(..), topTcLevel, pushTcLevel, isTopTcLevel,
      strictlyDeeperThan, sameDepthAs, fmvTcLevel,
      tcTypeLevel, tcTyVarLevel, maxTcLevel,

      --------------------------------
      -- MetaDetails
      UserTypeCtxt(..), pprUserTypeCtxt, isSigMaybe,
      TcTyVarDetails(..), pprTcTyVarDetails, vanillaSkolemTv, superSkolemTv,
      MetaDetails(Flexi, Indirect), MetaInfo(..),
      isImmutableTyVar, isSkolemTyVar, isMetaTyVar,  isMetaTyVarTy, isTyVarTy,
      isSigTyVar, isOverlappableTyVar,  isTyConableTyVar,
      isFskTyVar, isFmvTyVar, isFlattenTyVar,
      isAmbiguousTyVar, metaTyVarRef, metaTyVarInfo,
      isFlexi, isIndirect, isRuntimeUnkSkol,
      metaTyVarTcLevel, setMetaTyVarTcLevel, metaTyVarTcLevel_maybe,
      isTouchableMetaTyVar, isTouchableOrFmv,
      isFloatedTouchableMetaTyVar,

      --------------------------------
      -- Builders
      mkPhiTy, mkInfSigmaTy, mkSpecSigmaTy, mkSigmaTy,
      mkNakedTyConApp, mkNakedAppTys, mkNakedAppTy,
      mkNakedCastTy,

      --------------------------------
      -- Splitters
      -- These are important because they do not look through newtypes
      getTyVar,
      tcSplitForAllTy_maybe,
      tcSplitForAllTys, tcSplitPiTys, tcSplitForAllTyVarBndrs,
      tcSplitPhiTy, tcSplitPredFunTy_maybe,
      tcSplitFunTy_maybe, tcSplitFunTys, tcFunArgTy, tcFunResultTy, tcFunResultTyN,
      tcSplitFunTysN,
      tcSplitTyConApp, tcSplitTyConApp_maybe,
      tcRepSplitTyConApp_maybe, tcRepSplitTyConApp_maybe',
      tcTyConAppTyCon, tcTyConAppTyCon_maybe, tcTyConAppArgs,
      tcSplitAppTy_maybe, tcSplitAppTy, tcSplitAppTys, tcRepSplitAppTy_maybe,
      tcGetTyVar_maybe, tcGetTyVar, nextRole,
      tcSplitSigmaTy, tcSplitNestedSigmaTys, tcDeepSplitSigmaTy_maybe,

      ---------------------------------
      -- Predicates.
      -- Again, newtypes are opaque
      eqType, eqTypes, nonDetCmpType, nonDetCmpTypes, eqTypeX,
      pickyEqType, tcEqType, tcEqKind, tcEqTypeNoKindCheck, tcEqTypeVis,
      isSigmaTy, isRhoTy, isRhoExpTy, isOverloadedTy,
      isFloatingTy, isDoubleTy, isFloatTy, isIntTy, isWordTy, isStringTy,
      isIntegerTy, isBoolTy, isUnitTy, isCharTy, isCallStackTy, isCallStackPred,
      isTauTy, isTauTyCon, tcIsTyVarTy, tcIsForAllTy,
      isPredTy, isTyVarClassPred, isTyVarExposed, isInsolubleOccursCheck,
      checkValidClsArgs, hasTyVarHead,
      isRigidEqPred, isRigidTy,

      ---------------------------------
      -- Misc type manipulators

      deNoteType,
      orphNamesOfType, orphNamesOfCo,
      orphNamesOfTypes, orphNamesOfCoCon,
      getDFunTyKey,
      evVarPred_maybe, evVarPred,

      ---------------------------------
      -- Predicate types
      mkMinimalBySCs, transSuperClasses,
      pickQuantifiablePreds, pickCapturedPreds,
      immSuperClasses,
      isImprovementPred,

      -- * Finding type instances
      tcTyFamInsts,

      -- * Finding "exact" (non-dead) type variables
      exactTyCoVarsOfType, exactTyCoVarsOfTypes,
      splitDepVarsOfType, splitDepVarsOfTypes, TcDepVars(..), tcDepVarSet,
      anyRewritableTyVar,

      -- * Extracting bound variables
      allBoundVariables, allBoundVariabless,

      ---------------------------------
      -- Foreign import and export
      isFFIArgumentTy,     -- :: DynFlags -> Safety -> Type -> Bool
      isFFIImportResultTy, -- :: DynFlags -> Type -> Bool
      isFFIExportResultTy, -- :: Type -> Bool
      isFFIExternalTy,     -- :: Type -> Bool
      isFFIDynTy,          -- :: Type -> Type -> Bool
      isFFIPrimArgumentTy, -- :: DynFlags -> Type -> Bool
      isFFIPrimResultTy,   -- :: DynFlags -> Type -> Bool
      isFFILabelTy,        -- :: Type -> Bool
      isFFITy,             -- :: Type -> Bool
      isFunPtrTy,          -- :: Type -> Bool
      tcSplitIOType_maybe, -- :: Type -> Maybe Type

      --------------------------------
      -- Rexported from Kind
      Kind, typeKind,
      liftedTypeKind,
      constraintKind,
      isLiftedTypeKind, isUnliftedTypeKind, classifiesTypeWithValues,

      --------------------------------
      -- Rexported from Type
      Type, PredType, ThetaType, TyBinder, ArgFlag(..),

      mkForAllTy, mkForAllTys, mkInvForAllTys, mkSpecForAllTys, mkInvForAllTy,
      mkFunTy, mkFunTys,
      mkTyConApp, mkAppTy, mkAppTys,
      mkTyConTy, mkTyVarTy,
      mkTyVarTys,

      isClassPred, isEqPred, isNomEqPred, isIPPred,
      mkClassPred,
      isDictLikeTy,
      tcSplitDFunTy, tcSplitDFunHead, tcSplitMethodTy,
      isRuntimeRepVar, isKindLevPoly,
      isVisibleBinder, isInvisibleBinder,

      -- Type substitutions
      TCvSubst(..),         -- Representation visible to a few friends
      TvSubstEnv, emptyTCvSubst,
      zipTvSubst,
      mkTvSubstPrs, notElemTCvSubst, unionTCvSubst,
      getTvSubstEnv, setTvSubstEnv, getTCvInScope, extendTCvInScope,
      extendTCvInScopeList, extendTCvInScopeSet, extendTvSubstAndInScope,
      Type.lookupTyVar, Type.extendTCvSubst, Type.substTyVarBndr,
      Type.extendTvSubst,
      isInScope, mkTCvSubst, mkTvSubst, zipTyEnv, zipCoEnv,
      Type.substTy, substTys, substTyWith, substTyWithCoVars,
      substTyAddInScope,
      substTyUnchecked, substTysUnchecked, substThetaUnchecked,
      substTyWithUnchecked,
      substCoUnchecked, substCoWithUnchecked,
      substTheta,

      isUnliftedType,       -- Source types are always lifted
      isUnboxedTupleType,   -- Ditto
      isPrimitiveType,

      tcView, coreView,

      tyCoVarsOfType, tyCoVarsOfTypes, closeOverKinds,
      tyCoFVsOfType, tyCoFVsOfTypes,
      tyCoVarsOfTypeDSet, tyCoVarsOfTypesDSet, closeOverKindsDSet,
      tyCoVarsOfTypeList, tyCoVarsOfTypesList,
      noFreeVarsOfType,

      --------------------------------
      -- Transforming Types to TcTypes
      toTcType,    -- :: Type -> TcType
      toTcTypeBag, -- :: Bag EvVar -> Bag EvVar

      pprKind, pprParendKind, pprSigmaType,
      pprType, pprParendType, pprTypeApp, pprTyThingCategory, tyThingCategory,
      pprTheta, pprThetaArrowTy, pprClassPred,
      pprTvBndr, pprTvBndrs,

      TypeSize, sizeType, sizeTypes, toposortTyVars

      ) where

    #include "HsVersions.h"

    -- friends:
    import Kind
    import TyCoRep
    import Class
    import Var
    import ForeignCall
    import VarSet
    import Coercion
    import Type
    import RepType
    import TyCon

    -- others:
    import DynFlags
    import CoreFVs
    import Name -- hiding (varName)
                -- We use this to make dictionaries for type literals.
                -- Perhaps there's a better way to do this?
    import NameSet
    import VarEnv
    import PrelNames
    import TysWiredIn( coercibleClass, unitTyCon, unitTyConKey
                     , listTyCon, constraintKind )
    import BasicTypes
    import Util
    import Bag
    import Maybes
    import Outputable
    import FastString
    import ErrUtils( Validity(..), MsgDoc, isValid )
    import FV
    import qualified GHC.LanguageExtensions as LangExt

    import Data.IORef
    import Data.Functor.Identity

    {-
    ************************************************************************
    *                                                                      *
                  Types
    *                                                                      *
    ************************************************************************

    The type checker divides the generic Type world into the
    following more structured beasts:

    sigma ::= forall tyvars. phi
            -- A sigma type is a qualified type
            --
            -- Note that even if 'tyvars' is empty, theta
            -- may not be: e.g.   (?x::Int) => Int

            -- Note that 'sigma' is in prenex form:
            -- all the foralls are at the front.
            -- A 'phi' type has no foralls to the right of
            -- an arrow

    phi :: theta => rho

    rho ::= sigma -> rho
         |  tau

    -- A 'tau' type has no quantification anywhere
    -- Note that the args of a type constructor must be taus
    tau ::= tyvar
         |  tycon tau_1 .. tau_n
         |  tau_1 tau_2
         |  tau_1 -> tau_2

    -- In all cases, a (saturated) type synonym application is legal,
    -- provided it expands to the required form.

    Note [TcTyVars in the typechecker]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The typechecker uses a lot of type variables with special properties,
    notably being a unification variable with a mutable reference.  These
    use the 'TcTyVar' variant of Var.Var.

    However, the type checker and constraint solver can encounter type
    variables that use the 'TyVar' variant of Var.Var, for a couple of
    reasons:

      - When unifying or flattening under (forall a. ty)

      - When typechecking a class decl, say
           class C (a :: k) where
              foo :: T a -> Int
        We have first kind-check the header; fix k and (a:k) to be
        TyVars, bring 'k' and 'a' into scope, and kind check the
        signature for 'foo'.  In doing so we call solveEqualities to
        solve any kind equalities in foo's signature.  So the solver
        may see free occurrences of 'k'.

    It's convenient to simply treat these TyVars as skolem constants,
    which of course they are.  So

    * Var.tcTyVarDetails succeeds on a TyVar, returning
      vanillaSkolemTv, as well as on a TcTyVar.

    * tcIsTcTyVar returns True for both TyVar and TcTyVar variants
      of Var.Var.  The "tc" prefix means "a type variable that can be
      encountered by the typechecker".

    This is a bit of a change from an earlier era when we remoselessly
    insisted on real TcTyVars in the type checker.  But that seems
    unnecessary (for skolems, TyVars are fine) and it's now very hard
    to guarantee, with the advent of kind equalities.

    Note [Coercion variables in free variable lists]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    There are several places in the GHC codebase where functions like
    tyCoVarsOfType, tyCoVarsOfCt, et al. are used to compute the free type
    variables of a type. The "Co" part of these functions' names shouldn't be
    dismissed, as it is entirely possible that they will include coercion variables
    in addition to type variables! As a result, there are some places in TcType
    where we must take care to check that a variable is a _type_ variable (using
    isTyVar) before calling tcTyVarDetails--a partial function that is not defined
    for coercion variables--on the variable. Failing to do so led to
    GHC Trac #12785.
    -}

    -- See Note [TcTyVars in the typechecker]
    type TcTyVar = TyVar    -- Used only during type inference
    type TcCoVar = CoVar    -- Used only during type inference
    type TcType = Type      -- A TcType can have mutable type variables
    type TcTyCoVar = Var    -- Either a TcTyVar or a CoVar
            -- Invariant on ForAllTy in TcTypes:
            --      forall a. T
            -- a cannot occur inside a MutTyVar in T; that is,
            -- T is "flattened" before quantifying over a

    type TcTyVarBinder = TyVarBinder
    type TcTyCon       = TyCon   -- these can be the TcTyCon constructor

    -- These types do not have boxy type variables in them
    type TcPredType     = PredType
    type TcThetaType    = ThetaType
    type TcSigmaType    = TcType
    type TcRhoType      = TcType  -- Note [TcRhoType]
    type TcTauType      = TcType
    type TcKind         = Kind
    type TcTyVarSet     = TyVarSet
    type TcTyCoVarSet   = TyCoVarSet
    type TcDTyVarSet    = DTyVarSet
    type TcDTyCoVarSet  = DTyCoVarSet

    {- *********************************************************************
    *                                                                      *
              ExpType: an "expected type" in the type checker
    *                                                                      *
    ********************************************************************* -}

    -- | An expected type to check against during type-checking.
    -- See Note [ExpType] in TcMType, where you'll also find manipulators.
    data ExpType = Check TcType
                 | Infer !InferResult

    data InferResult
      = IR { ir_uniq :: Unique  -- For debugging only
           , ir_lvl  :: TcLevel -- See Note [TcLevel of ExpType] in TcMType
           , ir_inst :: Bool    -- True <=> deeply instantiate before returning
                                --           i.e. return a RhoType
                                -- False <=> do not instantiate before returning
                                --           i.e. return a SigmaType
           , ir_ref  :: IORef (Maybe TcType) }
             -- The type that fills in this hole should be a Type,
             -- that is, its kind should be (TYPE rr) for some rr

    type ExpSigmaType = ExpType
    type ExpRhoType   = ExpType

    instance Outputable ExpType where
      ppr (Check ty) = text "Check" <> braces (ppr ty)
      ppr (Infer ir) = ppr ir

    instance Outputable InferResult where
      ppr (IR { ir_uniq = u, ir_lvl = lvl
              , ir_inst = inst })
        = text "Infer" <> braces (ppr u <> comma <> ppr lvl <+> ppr inst)

    -- | Make an 'ExpType' suitable for checking.
    mkCheckExpType :: TcType -> ExpType
    mkCheckExpType = Check

    {- *********************************************************************
    *                                                                      *
              SyntaxOpType
    *                                                                      *
    ********************************************************************* -}

    -- | What to expect for an argument to a rebindable-syntax operator.
    -- Quite like 'Type', but allows for holes to be filled in by tcSyntaxOp.
    -- The callback called from tcSyntaxOp gets a list of types; the meaning
    -- of these types is determined by a left-to-right depth-first traversal
    -- of the 'SyntaxOpType' tree. So if you pass in
    --
    -- > SynAny `SynFun` (SynList `SynFun` SynType Int) `SynFun` SynAny
    --
    -- you'll get three types back: one for the first 'SynAny', the /element/
    -- type of the list, and one for the last 'SynAny'. You don't get anything
    -- for the 'SynType', because you've said positively that it should be an
    -- Int, and so it shall be.
    --
    -- This is defined here to avoid defining it in TcExpr.hs-boot.
    data SyntaxOpType
      = SynAny     -- ^ Any type
      | SynRho     -- ^ A rho type, deeply skolemised or instantiated as appropriate
      | SynList    -- ^ A list type. You get back the element type of the list
      | SynFun SyntaxOpType SyntaxOpType
                   -- ^ A function.
      | SynType ExpType   -- ^ A known type.
    infixr 0 `SynFun`

    -- | Like 'SynType' but accepts a regular TcType
    synKnownType :: TcType -> SyntaxOpType
    synKnownType = SynType . mkCheckExpType

    -- | Like 'mkFunTys' but for 'SyntaxOpType'
    mkSynFunTys :: [SyntaxOpType] -> ExpType -> SyntaxOpType
    mkSynFunTys arg_tys res_ty = foldr SynFun (SynType res_ty) arg_tys

    {-
    Note [TcRhoType]
    ~~~~~~~~~~~~~~~~
    A TcRhoType has no foralls or contexts at the top, or to the right of an arrow
      YES    (forall a. a->a) -> Int
      NO     forall a. a ->  Int
      NO     Eq a => a -> a
      NO     Int -> forall a. a -> Int

    ************************************************************************
    *                                                                      *
            TyVarDetails, MetaDetails, MetaInfo
    *                                                                      *
    ************************************************************************

    TyVarDetails gives extra info about type variables, used during type
    checking.  It's attached to mutable type variables only.
    It's knot-tied back to Var.hs.  There is no reason in principle
    why Var.hs shouldn't actually have the definition, but it "belongs" here.

    Note [Signature skolems]
    ~~~~~~~~~~~~~~~~~~~~~~~~
    A SigTv is a specialised variant of TauTv, with the following invarints:

        * A SigTv can be unified only with a TyVar,
          not with any other type

        * Its MetaDetails, if filled in, will always be another SigTv
          or a SkolemTv

    SigTvs are only distinguished to improve error messages.
    Consider this

      f :: forall a. [a] -> Int
      f (x::b : xs) = 3

    Here 'b' is a lexically scoped type variable, but it turns out to be
    the same as the skolem 'a'.  So we make them both SigTvs, which can unify
    with each other.

    Similarly consider
      data T (a:k1) = MkT (S a)
      data S (b:k2) = MkS (T b)
    When doing kind inference on {S,T} we don't want *skolems* for k1,k2,
    because they end up unifying; we want those SigTvs again.

    SigTvs are used *only* for pattern type signatures.

    Note [TyVars and TcTyVars during type checking]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The Var type has constructors TyVar and TcTyVar.  They are used
    as follows:

    * TcTyVar: used /only/ during type checking.  Should never appear
      afterwards.  May contain a mutable field, in the MetaTv case.

    * TyVar: is never seen by the constraint solver, except locally
      inside a type like (forall a. [a] ->[a]), where 'a' is a TyVar.
      We instantiate these with TcTyVars before exposing the type
      to the constraint solver.

    I have swithered about the latter invariant, excluding TyVars from the
    constraint solver.  It's not strictly essential, and indeed
    (historically but still there) Var.tcTyVarDetails returns
    vanillaSkolemTv for a TyVar.

    But ultimately I want to seeparate Type from TcType, and in that case
    we would need to enforce the separation.
    -}

    -- A TyVarDetails is inside a TyVar
    -- See Note [TyVars and TcTyVars]
    data TcTyVarDetails
      = SkolemTv      -- A skolem
           TcLevel    -- Level of the implication that binds it
           Bool       -- True <=> this skolem type variable can be overlapped
                      --          when looking up instances
                      -- See Note [Binding when looking up instances] in InstEnv

      | FlatSkol      -- A flatten-skolem.  It stands for the TcType, and zonking
           TcType     -- will replace it by that type.
                      -- See Note [The flattening story] in TcFlatten

      | RuntimeUnk    -- Stands for an as-yet-unknown type in the GHCi
                      -- interactive context

      | MetaTv { mtv_info  :: MetaInfo
               , mtv_ref   :: IORef MetaDetails
               , mtv_tclvl :: TcLevel }  -- See Note [TcLevel and untouchable type variables]

    vanillaSkolemTv, superSkolemTv :: TcTyVarDetails
    -- See Note [Binding when looking up instances] in InstEnv
    vanillaSkolemTv = SkolemTv (pushTcLevel topTcLevel) False  -- Might be instantiated
    superSkolemTv   = SkolemTv (pushTcLevel topTcLevel) True   -- Treat this as a completely distinct type

    -----------------------------
    data MetaDetails
      = Flexi  -- Flexi type variables unify to become Indirects
      | Indirect TcType

    data MetaInfo
       = TauTv         -- This MetaTv is an ordinary unification variable
                       -- A TauTv is always filled in with a tau-type, which
                       -- never contains any ForAlls.

       | SigTv         -- A variant of TauTv, except that it should not be
                       -- unified with a type, only with a type variable
                       -- See Note [Signature skolems]

       | FlatMetaTv    -- A flatten meta-tyvar
                       -- It is a meta-tyvar, but it is always untouchable, with level 0
                       -- See Note [The flattening story] in TcFlatten

    instance Outputable MetaDetails where
      ppr Flexi         = text "Flexi"
      ppr (Indirect ty) = text "Indirect" <+> ppr ty

    pprTcTyVarDetails :: TcTyVarDetails -> SDoc
    -- For debugging
    pprTcTyVarDetails (RuntimeUnk {})  = text "rt"
    pprTcTyVarDetails (FlatSkol {})    = text "fsk"
    pprTcTyVarDetails (SkolemTv lvl True)  = text "ssk" <> colon <> ppr lvl
    pprTcTyVarDetails (SkolemTv lvl False) = text "sk"  <> colon <> ppr lvl
    pprTcTyVarDetails (MetaTv { mtv_info = info, mtv_tclvl = tclvl })
      = pp_info <> colon <> ppr tclvl
      where
        pp_info = case info of
                    TauTv      -> text "tau"
                    SigTv      -> text "sig"
                    FlatMetaTv -> text "fuv"

    {- *********************************************************************
    *                                                                      *
              UserTypeCtxt
    *                                                                      *
    ********************************************************************* -}

    -------------------------------------
    -- UserTypeCtxt describes the origin of the polymorphic type
    -- in the places where we need to an expression has that type

    data UserTypeCtxt
      = FunSigCtxt      -- Function type signature, when checking the type
                        -- Also used for types in SPECIALISE pragmas
           Name              -- Name of the function
           Bool              -- True <=> report redundant constraints
                                -- This is usually True, but False for
                                --   * Record selectors (not important here)
                                --   * Class and instance methods.  Here
                                --     the code may legitimately be more
                                --     polymorphic than the signature
                                --     generated from the class
                                --     declaration

      | InfSigCtxt Name     -- Inferred type for function
      | ExprSigCtxt         -- Expression type signature
      | TypeAppCtxt         -- Visible type application
      | ConArgCtxt Name     -- Data constructor argument
      | TySynCtxt Name      -- RHS of a type synonym decl
      | PatSynCtxt Name     -- Type sig for a pattern synonym
      | PatSigCtxt          -- Type sig in pattern
                            --   eg  f (x::t) = ...
                            --   or  (x::t, y) = e
      | RuleSigCtxt Name    -- LHS of a RULE forall
                            --    RULE "foo" forall (x :: a -> a). f (Just x) = ...
      | ResSigCtxt          -- Result type sig
                            --      f x :: t = ....
      | ForSigCtxt Name     -- Foreign import or export signature
      | DefaultDeclCtxt     -- Types in a default declaration
      | InstDeclCtxt        -- An instance declaration
      | SpecInstCtxt        -- SPECIALISE instance pragma
      | ThBrackCtxt         -- Template Haskell type brackets [t| ... |]
      | GenSigCtxt          -- Higher-rank or impredicative situations
                            -- e.g. (f e) where f has a higher-rank type
                            -- We might want to elaborate this
      | GhciCtxt            -- GHCi command :kind <type>

      | ClassSCCtxt Name    -- Superclasses of a class
      | SigmaCtxt           -- Theta part of a normal for-all type
                            --      f :: <S> => a -> a
      | DataTyCtxt Name     -- The "stupid theta" part of a data decl
                            --      data <S> => T a = MkT a

    {-
    -- Notes re TySynCtxt
    -- We allow type synonyms that aren't types; e.g.  type List = []
    --
    -- If the RHS mentions tyvars that aren't in scope, we'll
    -- quantify over them:
    --      e.g.    type T = a->a
    -- will become  type T = forall a. a->a
    --
    -- With gla-exts that's right, but for H98 we should complain.
    -}

    pprUserTypeCtxt :: UserTypeCtxt -> SDoc
    pprUserTypeCtxt (FunSigCtxt n _)  = text "the type signature for" <+> quotes (ppr n)
    pprUserTypeCtxt (InfSigCtxt n)    = text "the inferred type for" <+> quotes (ppr n)
    pprUserTypeCtxt (RuleSigCtxt n)   = text "a RULE for" <+> quotes (ppr n)
    pprUserTypeCtxt ExprSigCtxt       = text "an expression type signature"
    pprUserTypeCtxt TypeAppCtxt       = text "a type argument"
    pprUserTypeCtxt (ConArgCtxt c)    = text "the type of the constructor" <+> quotes (ppr c)
    pprUserTypeCtxt (TySynCtxt c)     = text "the RHS of the type synonym" <+> quotes (ppr c)
    pprUserTypeCtxt ThBrackCtxt       = text "a Template Haskell quotation [t|...|]"
    pprUserTypeCtxt PatSigCtxt        = text "a pattern type signature"
    pprUserTypeCtxt ResSigCtxt        = text "a result type signature"
    pprUserTypeCtxt (ForSigCtxt n)    = text "the foreign declaration for" <+> quotes (ppr n)
    pprUserTypeCtxt DefaultDeclCtxt   = text "a type in a `default' declaration"
    pprUserTypeCtxt InstDeclCtxt      = text "an instance declaration"
    pprUserTypeCtxt SpecInstCtxt      = text "a SPECIALISE instance pragma"
    pprUserTypeCtxt GenSigCtxt        = text "a type expected by the context"
    pprUserTypeCtxt GhciCtxt          = text "a type in a GHCi command"
    pprUserTypeCtxt (ClassSCCtxt c)   = text "the super-classes of class" <+> quotes (ppr c)
    pprUserTypeCtxt SigmaCtxt         = text "the context of a polymorphic type"
    pprUserTypeCtxt (DataTyCtxt tc)   = text "the context of the data type declaration for" <+> quotes (ppr tc)
    pprUserTypeCtxt (PatSynCtxt n)    = text "the signature for pattern synonym" <+> quotes (ppr n)

    isSigMaybe :: UserTypeCtxt -> Maybe Name
    isSigMaybe (FunSigCtxt n _) = Just n
    isSigMaybe (ConArgCtxt n)   = Just n
    isSigMaybe (ForSigCtxt n)   = Just n
    isSigMaybe (PatSynCtxt n)   = Just n
    isSigMaybe _                = Nothing

    {- *********************************************************************
    *                                                                      *
                    Untoucable type variables
    *                                                                      *
    ********************************************************************* -}

    newtype TcLevel = TcLevel Int deriving( Eq, Ord )
      -- See Note [TcLevel and untouchable type variables] for what this Int is
      -- See also Note [TcLevel assignment]

    {-
    Note [TcLevel and untouchable type variables]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    * Each unification variable (MetaTv)
      and each Implication
      has a level number (of type TcLevel)

    * INVARIANTS.  In a tree of Implications,

        (ImplicInv) The level number of an Implication is
                    STRICTLY GREATER THAN that of its parent

        (MetaTvInv) The level number of a unification variable is
                    LESS THAN OR EQUAL TO that of its parent
                    implication

    * A unification variable is *touchable* if its level number
      is EQUAL TO that of its immediate parent implication.

    * INVARIANT
        (GivenInv)  The free variables of the ic_given of an
                    implication are all untouchable; ie their level
                    numbers are LESS THAN the ic_tclvl of the implication

    Note [Skolem escape prevention]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We only unify touchable unification variables.  Because of
    (MetaTvInv), there can be no occurrences of the variable further out,
    so the unification can't cause the skolems to escape. Example:
         data T = forall a. MkT a (a->Int)
         f x (MkT v f) = length [v,x]
    We decide (x::alpha), and generate an implication like
          [1]forall a. (a ~ alpha[0])
    But we must not unify alpha:=a, because the skolem would escape.

    For the cases where we DO want to unify, we rely on floating the
    equality.   Example (with same T)
         g x (MkT v f) = x && True
    We decide (x::alpha), and generate an implication like
          [1]forall a. (Bool ~ alpha[0])
    We do NOT unify directly, bur rather float out (if the constraint
    does not mention 'a') to get
          (Bool ~ alpha[0]) /\ [1]forall a.()
    and NOW we can unify alpha.

    The same idea of only unifying touchables solves another problem.
    Suppose we had
       (F Int ~ uf[0])  /\  [1](forall a. C a => F Int ~ beta[1])
    In this example, beta is touchable inside the implication. The
    first solveSimpleWanteds step leaves 'uf' un-unified. Then we move inside
    the implication where a new constraint
           uf  ~  beta
    emerges. If we (wrongly) spontaneously solved it to get uf := beta,
    the whole implication disappears but when we pop out again we are left with
    (F Int ~ uf) which will be unified by our final zonking stage and
    uf will get unified *once more* to (F Int).

    Note [TcLevel assignment]
    ~~~~~~~~~~~~~~~~~~~~~~~~~
    We arrange the TcLevels like this

       0   Level for flatten meta-vars
       1   Top level
       2   First-level implication constraints
       3   Second-level implication constraints
       ...etc...

    The flatten meta-vars are all at level 0, just to make them untouchable.
    -}

    maxTcLevel :: TcLevel -> TcLevel -> TcLevel
    maxTcLevel (TcLevel a) (TcLevel b) = TcLevel (a `max` b)

    fmvTcLevel :: TcLevel -> TcLevel
    -- See Note [TcLevel assignment]
    fmvTcLevel _ = TcLevel 0

    topTcLevel :: TcLevel
    -- See Note [TcLevel assignment]
    topTcLevel = TcLevel 1   -- 1 = outermost level

    isTopTcLevel :: TcLevel -> Bool
    isTopTcLevel (TcLevel 1) = True
    isTopTcLevel _           = False

    pushTcLevel :: TcLevel -> TcLevel
    -- See Note [TcLevel assignment]
    pushTcLevel (TcLevel us) = TcLevel (us + 1)

    strictlyDeeperThan :: TcLevel -> TcLevel -> Bool
    strictlyDeeperThan (TcLevel tv_tclvl) (TcLevel ctxt_tclvl)
      = tv_tclvl > ctxt_tclvl

    sameDepthAs :: TcLevel -> TcLevel -> Bool
    sameDepthAs (TcLevel ctxt_tclvl) (TcLevel tv_tclvl)
      = ctxt_tclvl == tv_tclvl   -- NB: invariant ctxt_tclvl >= tv_tclvl
                                 --     So <= would be equivalent

    checkTcLevelInvariant :: TcLevel -> TcLevel -> Bool
    -- Checks (MetaTvInv) from Note [TcLevel and untouchable type variables]
    checkTcLevelInvariant (TcLevel ctxt_tclvl) (TcLevel tv_tclvl)
      = ctxt_tclvl >= tv_tclvl

    tcTyVarLevel :: TcTyVar -> TcLevel
    tcTyVarLevel tv
      = ASSERT2( tcIsTcTyVar tv, ppr tv )
        case tcTyVarDetails tv of
              MetaTv { mtv_tclvl = tv_lvl } -> tv_lvl
              SkolemTv tv_lvl _             -> tv_lvl
              FlatSkol ty                   -> tcTypeLevel ty
              RuntimeUnk                    -> topTcLevel

    tcTypeLevel :: TcType -> TcLevel
    -- Max level of any free var of the type
    tcTypeLevel ty
      = foldDVarSet add topTcLevel (tyCoVarsOfTypeDSet ty)
      where
        add v lvl
          | isTcTyVar v = lvl `maxTcLevel` tcTyVarLevel v
          | otherwise   = lvl

    instance Outputable TcLevel where
      ppr (TcLevel us) = ppr us

    {- *********************************************************************
    *                                                                      *
        Finding type family instances
    *                                                                      *
    ************************************************************************
    -}

    -- | Finds outermost type-family applications occuring in a type,
    -- after expanding synonyms.  In the list (F, tys) that is returned
    -- we guarantee that tys matches F's arity.  For example, given
    --    type family F a :: * -> *    (arity 1)
    -- calling tcTyFamInsts on (Maybe (F Int Bool) will return
    --     (F, [Int]), not (F, [Int,Bool])
    --
    -- This is important for its use in deciding termination of type
    -- instances (see Trac #11581).  E.g.
    --    type instance G [Int] = ...(F Int <big type>)...
    -- we don't need to take <big type> into account when asking if
    -- the calls on the RHS are smaller than the LHS
    tcTyFamInsts :: Type -> [(TyCon, [Type])]
    tcTyFamInsts ty
      | Just exp_ty <- tcView ty    = tcTyFamInsts exp_ty
    tcTyFamInsts (TyVarTy _)        = []
    tcTyFamInsts (TyConApp tc tys)
      | isTypeFamilyTyCon tc        = [(tc, take (tyConArity tc) tys)]
      | otherwise                   = concat (map tcTyFamInsts tys)
    tcTyFamInsts (LitTy {})         = []
    tcTyFamInsts (ForAllTy bndr ty) = tcTyFamInsts (binderKind bndr)
                                      ++ tcTyFamInsts ty
    tcTyFamInsts (FunTy ty1 ty2)    = tcTyFamInsts ty1 ++ tcTyFamInsts ty2
    tcTyFamInsts (AppTy ty1 ty2)    = tcTyFamInsts ty1 ++ tcTyFamInsts ty2
    tcTyFamInsts (CastTy ty _)      = tcTyFamInsts ty
    tcTyFamInsts (CoercionTy _)     = []  -- don't count tyfams in coercions,
                                          -- as they never get normalized, anyway

    {-
    ************************************************************************
    *                                                                      *
              The "exact" free variables of a type
    *                                                                      *
    ************************************************************************

    Note [Silly type synonym]
    ~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider
      type T a = Int
    What are the free tyvars of (T x)?  Empty, of course!
    Here's the example that Ralf Laemmel showed me:
      foo :: (forall a. C u a -> C u a) -> u
      mappend :: Monoid u => u -> u -> u

      bar :: Monoid u => u
      bar = foo (\t -> t `mappend` t)
    We have to generalise at the arg to f, and we don't
    want to capture the constraint (Monad (C u a)) because
    it appears to mention a.  Pretty silly, but it was useful to him.

    exactTyCoVarsOfType is used by the type checker to figure out exactly
    which type variables are mentioned in a type.  It's also used in the
    smart-app checking code --- see TcExpr.tcIdApp

    On the other hand, consider a *top-level* definition
      f = (\x -> x) :: T a -> T a
    If we don't abstract over 'a' it'll get fixed to GHC.Prim.Any, and then
    if we have an application like (f "x") we get a confusing error message
    involving Any.  So the conclusion is this: when generalising
      - at top level use tyCoVarsOfType
      - in nested bindings use exactTyCoVarsOfType
    See Trac #1813 for example.
    -}

    exactTyCoVarsOfType :: Type -> TyCoVarSet
    -- Find the free type variables (of any kind)
    -- but *expand* type synonyms.  See Note [Silly type synonym] above.
    exactTyCoVarsOfType ty
      = go ty
      where
        go ty | Just ty' <- tcView ty = go ty'  -- This is the key line
        go (TyVarTy tv)         = unitVarSet tv `unionVarSet` go (tyVarKind tv)
        go (TyConApp _ tys)     = exactTyCoVarsOfTypes tys
        go (LitTy {})           = emptyVarSet
        go (AppTy fun arg)      = go fun `unionVarSet` go arg
        go (FunTy arg res)      = go arg `unionVarSet` go res
        go (ForAllTy bndr ty)   = delBinderVar (go ty) bndr `unionVarSet` go (binderKind bndr)
        go (CastTy ty co)       = go ty `unionVarSet` goCo co
        go (CoercionTy co)      = goCo co

        goCo (Refl _ ty)        = go ty
        goCo (TyConAppCo _ _ args)= goCos args
        goCo (AppCo co arg)     = goCo co `unionVarSet` goCo arg
        goCo (ForAllCo tv k_co co)
          = goCo co `delVarSet` tv `unionVarSet` goCo k_co
        goCo (FunCo _ co1 co2)   = goCo co1 `unionVarSet` goCo co2
        goCo (CoVarCo v)         = unitVarSet v `unionVarSet` go (varType v)
        goCo (AxiomInstCo _ _ args) = goCos args
        goCo (UnivCo p _ t1 t2)  = goProv p `unionVarSet` go t1 `unionVarSet` go t2
        goCo (SymCo co)          = goCo co
        goCo (TransCo co1 co2)   = goCo co1 `unionVarSet` goCo co2
        goCo (NthCo _ co)        = goCo co
        goCo (LRCo _ co)         = goCo co
        goCo (InstCo co arg)     = goCo co `unionVarSet` goCo arg
        goCo (CoherenceCo c1 c2) = goCo c1 `unionVarSet` goCo c2
        goCo (KindCo co)         = goCo co
        goCo (SubCo co)          = goCo co
        goCo (AxiomRuleCo _ c)   = goCos c

        goCos cos = foldr (unionVarSet . goCo) emptyVarSet cos

        goProv UnsafeCoerceProv     = emptyVarSet
        goProv (PhantomProv kco)    = goCo kco
        goProv (ProofIrrelProv kco) = goCo kco
        goProv (PluginProv _)       = emptyVarSet
        goProv (HoleProv _)         = emptyVarSet

    exactTyCoVarsOfTypes :: [Type] -> TyVarSet
    exactTyCoVarsOfTypes tys = mapUnionVarSet exactTyCoVarsOfType tys

    anyRewritableTyVar :: Bool -> (TcTyVar -> Bool)
                       -> TcType -> Bool
    -- (anyRewritableTyVar ignore_cos pred ty) returns True
    --    if the 'pred' returns True of free TyVar in 'ty'
    -- Do not look inside casts and coercions if 'ignore_cos' is True
    -- See Note [anyRewritableTyVar]
    anyRewritableTyVar ignore_cos pred ty
      = go emptyVarSet ty
      where
        go_tv bound tv | tv `elemVarSet` bound = False
                       | otherwise             = pred tv

        go bound (TyVarTy tv)     = go_tv bound tv
        go _     (LitTy {})       = False
        go bound (TyConApp _ tys) = any (go bound) tys
        go bound (AppTy fun arg)  = go bound fun || go bound arg
        go bound (FunTy arg res)  = go bound arg || go bound res
        go bound (ForAllTy tv ty) = go (bound `extendVarSet` binderVar tv) ty
        go bound (CastTy ty co)   = go bound ty || go_co bound co
        go bound (CoercionTy co)  = go_co bound co

        go_co bound co
          | ignore_cos = False
          | otherwise  = anyVarSet (go_tv bound) (tyCoVarsOfCo co)
          -- We don't have an equivalent of anyRewritableTyVar for coercions
          -- (at least not yet) so take the free vars and test them

    {- Note [anyRewritableTyVar]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    anyRewritableTyVar is used during kick-out from the inert set,
    to decide if, given a new equality (a ~ ty), we should kick out
    a constraint C.  Rather than gather free variables and see if 'a'
    is among them, we instead pass in a predicate; this is just efficiency.
    -}

    {- *********************************************************************
    *                                                                      *
              Bound variables in a type
    *                                                                      *
    ********************************************************************* -}

    -- | Find all variables bound anywhere in a type.
    -- See also Note [Scope-check inferred kinds] in TcHsType
    allBoundVariables :: Type -> TyVarSet
    allBoundVariables ty = fvVarSet $ go ty
      where
        go :: Type -> FV
        go (TyVarTy tv)     = go (tyVarKind tv)
        go (TyConApp _ tys) = mapUnionFV go tys
        go (AppTy t1 t2)    = go t1 `unionFV` go t2
        go (FunTy t1 t2)    = go t1 `unionFV` go t2
        go (ForAllTy (TvBndr tv _) t2) = FV.unitFV tv `unionFV`
                                        go (tyVarKind tv) `unionFV` go t2
        go (LitTy {})       = emptyFV
        go (CastTy ty _)    = go ty
        go (CoercionTy {})  = emptyFV
          -- any types mentioned in a coercion should also be mentioned in
          -- a type.

    allBoundVariabless :: [Type] -> TyVarSet
    allBoundVariabless = mapUnionVarSet allBoundVariables

    {- *********************************************************************
    *                                                                      *
              Type and kind variables in a type
    *                                                                      *
    ********************************************************************* -}

    data TcDepVars  -- See Note [Dependent type variables]
                    -- See Note [TcDepVars determinism]
      = DV { dv_kvs :: DTyCoVarSet  -- "kind" variables (dependent)
           , dv_tvs :: DTyVarSet    -- "type" variables (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]
        }

    tcDepVarSet :: TcDepVars -> TyVarSet
    -- Actually can contain CoVars, but never mind
    tcDepVarSet (DV { dv_kvs = kvs, dv_tvs = tvs })
      = dVarSetToVarSet kvs `unionVarSet` dVarSetToVarSet tvs

    instance Monoid TcDepVars where
       mempty = DV { dv_kvs = emptyDVarSet, dv_tvs = emptyDVarSet }
       mappend (DV { dv_kvs = kv1, dv_tvs = tv1 })
               (DV { dv_kvs = kv2, dv_tvs = tv2 })
              = DV { dv_kvs = kv1 `unionDVarSet` kv2
                   , dv_tvs = tv1 `unionDVarSet` tv2}

    instance Outputable TcDepVars where
      ppr (DV {dv_kvs = kvs, dv_tvs = tvs })
        = text "DV" <+> braces (sep [ text "dv_kvs =" <+> ppr kvs
                                    , text "dv_tvs =" <+> ppr tvs ])

    {- Note [Dependent type variables]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    In Haskell type inference we quantify over type variables; but we only
    quantify over /kind/ variables when -XPolyKinds is on. So when
    collecting the free vars of a type, 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
         PLUS any free coercion variables

      - "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 TcDepVars record:
      DV { dv_kvs: Variables free in the kind of a free type variable
                   or of a forall-bound type variable
         , dv_tvs: Variables sytactically 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.

    * Both sets are un-ordered, of course.

    * 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 [TcDepVars 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 UniqDFM.
    -}

    -- | Like 'splitDepVarsOfType', but over a list of types
    splitDepVarsOfTypes :: [Type] -> TcDepVars
    splitDepVarsOfTypes = foldMap splitDepVarsOfType

    -- | Worker for 'splitDepVarsOfType'. This might output the same var
    -- in both sets, if it's used in both a type and a kind.
    -- See Note [TcDepVars determinism]
    -- See Note [Dependent type variables]
    splitDepVarsOfType :: Type -> TcDepVars
    splitDepVarsOfType = go
      where
        go (TyVarTy tv)     = DV { dv_kvs =tyCoVarsOfTypeDSet $ tyVarKind tv
                                 , dv_tvs = unitDVarSet tv }
        go (AppTy t1 t2)    = go t1 `mappend` go t2
        go (TyConApp _ tys) = foldMap go tys
        go (FunTy arg res)  = go arg `mappend` go res
        go (LitTy {})       = mempty
        go (CastTy ty co)   = go ty `mappend` go_co co
        go (CoercionTy co)  = go_co co
        go (ForAllTy (TvBndr tv _) ty)
          = let DV { dv_kvs = kvs, dv_tvs = tvs } = go ty in
            DV { dv_kvs = (kvs `delDVarSet` tv)
                          `extendDVarSetList` tyCoVarsOfTypeList (tyVarKind tv)
               , dv_tvs = tvs `delDVarSet` tv }

        go_co co = DV { dv_kvs = tyCoVarsOfCoDSet co
                      , dv_tvs = emptyDVarSet }

    {-
    ************************************************************************
    *                                                                      *
                    Predicates
    *                                                                      *
    ************************************************************************
    -}

    tcIsTcTyVar :: TcTyVar -> Bool
    -- See Note [TcTyVars in the typechecker]
    tcIsTcTyVar tv = isTyVar tv

    isTouchableOrFmv :: TcLevel -> TcTyVar -> Bool
    isTouchableOrFmv ctxt_tclvl tv
      = ASSERT2( tcIsTcTyVar tv, ppr tv )
        case tcTyVarDetails tv of
          MetaTv { mtv_tclvl = tv_tclvl, mtv_info = info }
            -> ASSERT2( checkTcLevelInvariant ctxt_tclvl tv_tclvl,
                        ppr tv $$ ppr tv_tclvl $$ ppr ctxt_tclvl )
               case info of
                 FlatMetaTv -> True
                 _          -> tv_tclvl `sameDepthAs` ctxt_tclvl
          _          -> False

    isTouchableMetaTyVar :: TcLevel -> TcTyVar -> Bool
    isTouchableMetaTyVar ctxt_tclvl tv
      | isTyVar tv -- See Note [Coercion variables in free variable lists]
      = ASSERT2( tcIsTcTyVar tv, ppr tv )
        case tcTyVarDetails tv of
          MetaTv { mtv_tclvl = tv_tclvl }
            -> ASSERT2( checkTcLevelInvariant ctxt_tclvl tv_tclvl,
                        ppr tv $$ ppr tv_tclvl $$ ppr ctxt_tclvl )
               tv_tclvl `sameDepthAs` ctxt_tclvl
          _ -> False
      | otherwise = False

    isFloatedTouchableMetaTyVar :: TcLevel -> TcTyVar -> Bool
    isFloatedTouchableMetaTyVar ctxt_tclvl tv
      | isTyVar tv -- See Note [Coercion variables in free variable lists]
      = ASSERT2( tcIsTcTyVar tv, ppr tv )
        case tcTyVarDetails tv of
          MetaTv { mtv_tclvl = tv_tclvl } -> tv_tclvl `strictlyDeeperThan` ctxt_tclvl
          _ -> False
      | otherwise = False

    isImmutableTyVar :: TyVar -> Bool
    isImmutableTyVar tv = isSkolemTyVar tv

    isTyConableTyVar, isSkolemTyVar, isOverlappableTyVar,
      isMetaTyVar, isAmbiguousTyVar,
      isFmvTyVar, isFskTyVar, isFlattenTyVar :: TcTyVar -> Bool

    isTyConableTyVar tv
            -- True of a meta-type variable that can be filled in
            -- with a type constructor application; in particular,
            -- not a SigTv
      | isTyVar tv -- See Note [Coercion variables in free variable lists]
      = ASSERT2( tcIsTcTyVar tv, ppr tv )
        case tcTyVarDetails tv of
            MetaTv { mtv_info = SigTv } -> False
            _                           -> True
      | otherwise = True

    isFmvTyVar tv
      = ASSERT2( tcIsTcTyVar tv, ppr tv )
        case tcTyVarDetails tv of
            MetaTv { mtv_info = FlatMetaTv } -> True
            _                                -> False

    -- | True of both given and wanted flatten-skolems (fak and usk)
    isFlattenTyVar tv
      = ASSERT2( tcIsTcTyVar tv, ppr tv )
        case tcTyVarDetails tv of
            FlatSkol {}                      -> True
            MetaTv { mtv_info = FlatMetaTv } -> True
            _                                -> False

    -- | True of FlatSkol skolems only
    isFskTyVar tv
      = ASSERT2( tcIsTcTyVar tv, ppr tv )
        case tcTyVarDetails tv of
            FlatSkol {} -> True
            _           -> False

    isSkolemTyVar tv
      = ASSERT2( tcIsTcTyVar tv, ppr tv )
        case tcTyVarDetails tv of
            MetaTv {} -> False
            _other    -> True

    isOverlappableTyVar tv
      | isTyVar tv -- See Note [Coercion variables in free variable lists]
      = ASSERT2( tcIsTcTyVar tv, ppr tv )
        case tcTyVarDetails tv of
            SkolemTv _ overlappable -> overlappable
            _                       -> False
      | otherwise = False

    isMetaTyVar tv
      | isTyVar tv -- See Note [Coercion variables in free variable lists]
      = ASSERT2( tcIsTcTyVar tv, ppr tv )
        case tcTyVarDetails tv of
            MetaTv {} -> True
            _         -> False
      | otherwise = False

    -- isAmbiguousTyVar is used only when reporting type errors
    -- It picks out variables that are unbound, namely meta
    -- type variables and the RuntimUnk variables created by
    -- RtClosureInspect.zonkRTTIType.  These are "ambiguous" in
    -- the sense that they stand for an as-yet-unknown type
    isAmbiguousTyVar tv
      | isTyVar tv -- See Note [Coercion variables in free variable lists]
      = case tcTyVarDetails tv of
            MetaTv {}     -> True
            RuntimeUnk {} -> True
            _             -> False
      | otherwise = False

    isMetaTyVarTy :: TcType -> Bool
    isMetaTyVarTy (TyVarTy tv) = isMetaTyVar tv
    isMetaTyVarTy _            = False

    metaTyVarInfo :: TcTyVar -> MetaInfo
    metaTyVarInfo tv
      = case tcTyVarDetails tv of
          MetaTv { mtv_info = info } -> info
          _ -> pprPanic "metaTyVarInfo" (ppr tv)

    metaTyVarTcLevel :: TcTyVar -> TcLevel
    metaTyVarTcLevel tv
      = case tcTyVarDetails tv of
          MetaTv { mtv_tclvl = tclvl } -> tclvl
          _ -> pprPanic "metaTyVarTcLevel" (ppr tv)

    metaTyVarTcLevel_maybe :: TcTyVar -> Maybe TcLevel
    metaTyVarTcLevel_maybe tv
      = case tcTyVarDetails tv of
          MetaTv { mtv_tclvl = tclvl } -> Just tclvl
          _                            -> Nothing

    metaTyVarRef :: TyVar -> IORef MetaDetails
    metaTyVarRef tv
      = case tcTyVarDetails tv of
            MetaTv { mtv_ref = ref } -> ref
            _ -> pprPanic "metaTyVarRef" (ppr tv)

    setMetaTyVarTcLevel :: TcTyVar -> TcLevel -> TcTyVar
    setMetaTyVarTcLevel tv tclvl
      = case tcTyVarDetails tv of
          details@(MetaTv {}) -> setTcTyVarDetails tv (details { mtv_tclvl = tclvl })
          _ -> pprPanic "metaTyVarTcLevel" (ppr tv)

    isSigTyVar :: Var -> Bool
    isSigTyVar tv
      = case tcTyVarDetails tv of
            MetaTv { mtv_info = SigTv } -> True
            _                           -> False

    isFlexi, isIndirect :: MetaDetails -> Bool
    isFlexi Flexi = True
    isFlexi _     = False

    isIndirect (Indirect _) = True
    isIndirect _            = False

    isRuntimeUnkSkol :: TyVar -> Bool
    -- Called only in TcErrors; see Note [Runtime skolems] there
    isRuntimeUnkSkol x
      | RuntimeUnk <- tcTyVarDetails x = True
      | otherwise                      = False

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Tau, sigma and rho}
    *                                                                      *
    ************************************************************************
    -}

    mkSigmaTy :: [TyVarBinder] -> [PredType] -> Type -> Type
    mkSigmaTy bndrs theta tau = mkForAllTys bndrs (mkPhiTy theta tau)

    -- | Make a sigma ty where all type variables are 'Inferred'. That is,
    -- they cannot be used with visible type application.
    mkInfSigmaTy :: [TyVar] -> [PredType] -> Type -> Type
    mkInfSigmaTy tyvars ty = mkSigmaTy (mkTyVarBinders Inferred tyvars) ty

    -- | Make a sigma ty where all type variables are "specified". That is,
    -- they can be used with visible type application
    mkSpecSigmaTy :: [TyVar] -> [PredType] -> Type -> Type
    mkSpecSigmaTy tyvars ty = mkSigmaTy (mkTyVarBinders Specified tyvars) ty

    mkPhiTy :: [PredType] -> Type -> Type
    mkPhiTy = mkFunTys

    ---------------
    getDFunTyKey :: Type -> OccName -- Get some string from a type, to be used to
                                    -- construct a dictionary function name
    getDFunTyKey ty | Just ty' <- coreView ty = getDFunTyKey ty'
    getDFunTyKey (TyVarTy tv)            = getOccName tv
    getDFunTyKey (TyConApp tc _)         = getOccName tc
    getDFunTyKey (LitTy x)               = getDFunTyLitKey x
    getDFunTyKey (AppTy fun _)           = getDFunTyKey fun
    getDFunTyKey (FunTy _ _)             = getOccName funTyCon
    getDFunTyKey (ForAllTy _ t)          = getDFunTyKey t
    getDFunTyKey (CastTy ty _)           = getDFunTyKey ty
    getDFunTyKey t@(CoercionTy _)        = pprPanic "getDFunTyKey" (ppr t)

    getDFunTyLitKey :: TyLit -> OccName
    getDFunTyLitKey (NumTyLit n) = mkOccName Name.varName (show n)
    getDFunTyLitKey (StrTyLit n) = mkOccName Name.varName (show n)  -- hm

    ---------------
    mkNakedTyConApp :: TyCon -> [Type] -> Type
    -- Builds a TyConApp
    --   * without being strict in TyCon,
    --   * without satisfying the invariants of TyConApp
    -- A subsequent zonking will establish the invariants
    -- See Note [Type-checking inside the knot] in TcHsType
    mkNakedTyConApp tc tys = TyConApp tc tys

    mkNakedAppTys :: Type -> [Type] -> Type
    -- See Note [Type-checking inside the knot] in TcHsType
    mkNakedAppTys ty1                []   = ty1
    mkNakedAppTys (TyConApp tc tys1) tys2 = mkNakedTyConApp tc (tys1 ++ tys2)
    mkNakedAppTys ty1                tys2 = foldl AppTy ty1 tys2

    mkNakedAppTy :: Type -> Type -> Type
    -- See Note [Type-checking inside the knot] in TcHsType
    mkNakedAppTy ty1 ty2 = mkNakedAppTys ty1 [ty2]

    mkNakedCastTy :: Type -> Coercion -> Type
    -- Do simple, fast compaction; especially dealing with Refl
    -- for which it's plain stupid to create a cast
    -- This simple function killed off a huge number of Refl casts
    -- in types, at birth.
    -- Note that it's fine to do this even for a "mkNaked" function,
    -- because we don't look at TyCons.  isReflCo checks if the coercion
    -- is structurally Refl; it does not check for shape k ~ k.
    mkNakedCastTy ty co | isReflCo co = ty
    mkNakedCastTy (CastTy ty co1) co2 = CastTy ty (co1 `mkTransCo` co2)
    mkNakedCastTy ty co = CastTy ty co

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Expanding and splitting}
    *                                                                      *
    ************************************************************************

    These tcSplit functions are like their non-Tc analogues, but
            *) they do not look through newtypes

    However, they are non-monadic and do not follow through mutable type
    variables.  It's up to you to make sure this doesn't matter.
    -}

    -- | Splits a forall type into a list of 'TyBinder's and the inner type.
    -- Always succeeds, even if it returns an empty list.
    tcSplitPiTys :: Type -> ([TyBinder], Type)
    tcSplitPiTys = splitPiTys

    tcSplitForAllTy_maybe :: Type -> Maybe (TyVarBinder, Type)
    tcSplitForAllTy_maybe ty | Just ty' <- tcView ty = tcSplitForAllTy_maybe ty'
    tcSplitForAllTy_maybe (ForAllTy tv ty) = Just (tv, ty)
    tcSplitForAllTy_maybe _                = Nothing

    -- | Like 'tcSplitPiTys', but splits off only named binders, returning
    -- just the tycovars.
    tcSplitForAllTys :: Type -> ([TyVar], Type)
    tcSplitForAllTys = splitForAllTys

    -- | Like 'tcSplitForAllTys', but splits off only named binders.
    tcSplitForAllTyVarBndrs :: Type -> ([TyVarBinder], Type)
    tcSplitForAllTyVarBndrs = splitForAllTyVarBndrs

    -- | Is this a ForAllTy with a named binder?
    tcIsForAllTy :: Type -> Bool
    tcIsForAllTy ty | Just ty' <- tcView ty = tcIsForAllTy ty'
    tcIsForAllTy (ForAllTy {}) = True
    tcIsForAllTy _             = False

    tcSplitPredFunTy_maybe :: Type -> Maybe (PredType, Type)
    -- Split off the first predicate argument from a type
    tcSplitPredFunTy_maybe ty
      | Just ty' <- tcView ty = tcSplitPredFunTy_maybe ty'
    tcSplitPredFunTy_maybe (FunTy arg res)
      | isPredTy arg = Just (arg, res)
    tcSplitPredFunTy_maybe _
      = Nothing

    tcSplitPhiTy :: Type -> (ThetaType, Type)
    tcSplitPhiTy ty
      = split ty []
      where
        split ty ts
          = case tcSplitPredFunTy_maybe ty of
              Just (pred, ty) -> split ty (pred:ts)
              Nothing         -> (reverse ts, ty)

    -- | Split a sigma type into its parts.
    tcSplitSigmaTy :: Type -> ([TyVar], ThetaType, Type)
    tcSplitSigmaTy ty = case tcSplitForAllTys ty of
                            (tvs, rho) -> case tcSplitPhiTy rho of
                                            (theta, tau) -> (tvs, theta, tau)

    -- | Split a sigma type into its parts, going underneath as many @ForAllTy@s
    -- as possible. For example, given this type synonym:
    --
    -- @
    -- type Traversal s t a b = forall f. Applicative f => (a -> f b) -> s -> f t
    -- @
    --
    -- if you called @tcSplitSigmaTy@ on this type:
    --
    -- @
    -- forall s t a b. Each s t a b => Traversal s t a b
    -- @
    --
    -- then it would return @([s,t,a,b], [Each s t a b], Traversal s t a b)@. But
    -- if you instead called @tcSplitNestedSigmaTys@ on the type, it would return
    -- @([s,t,a,b,f], [Each s t a b, Applicative f], (a -> f b) -> s -> f t)@.
    tcSplitNestedSigmaTys :: Type -> ([TyVar], ThetaType, Type)
    -- NB: This is basically a pure version of deeplyInstantiate (from Inst) that
    -- doesn't compute an HsWrapper.
    tcSplitNestedSigmaTys ty
        -- If there's a forall, split it apart and try splitting the rho type
        -- underneath it.
      | Just (arg_tys, tvs1, theta1, rho1) <- tcDeepSplitSigmaTy_maybe ty
      = let (tvs2, theta2, rho2) = tcSplitNestedSigmaTys rho1
        in (tvs1 ++ tvs2, theta1 ++ theta2, mkFunTys arg_tys rho2)
        -- If there's no forall, we're done.
      | otherwise = ([], [], ty)

    -----------------------
    tcDeepSplitSigmaTy_maybe
      :: TcSigmaType -> Maybe ([TcType], [TyVar], ThetaType, TcSigmaType)
    -- Looks for a *non-trivial* quantified type, under zero or more function arrows
    -- By "non-trivial" we mean either tyvars or constraints are non-empty

    tcDeepSplitSigmaTy_maybe ty
      | Just (arg_ty, res_ty)           <- tcSplitFunTy_maybe ty
      , Just (arg_tys, tvs, theta, rho) <- tcDeepSplitSigmaTy_maybe res_ty
      = Just (arg_ty:arg_tys, tvs, theta, rho)

      | (tvs, theta, rho) <- tcSplitSigmaTy ty
      , not (null tvs && null theta)
      = Just ([], tvs, theta, rho)

      | otherwise = Nothing

    -----------------------
    tcTyConAppTyCon :: Type -> TyCon
    tcTyConAppTyCon ty
      = case tcTyConAppTyCon_maybe ty of
          Just tc -> tc
          Nothing -> pprPanic "tcTyConAppTyCon" (pprType ty)

    -- | Like 'tcRepSplitTyConApp_maybe', but only returns the 'TyCon'.
    tcTyConAppTyCon_maybe :: Type -> Maybe TyCon
    tcTyConAppTyCon_maybe ty
      | Just ty' <- tcView ty = tcTyConAppTyCon_maybe ty'
    tcTyConAppTyCon_maybe (TyConApp tc _)
      = Just tc
    tcTyConAppTyCon_maybe (FunTy _ _)
      = Just funTyCon
    tcTyConAppTyCon_maybe _
      = Nothing

    tcTyConAppArgs :: Type -> [Type]
    tcTyConAppArgs ty = case tcSplitTyConApp_maybe ty of
                            Just (_, args) -> args
                            Nothing        -> pprPanic "tcTyConAppArgs" (pprType ty)

    tcSplitTyConApp :: Type -> (TyCon, [Type])
    tcSplitTyConApp ty = case tcSplitTyConApp_maybe ty of
                            Just stuff -> stuff
                            Nothing    -> pprPanic "tcSplitTyConApp" (pprType ty)

    -- | Like 'tcRepSplitTyConApp_maybe', but returns 'Nothing' if,
    --
    -- 1. the type is structurally not a type constructor application, or
    --
    -- 2. the type is a function type (e.g. application of 'funTyCon'), but we
    --    currently don't even enough information to fully determine its RuntimeRep
    --    variables. For instance, @FunTy (a :: k) Int@.
    --
    -- By contrast 'tcRepSplitTyConApp_maybe' panics in the second case.
    --
    -- The behavior here is needed during canonicalization; see Note [FunTy and
    -- decomposing tycon applications] in TcCanonical for details.
    tcRepSplitTyConApp_maybe' :: HasCallStack => Type -> Maybe (TyCon, [Type])
    tcRepSplitTyConApp_maybe' (TyConApp tc tys)          = Just (tc, tys)
    tcRepSplitTyConApp_maybe' (FunTy arg res)
      | Just arg_rep <- getRuntimeRep_maybe arg
      , Just res_rep <- getRuntimeRep_maybe res
      = Just (funTyCon, [arg_rep, res_rep, arg, res])
    tcRepSplitTyConApp_maybe' _                          = Nothing

    -----------------------
    tcSplitFunTys :: Type -> ([Type], Type)
    tcSplitFunTys ty = case tcSplitFunTy_maybe ty of
                            Nothing        -> ([], ty)
                            Just (arg,res) -> (arg:args, res')
                                           where
                                              (args,res') = tcSplitFunTys res

    tcSplitFunTy_maybe :: Type -> Maybe (Type, Type)
    tcSplitFunTy_maybe ty | Just ty' <- tcView ty         = tcSplitFunTy_maybe ty'
    tcSplitFunTy_maybe (FunTy arg res) | not (isPredTy arg) = Just (arg, res)
    tcSplitFunTy_maybe _                                    = Nothing
            -- Note the typeKind guard
            -- Consider     (?x::Int) => Bool
            -- We don't want to treat this as a function type!
            -- A concrete example is test tc230:
            --      f :: () -> (?p :: ()) => () -> ()
            --
            --      g = f () ()

    tcSplitFunTysN :: Arity                      -- N: Number of desired args
                   -> TcRhoType
                   -> Either Arity               -- Number of missing arrows
                            ([TcSigmaType],      -- Arg types (always N types)
                             TcSigmaType)        -- The rest of the type
    -- ^ Split off exactly the specified number argument types
    -- Returns
    --  (Left m) if there are 'm' missing arrows in the type
    --  (Right (tys,res)) if the type looks like t1 -> ... -> tn -> res
    tcSplitFunTysN n ty
     | n == 0
     = Right ([], ty)
     | Just (arg,res) <- tcSplitFunTy_maybe ty
     = case tcSplitFunTysN (n-1) res of
         Left m            -> Left m
         Right (args,body) -> Right (arg:args, body)
     | otherwise
     = Left n

    tcSplitFunTy :: Type -> (Type, Type)
    tcSplitFunTy  ty = expectJust "tcSplitFunTy" (tcSplitFunTy_maybe ty)

    tcFunArgTy :: Type -> Type
    tcFunArgTy    ty = fst (tcSplitFunTy ty)

    tcFunResultTy :: Type -> Type
    tcFunResultTy ty = snd (tcSplitFunTy ty)

    -- | Strips off n *visible* arguments and returns the resulting type
    tcFunResultTyN :: HasDebugCallStack => Arity -> Type -> Type
    tcFunResultTyN n ty
      | Right (_, res_ty) <- tcSplitFunTysN n ty
      = res_ty
      | otherwise
      = pprPanic "tcFunResultTyN" (ppr n <+> ppr ty)

    -----------------------
    tcSplitAppTy_maybe :: Type -> Maybe (Type, Type)
    tcSplitAppTy_maybe ty | Just ty' <- tcView ty = tcSplitAppTy_maybe ty'
    tcSplitAppTy_maybe ty = tcRepSplitAppTy_maybe ty

    tcSplitAppTy :: Type -> (Type, Type)
    tcSplitAppTy ty = case tcSplitAppTy_maybe ty of
                        Just stuff -> stuff
                        Nothing    -> pprPanic "tcSplitAppTy" (pprType ty)

    tcSplitAppTys :: Type -> (Type, [Type])
    tcSplitAppTys ty
      = go ty []
      where
        go ty args = case tcSplitAppTy_maybe ty of
                       Just (ty', arg) -> go ty' (arg:args)
                       Nothing         -> (ty,args)

    -----------------------
    tcGetTyVar_maybe :: Type -> Maybe TyVar
    tcGetTyVar_maybe ty | Just ty' <- tcView ty = tcGetTyVar_maybe ty'
    tcGetTyVar_maybe (TyVarTy tv)   = Just tv
    tcGetTyVar_maybe _              = Nothing

    tcGetTyVar :: String -> Type -> TyVar
    tcGetTyVar msg ty = expectJust msg (tcGetTyVar_maybe ty)

    tcIsTyVarTy :: Type -> Bool
    tcIsTyVarTy ty | Just ty' <- tcView ty = tcIsTyVarTy ty'
    tcIsTyVarTy (CastTy ty _) = tcIsTyVarTy ty  -- look through casts, as
                                                -- this is only used for
                                                -- e.g., FlexibleContexts
    tcIsTyVarTy (TyVarTy _)   = True
    tcIsTyVarTy _             = False

    -----------------------
    tcSplitDFunTy :: Type -> ([TyVar], [Type], Class, [Type])
    -- Split the type of a dictionary function
    -- We don't use tcSplitSigmaTy,  because a DFun may (with NDP)
    -- have non-Pred arguments, such as
    --     df :: forall m. (forall b. Eq b => Eq (m b)) -> C m
    --
    -- Also NB splitFunTys, not tcSplitFunTys;
    -- the latter  specifically stops at PredTy arguments,
    -- and we don't want to do that here
    tcSplitDFunTy ty
      = case tcSplitForAllTys ty   of { (tvs, rho)    ->
        case splitFunTys rho       of { (theta, tau)  ->
        case tcSplitDFunHead tau   of { (clas, tys)   ->
        (tvs, theta, clas, tys) }}}

    tcSplitDFunHead :: Type -> (Class, [Type])
    tcSplitDFunHead = getClassPredTys

    tcSplitMethodTy :: Type -> ([TyVar], PredType, Type)
    -- A class method (selector) always has a type like
    --   forall as. C as => blah
    -- So if the class looks like
    --   class C a where
    --     op :: forall b. (Eq a, Ix b) => a -> b
    -- the class method type looks like
    --  op :: forall a. C a => forall b. (Eq a, Ix b) => a -> b
    --
    -- tcSplitMethodTy just peels off the outer forall and
    -- that first predicate
    tcSplitMethodTy ty
      | (sel_tyvars,sel_rho) <- tcSplitForAllTys ty
      , Just (first_pred, local_meth_ty) <- tcSplitPredFunTy_maybe sel_rho
      = (sel_tyvars, first_pred, local_meth_ty)
      | otherwise
      = pprPanic "tcSplitMethodTy" (ppr ty)

    {- *********************************************************************
    *                                                                      *
                Type equalities
    *                                                                      *
    ********************************************************************* -}

    tcEqKind :: TcKind -> TcKind -> Bool
    tcEqKind = tcEqType

    tcEqType :: TcType -> TcType -> Bool
    -- tcEqType is a proper implements the same Note [Non-trivial definitional
    -- equality] (in TyCoRep) as `eqType`, but Type.eqType believes (* ==
    -- Constraint), and that is NOT what we want in the type checker!
    tcEqType ty1 ty2
      = isNothing (tc_eq_type tcView ki1 ki2) &&
        isNothing (tc_eq_type tcView ty1 ty2)
      where
        ki1 = typeKind ty1
        ki2 = typeKind ty2

    -- | Just like 'tcEqType', but will return True for types of different kinds
    -- as long as their non-coercion structure is identical.
    tcEqTypeNoKindCheck :: TcType -> TcType -> Bool
    tcEqTypeNoKindCheck ty1 ty2
      = isNothing $ tc_eq_type tcView ty1 ty2

    -- | Like 'tcEqType', but returns information about whether the difference
    -- is visible in the case of a mismatch.
    -- @Nothing@    : the types are equal
    -- @Just True@  : the types differ, and the point of difference is visible
    -- @Just False@ : the types differ, and the point of difference is invisible
    tcEqTypeVis :: TcType -> TcType -> Maybe Bool
    tcEqTypeVis ty1 ty2
      = tc_eq_type tcView ty1 ty2 <!> invis (tc_eq_type tcView ki1 ki2)
      where
        ki1 = typeKind ty1
        ki2 = typeKind ty2

          -- convert Just True to Just False
        invis :: Maybe Bool -> Maybe Bool
        invis = fmap (const False)

    (<!>) :: Maybe Bool -> Maybe Bool -> Maybe Bool
    Nothing        <!> x         = x
    Just True      <!> _         = Just True
    Just _vis      <!> Just True = Just True
    Just vis       <!> _         = Just vis
    infixr 3 <!>

    -- | Real worker for 'tcEqType'. No kind check!
    tc_eq_type :: (TcType -> Maybe TcType)  -- ^ @tcView@, if you want unwrapping
               -> Type -> Type -> Maybe Bool
    tc_eq_type view_fun orig_ty1 orig_ty2 = go True orig_env orig_ty1 orig_ty2
      where
        go :: Bool -> RnEnv2 -> Type -> Type -> Maybe Bool
        go vis env t1 t2 | Just t1' <- view_fun t1 = go vis env t1' t2
        go vis env t1 t2 | Just t2' <- view_fun t2 = go vis env t1 t2'

        go vis env (TyVarTy tv1)       (TyVarTy tv2)
          = check vis $ rnOccL env tv1 == rnOccR env tv2

        go vis _   (LitTy lit1)        (LitTy lit2)
          = check vis $ lit1 == lit2

        go vis env (ForAllTy (TvBndr tv1 vis1) ty1)
                   (ForAllTy (TvBndr tv2 vis2) ty2)
          = go (isVisibleArgFlag vis1) env (tyVarKind tv1) (tyVarKind tv2)
              <!> go vis (rnBndr2 env tv1 tv2) ty1 ty2
              <!> check vis (vis1 == vis2)
        -- Make sure we handle all FunTy cases since falling through to the
        -- AppTy case means that tcRepSplitAppTy_maybe may see an unzonked
        -- kind variable, which causes things to blow up.
        go vis env (FunTy arg1 res1) (FunTy arg2 res2)
          = go vis env arg1 arg2 <!> go vis env res1 res2
        go vis env ty (FunTy arg res)
          = eqFunTy vis env arg res ty
        go vis env (FunTy arg res) ty
          = eqFunTy vis env arg res ty

          -- See Note [Equality on AppTys] in Type
        go vis env (AppTy s1 t1)        ty2
          | Just (s2, t2) <- tcRepSplitAppTy_maybe ty2
          = go vis env s1 s2 <!> go vis env t1 t2
        go vis env ty1                  (AppTy s2 t2)
          | Just (s1, t1) <- tcRepSplitAppTy_maybe ty1
          = go vis env s1 s2 <!> go vis env t1 t2
        go vis env (TyConApp tc1 ts1)   (TyConApp tc2 ts2)
          = check vis (tc1 == tc2) <!> gos (tc_vis vis tc1) env ts1 ts2
        go vis env (CastTy t1 _)        t2              = go vis env t1 t2
        go vis env t1                   (CastTy t2 _)   = go vis env t1 t2
        go _   _   (CoercionTy {})      (CoercionTy {}) = Nothing
        go vis _   _                    _               = Just vis

        gos _      _   []       []       = Nothing
        gos (v:vs) env (t1:ts1) (t2:ts2) = go v env t1 t2 <!> gos vs env ts1 ts2
        gos (v:_)  _   _        _        = Just v
        gos _      _   _        _        = panic "tc_eq_type"

        tc_vis :: Bool -> TyCon -> [Bool]
        tc_vis True tc = viss ++ repeat True
           -- the repeat True is necessary because tycons can legitimately
           -- be oversaturated
          where
            bndrs = tyConBinders tc
            viss  = map (isVisibleArgFlag . tyConBinderArgFlag) bndrs
        tc_vis False _ = repeat False  -- if we're not in a visible context, our args
                                       -- aren't either

        check :: Bool -> Bool -> Maybe Bool
        check _   True  = Nothing
        check vis False = Just vis

        orig_env = mkRnEnv2 $ mkInScopeSet $ tyCoVarsOfTypes [orig_ty1, orig_ty2]

        -- @eqFunTy arg res ty@ is True when @ty@ equals @FunTy arg res@. This is
        -- sometimes hard to know directly because @ty@ might have some casts
        -- obscuring the FunTy. And 'splitAppTy' is difficult because we can't
        -- always extract a RuntimeRep (see Note [xyz]) if the kind of the arg or
        -- res is unzonked/unflattened. Thus this function, which handles this
        -- corner case.
        eqFunTy :: Bool -> RnEnv2 -> Type -> Type -> Type -> Maybe Bool
        eqFunTy vis env arg res (FunTy arg' res')
          = go vis env arg arg' <!> go vis env res res'
        eqFunTy vis env arg res ty@(AppTy{})
          | Just (tc, [_, _, arg', res']) <- get_args ty []
          , tc == funTyCon
          = go vis env arg arg' <!> go vis env res res'
          where
            get_args :: Type -> [Type] -> Maybe (TyCon, [Type])
            get_args (AppTy f x)       args = get_args f (x:args)
            get_args (CastTy t _)      args = get_args t args
            get_args (TyConApp tc tys) args = Just (tc, tys ++ args)
            get_args _                 _    = Nothing
        eqFunTy vis _ _ _ _
          = Just vis

    -- | Like 'pickyEqTypeVis', but returns a Bool for convenience
    pickyEqType :: TcType -> TcType -> Bool
    -- Check when two types _look_ the same, _including_ synonyms.
    -- So (pickyEqType String [Char]) returns False
    -- This ignores kinds and coercions, because this is used only for printing.
    pickyEqType ty1 ty2
      = isNothing $
        tc_eq_type (const Nothing) ty1 ty2

    {- *********************************************************************
    *                                                                      *
                           Predicate types
    *                                                                      *
    ************************************************************************

    Deconstructors and tests on predicate types

    Note [Kind polymorphic type classes]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        class C f where...   -- C :: forall k. k -> Constraint
        g :: forall (f::*). C f => f -> f

    Here the (C f) in the signature is really (C * f), and we
    don't want to complain that the * isn't a type variable!
    -}

    isTyVarClassPred :: PredType -> Bool
    isTyVarClassPred ty = case getClassPredTys_maybe ty of
        Just (_, tys) -> all isTyVarTy tys
        _             -> False

    -------------------------
    checkValidClsArgs :: Bool -> Class -> [KindOrType] -> Bool
    -- If the Bool is True (flexible contexts), return True (i.e. ok)
    -- Otherwise, check that the type (not kind) args are all headed by a tyvar
    --   E.g. (Eq a) accepted, (Eq (f a)) accepted, but (Eq Int) rejected
    -- This function is here rather than in TcValidity because it is
    -- called from TcSimplify, which itself is imported by TcValidity
    checkValidClsArgs flexible_contexts cls kts
      | flexible_contexts = True
      | otherwise         = all hasTyVarHead tys
      where
        tys = filterOutInvisibleTypes (classTyCon cls) kts

    hasTyVarHead :: Type -> Bool
    -- Returns true of (a t1 .. tn), where 'a' is a type variable
    hasTyVarHead ty                 -- Haskell 98 allows predicates of form
      | tcIsTyVarTy ty = True       --      C (a ty1 .. tyn)
      | otherwise                   -- where a is a type variable
      = case tcSplitAppTy_maybe ty of
           Just (ty, _) -> hasTyVarHead ty
           Nothing      -> False

    evVarPred_maybe :: EvVar -> Maybe PredType
    evVarPred_maybe v = if isPredTy ty then Just ty else Nothing
      where ty = varType v

    evVarPred :: EvVar -> PredType
    evVarPred var
     | debugIsOn
      = case evVarPred_maybe var of
          Just pred -> pred
          Nothing   -> pprPanic "tcEvVarPred" (ppr var <+> ppr (varType var))
     | otherwise
      = varType var

    ------------------
    -- | When inferring types, should we quantify over a given predicate?
    -- Generally true of classes; generally false of equality constraints.
    -- Equality constraints that mention quantified type variables and
    -- implicit variables complicate the story. See Notes
    -- [Inheriting implicit parameters] and [Quantifying over equality constraints]
    pickQuantifiablePreds
      :: TyVarSet           -- Quantifying over these
      -> TcThetaType        -- Proposed constraints to quantify
      -> TcThetaType        -- A subset that we can actually quantify
    -- This function decides whether a particular constraint should be
    -- quantified over, given the type variables that are being quantified
    pickQuantifiablePreds qtvs theta
      = let flex_ctxt = True in  -- Quantify over non-tyvar constraints, even without
                                 -- -XFlexibleContexts: see Trac #10608, #10351
             -- flex_ctxt <- xoptM Opt_FlexibleContexts
        filter (pick_me flex_ctxt) theta
      where
        pick_me flex_ctxt pred
          = case classifyPredType pred of

              ClassPred cls tys
                | Just {} <- isCallStackPred pred
                  -- NEVER infer a CallStack constraint
                  -- Otherwise, we let the constraints bubble up to be
                  -- solved from the outer context, or be defaulted when we
                  -- reach the top-level.
                  -- see Note [Overview of implicit CallStacks]
                  -> False

                | isIPClass cls    -> True -- See note [Inheriting implicit parameters]

                | otherwise
                  -> pick_cls_pred flex_ctxt cls tys

              EqPred ReprEq ty1 ty2 -> pick_cls_pred flex_ctxt coercibleClass [ty1, ty2]
                -- representational equality is like a class constraint

              EqPred NomEq ty1 ty2  -> quant_fun ty1 || quant_fun ty2
              IrredPred ty          -> tyCoVarsOfType ty `intersectsVarSet` qtvs

        pick_cls_pred flex_ctxt cls tys
          = tyCoVarsOfTypes tys `intersectsVarSet` qtvs
            && (checkValidClsArgs flex_ctxt cls tys)
               -- Only quantify over predicates that checkValidType
               -- will pass!  See Trac #10351.

        -- See Note [Quantifying over equality constraints]
        quant_fun ty
          = case tcSplitTyConApp_maybe ty of
              Just (tc, tys) | isTypeFamilyTyCon tc
                             -> tyCoVarsOfTypes tys `intersectsVarSet` qtvs
              _ -> False

    pickCapturedPreds
      :: TyVarSet           -- Quantifying over these
      -> TcThetaType        -- Proposed constraints to quantify
      -> TcThetaType        -- A subset that we can actually quantify
    -- A simpler version of pickQuantifiablePreds, used to winnow down
    -- the inferred constrains of a group of bindings, into those for
    -- one particular identifier
    pickCapturedPreds qtvs theta
      = filter captured theta
      where
        captured pred = isIPPred pred || (tyCoVarsOfType pred `intersectsVarSet` qtvs)

    -- Superclasses

    type PredWithSCs = (PredType, [PredType])

    mkMinimalBySCs :: [PredType] -> [PredType]
    -- Remove predicates that can be deduced from others by superclasses,
    -- including duplicate predicates. The result is a subset of the input.
    mkMinimalBySCs ptys = go preds_with_scs []
     where
       preds_with_scs :: [PredWithSCs]
       preds_with_scs = [ (pred, pred : transSuperClasses pred)
                        | pred <- ptys ]

       go :: [PredWithSCs]   -- Work list
          -> [PredWithSCs]   -- Accumulating result
          -> [PredType]
       go [] min_preds = map fst min_preds
       go (work_item@(p,_) : work_list) min_preds
         | p `in_cloud` work_list || p `in_cloud` min_preds
         = go work_list min_preds
         | otherwise
         = go work_list (work_item : min_preds)

       in_cloud :: PredType -> [PredWithSCs] -> Bool
       in_cloud p ps = or [ p `eqType` p' | (_, scs) <- ps, p' <- scs ]

    transSuperClasses :: PredType -> [PredType]
    -- (transSuperClasses p) returns (p's superclasses) not including p
    -- Stop if you encounter the same class again
    -- See Note [Expanding superclasses]
    transSuperClasses p
      = go emptyNameSet p
      where
        go :: NameSet -> PredType -> [PredType]
        go rec_clss p
           | ClassPred cls tys <- classifyPredType p
           , let cls_nm = className cls
           , not (cls_nm `elemNameSet` rec_clss)
           , let rec_clss' | isCTupleClass cls = rec_clss
                           | otherwise         = rec_clss `extendNameSet` cls_nm
           = [ p' | sc <- immSuperClasses cls tys
                  , p'  <- sc : go rec_clss' sc ]
           | otherwise
           = []

    immSuperClasses :: Class -> [Type] -> [PredType]
    immSuperClasses cls tys
      = substTheta (zipTvSubst tyvars tys) sc_theta
      where
        (tyvars,sc_theta,_,_) = classBigSig cls

    isImprovementPred :: PredType -> Bool
    -- Either it's an equality, or has some functional dependency
    isImprovementPred ty
      = case classifyPredType ty of
          EqPred NomEq t1 t2 -> not (t1 `tcEqType` t2)
          EqPred ReprEq _ _  -> False
          ClassPred cls _    -> classHasFds cls
          IrredPred {}       -> True -- Might have equalities after reduction?

    {- Note [Expanding superclasses]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    When we expand superclasses, we use the following algorithm:

    expand( so_far, pred ) returns the transitive superclasses of pred,
                                   not including pred itself
     1. If pred is not a class constraint, return empty set
           Otherwise pred = C ts
     2. If C is in so_far, return empty set (breaks loops)
     3. Find the immediate superclasses constraints of (C ts)
     4. For each such sc_pred, return (sc_pred : expand( so_far+C, D ss )

    Notice that

     * With normal Haskell-98 classes, the loop-detector will never bite,
       so we'll get all the superclasses.

     * Since there is only a finite number of distinct classes, expansion
       must terminate.

     * The loop breaking is a bit conservative. Notably, a tuple class
       could contain many times without threatening termination:
          (Eq a, (Ord a, Ix a))
       And this is try of any class that we can statically guarantee
       as non-recursive (in some sense).  For now, we just make a special
       case for tuples.  Something better would be cool.

    See also TcTyDecls.checkClassCycles.

    Note [Inheriting implicit parameters]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider this:

            f x = (x::Int) + ?y

    where f is *not* a top-level binding.
    From the RHS of f we'll get the constraint (?y::Int).
    There are two types we might infer for f:

            f :: Int -> Int

    (so we get ?y from the context of f's definition), or

            f :: (?y::Int) => Int -> Int

    At first you might think the first was better, because then
    ?y behaves like a free variable of the definition, rather than
    having to be passed at each call site.  But of course, the WHOLE
    IDEA is that ?y should be passed at each call site (that's what
    dynamic binding means) so we'd better infer the second.

    BOTTOM LINE: when *inferring types* you must quantify over implicit
    parameters, *even if* they don't mention the bound type variables.
    Reason: because implicit parameters, uniquely, have local instance
    declarations. See pickQuantifiablePreds.

    Note [Quantifying over equality constraints]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Should we quantify over an equality constraint (s ~ t)?  In general, we don't.
    Doing so may simply postpone a type error from the function definition site to
    its call site.  (At worst, imagine (Int ~ Bool)).

    However, consider this
             forall a. (F [a] ~ Int) => blah
    Should we quantify over the (F [a] ~ Int).  Perhaps yes, because at the call
    site we will know 'a', and perhaps we have instance  F [Bool] = Int.
    So we *do* quantify over a type-family equality where the arguments mention
    the quantified variables.

    ************************************************************************
    *                                                                      *
    \subsection{Predicates}
    *                                                                      *
    ************************************************************************
    -}

    isSigmaTy :: TcType -> Bool
    -- isSigmaTy returns true of any qualified type.  It doesn't
    -- *necessarily* have any foralls.  E.g
    --        f :: (?x::Int) => Int -> Int
    isSigmaTy ty | Just ty' <- tcView ty = isSigmaTy ty'
    isSigmaTy (ForAllTy {}) = True
    isSigmaTy (FunTy a _)   = isPredTy a
    isSigmaTy _             = False

    isRhoTy :: TcType -> Bool   -- True of TcRhoTypes; see Note [TcRhoType]
    isRhoTy ty | Just ty' <- tcView ty = isRhoTy ty'
    isRhoTy (ForAllTy {}) = False
    isRhoTy (FunTy a r)   = not (isPredTy a) && isRhoTy r
    isRhoTy _             = True

    -- | Like 'isRhoTy', but also says 'True' for 'Infer' types
    isRhoExpTy :: ExpType -> Bool
    isRhoExpTy (Check ty) = isRhoTy ty
    isRhoExpTy (Infer {}) = True

    isOverloadedTy :: Type -> Bool
    -- Yes for a type of a function that might require evidence-passing
    -- Used only by bindLocalMethods
    isOverloadedTy ty | Just ty' <- tcView ty = isOverloadedTy ty'
    isOverloadedTy (ForAllTy _  ty) = isOverloadedTy ty
    isOverloadedTy (FunTy a _)      = isPredTy a
    isOverloadedTy _                = False

    isFloatTy, isDoubleTy, isIntegerTy, isIntTy, isWordTy, isBoolTy,
        isUnitTy, isCharTy, isAnyTy :: Type -> Bool
    isFloatTy      = is_tc floatTyConKey
    isDoubleTy     = is_tc doubleTyConKey
    isIntegerTy    = is_tc integerTyConKey
    isIntTy        = is_tc intTyConKey
    isWordTy       = is_tc wordTyConKey
    isBoolTy       = is_tc boolTyConKey
    isUnitTy       = is_tc unitTyConKey
    isCharTy       = is_tc charTyConKey
    isAnyTy        = is_tc anyTyConKey

    -- | Does a type represent a floating-point number?
    isFloatingTy :: Type -> Bool
    isFloatingTy ty = isFloatTy ty || isDoubleTy ty

    -- | Is a type 'String'?
    isStringTy :: Type -> Bool
    isStringTy ty
      = case tcSplitTyConApp_maybe ty of
          Just (tc, [arg_ty]) -> tc == listTyCon && isCharTy arg_ty
          _                   -> False

    -- | Is a type a 'CallStack'?
    isCallStackTy :: Type -> Bool
    isCallStackTy ty
      | Just tc <- tyConAppTyCon_maybe ty
      = tc `hasKey` callStackTyConKey
      | otherwise
      = False

    -- | Is a 'PredType' a 'CallStack' implicit parameter?
    --
    -- If so, return the name of the parameter.
    isCallStackPred :: PredType -> Maybe FastString
    isCallStackPred pred
      | Just (str, ty) <- isIPPred_maybe pred
      , isCallStackTy ty
      = Just str
      | otherwise
      = Nothing

    is_tc :: Unique -> Type -> Bool
    -- Newtypes are opaque to this
    is_tc uniq ty = case tcSplitTyConApp_maybe ty of
                            Just (tc, _) -> uniq == getUnique tc
                            Nothing      -> False

    -- | Does the given tyvar appear in the given type outside of any
    -- non-newtypes? Assume we're looking for @a@. Says "yes" for
    -- @a@, @N a@, @b a@, @a b@, @b (N a)@. Says "no" for
    -- @[a]@, @Maybe a@, @T a@, where @N@ is a newtype and @T@ is a datatype.
    isTyVarExposed :: TcTyVar -> TcType -> Bool
    isTyVarExposed tv (TyVarTy tv')   = tv == tv'
    isTyVarExposed tv (TyConApp tc tys)
      | isNewTyCon tc                 = any (isTyVarExposed tv) tys
      | otherwise                     = False
    isTyVarExposed _  (LitTy {})      = False
    isTyVarExposed tv (AppTy fun arg) = isTyVarExposed tv fun
                                     || isTyVarExposed tv arg
    isTyVarExposed _  (ForAllTy {})   = False
    isTyVarExposed _  (FunTy {})      = False
    isTyVarExposed tv (CastTy ty _)   = isTyVarExposed tv ty
    isTyVarExposed _  (CoercionTy {}) = False

    -- | Is the equality
    --        a ~r ...a....
    -- definitely insoluble or not?
    --      a ~r Maybe a      -- Definitely insoluble
    --      a ~N ...(F a)...  -- Not definitely insoluble
    --                        -- Perhaps (F a) reduces to Int
    --      a ~R ...(N a)...  -- Not definitely insoluble
    --                        -- Perhaps newtype N a = MkN Int
    -- See Note [Occurs check error] in
    -- TcCanonical for the motivation for this function.
    isInsolubleOccursCheck :: EqRel -> TcTyVar -> TcType -> Bool
    isInsolubleOccursCheck eq_rel tv ty
      = go ty
      where
        go ty | Just ty' <- tcView ty = go ty'
        go (TyVarTy tv') = tv == tv' || go (tyVarKind tv')
        go (LitTy {})    = False
        go (AppTy t1 t2) = go t1 || go t2
        go (FunTy t1 t2) = go t1 || go t2
        go (ForAllTy (TvBndr tv' _) inner_ty)
          | tv' == tv = False
          | otherwise = go (tyVarKind tv') || go inner_ty
        go (CastTy ty _)  = go ty   -- ToDo: what about the coercion
        go (CoercionTy _) = False   -- ToDo: what about the coercion
        go (TyConApp tc tys)
          | isGenerativeTyCon tc role = any go tys
          | otherwise                 = False

        role = eqRelRole eq_rel

    isRigidTy :: TcType -> Bool
    isRigidTy ty
      | Just (tc,_) <- tcSplitTyConApp_maybe ty = isGenerativeTyCon tc Nominal
      | Just {} <- tcSplitAppTy_maybe ty        = True
      | isForAllTy ty                           = True
      | otherwise                               = False

    isRigidEqPred :: TcLevel -> PredTree -> Bool
    -- ^ True of all Nominal equalities that are solidly insoluble
    -- This means all equalities *except*
    --   * Meta-tv non-SigTv on LHS
    --   * Meta-tv SigTv on LHS, tyvar on right
    isRigidEqPred tc_lvl (EqPred NomEq ty1 _)
      | Just tv1 <- tcGetTyVar_maybe ty1
      = ASSERT2( tcIsTcTyVar tv1, ppr tv1 )
        not (isMetaTyVar tv1) || isTouchableMetaTyVar tc_lvl tv1

      | otherwise  -- LHS is not a tyvar
      = True

    isRigidEqPred _ _ = False  -- Not an equality

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Transformation of Types to TcTypes}
    *                                                                      *
    ************************************************************************
    -}

    toTcType :: Type -> TcType
    -- The constraint solver expects EvVars to have TcType, in which the
    -- free type variables are TcTyVars. So we convert from Type to TcType here
    -- A bit tiresome; but one day I expect the two types to be entirely separate
    -- in which case we'll definitely need to do this
    toTcType = runIdentity . to_tc_type emptyVarSet

    toTcTypeBag :: Bag EvVar -> Bag EvVar -- All TyVars are transformed to TcTyVars
    toTcTypeBag evvars = mapBag (\tv -> setTyVarKind tv (toTcType (tyVarKind tv))) evvars

    to_tc_mapper :: TyCoMapper VarSet Identity
    to_tc_mapper
      = TyCoMapper { tcm_smart    = False   -- more efficient not to use smart ctors
                   , tcm_tyvar    = tyvar
                   , tcm_covar    = covar
                   , tcm_hole     = hole
                   , tcm_tybinder = tybinder }
      where
        tyvar :: VarSet -> TyVar -> Identity Type
        tyvar ftvs tv
          | Just var <- lookupVarSet ftvs tv = return $ TyVarTy var
          | isTcTyVar tv = TyVarTy <$> updateTyVarKindM (to_tc_type ftvs) tv
          | otherwise
          = do { kind' <- to_tc_type ftvs (tyVarKind tv)
               ; return $ TyVarTy $ mkTcTyVar (tyVarName tv) kind' vanillaSkolemTv }

        covar :: VarSet -> CoVar -> Identity Coercion
        covar ftvs cv
          | Just var <- lookupVarSet ftvs cv = return $ CoVarCo var
          | otherwise = CoVarCo <$> updateVarTypeM (to_tc_type ftvs) cv

        hole :: VarSet -> CoercionHole -> Role -> Type -> Type
             -> Identity Coercion
        hole ftvs h r t1 t2 = mkHoleCo h r <$> to_tc_type ftvs t1
                                           <*> to_tc_type ftvs t2

        tybinder :: VarSet -> TyVar -> ArgFlag -> Identity (VarSet, TyVar)
        tybinder ftvs tv _vis = do { kind' <- to_tc_type ftvs (tyVarKind tv)
                                   ; let tv' = mkTcTyVar (tyVarName tv) kind'
                                                         vanillaSkolemTv
                                   ; return (ftvs `extendVarSet` tv', tv') }

    to_tc_type :: VarSet -> Type -> Identity TcType
    to_tc_type = mapType to_tc_mapper

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Misc}
    *                                                                      *
    ************************************************************************

    Note [Visible type application]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    GHC implements a generalisation of the algorithm described in the
    "Visible Type Application" paper (available from
    http://www.cis.upenn.edu/~sweirich/publications.html). A key part
    of that algorithm is to distinguish user-specified variables from inferred
    variables. For example, the following should typecheck:

      f :: forall a b. a -> b -> b
      f = const id

      g = const id

      x = f @Int @Bool 5 False
      y = g 5 @Bool False

    The idea is that we wish to allow visible type application when we are
    instantiating a specified, fixed variable. In practice, specified, fixed
    variables are either written in a type signature (or
    annotation), OR are imported from another module. (We could do better here,
    for example by doing SCC analysis on parts of a module and considering any
    type from outside one's SCC to be fully specified, but this is very confusing to
    users. The simple rule above is much more straightforward and predictable.)

    So, both of f's quantified variables are specified and may be instantiated.
    But g has no type signature, so only id's variable is specified (because id
    is imported). We write the type of g as forall {a}. a -> forall b. b -> b.
    Note that the a is in braces, meaning it cannot be instantiated with
    visible type application.

    Tracking specified vs. inferred variables is done conveniently by a field
    in TyBinder.

    -}

    deNoteType :: Type -> Type
    -- Remove all *outermost* type synonyms and other notes
    deNoteType ty | Just ty' <- coreView ty = deNoteType ty'
    deNoteType ty = ty

    {-
    Find the free tycons and classes of a type.  This is used in the front
    end of the compiler.
    -}

    {-
    ************************************************************************
    *                                                                      *
    \subsection[TysWiredIn-ext-type]{External types}
    *                                                                      *
    ************************************************************************

    The compiler's foreign function interface supports the passing of a
    restricted set of types as arguments and results (the restricting factor
    being the )
    -}

    tcSplitIOType_maybe :: Type -> Maybe (TyCon, Type)
    -- (tcSplitIOType_maybe t) returns Just (IO,t',co)
    --              if co : t ~ IO t'
    --              returns Nothing otherwise
    tcSplitIOType_maybe ty
      = case tcSplitTyConApp_maybe ty of
            Just (io_tycon, [io_res_ty])
             | io_tycon `hasKey` ioTyConKey ->
                Just (io_tycon, io_res_ty)
            _ ->
                Nothing

    isFFITy :: Type -> Bool
    -- True for any TyCon that can possibly be an arg or result of an FFI call
    isFFITy ty = isValid (checkRepTyCon legalFFITyCon ty)

    isFFIArgumentTy :: DynFlags -> Safety -> Type -> Validity
    -- Checks for valid argument type for a 'foreign import'
    isFFIArgumentTy dflags safety ty
       = checkRepTyCon (legalOutgoingTyCon dflags safety) ty

    isFFIExternalTy :: Type -> Validity
    -- Types that are allowed as arguments of a 'foreign export'
    isFFIExternalTy ty = checkRepTyCon legalFEArgTyCon ty

    isFFIImportResultTy :: DynFlags -> Type -> Validity
    isFFIImportResultTy dflags ty
      = checkRepTyCon (legalFIResultTyCon dflags) ty

    isFFIExportResultTy :: Type -> Validity
    isFFIExportResultTy ty = checkRepTyCon legalFEResultTyCon ty

    isFFIDynTy :: Type -> Type -> Validity
    -- The type in a foreign import dynamic must be Ptr, FunPtr, or a newtype of
    -- either, and the wrapped function type must be equal to the given type.
    -- We assume that all types have been run through normaliseFfiType, so we don't
    -- need to worry about expanding newtypes here.
    isFFIDynTy expected ty
        -- Note [Foreign import dynamic]
        -- In the example below, expected would be 'CInt -> IO ()', while ty would
        -- be 'FunPtr (CDouble -> IO ())'.
        | Just (tc, [ty']) <- splitTyConApp_maybe ty
        , tyConUnique tc `elem` [ptrTyConKey, funPtrTyConKey]
        , eqType ty' expected
        = IsValid
        | otherwise
        = NotValid (vcat [ text "Expected: Ptr/FunPtr" <+> pprParendType expected <> comma
                         , text "  Actual:" <+> ppr ty ])

    isFFILabelTy :: Type -> Validity
    -- The type of a foreign label must be Ptr, FunPtr, or a newtype of either.
    isFFILabelTy ty = checkRepTyCon ok ty
      where
        ok tc | tc `hasKey` funPtrTyConKey || tc `hasKey` ptrTyConKey
              = IsValid
              | otherwise
              = NotValid (text "A foreign-imported address (via &foo) must have type (Ptr a) or (FunPtr a)")

    isFFIPrimArgumentTy :: DynFlags -> Type -> Validity
    -- Checks for valid argument type for a 'foreign import prim'
    -- Currently they must all be simple unlifted types, or the well-known type
    -- Any, which can be used to pass the address to a Haskell object on the heap to
    -- the foreign function.
    isFFIPrimArgumentTy dflags ty
      | isAnyTy ty = IsValid
      | otherwise  = checkRepTyCon (legalFIPrimArgTyCon dflags) ty

    isFFIPrimResultTy :: DynFlags -> Type -> Validity
    -- Checks for valid result type for a 'foreign import prim' Currently
    -- it must be an unlifted type, including unboxed tuples, unboxed
    -- sums, or the well-known type Any.
    isFFIPrimResultTy dflags ty
      | isAnyTy ty = IsValid
      | otherwise = checkRepTyCon (legalFIPrimResultTyCon dflags) ty

    isFunPtrTy :: Type -> Bool
    isFunPtrTy ty
      | Just (tc, [_]) <- splitTyConApp_maybe ty
      = tc `hasKey` funPtrTyConKey
      | otherwise
      = False

    -- normaliseFfiType gets run before checkRepTyCon, so we don't
    -- need to worry about looking through newtypes or type functions
    -- here; that's already been taken care of.
    checkRepTyCon :: (TyCon -> Validity) -> Type -> Validity
    checkRepTyCon check_tc ty
      = case splitTyConApp_maybe ty of
          Just (tc, tys)
            | isNewTyCon tc -> NotValid (hang msg 2 (mk_nt_reason tc tys $$ nt_fix))
            | otherwise     -> case check_tc tc of
                                 IsValid        -> IsValid
                                 NotValid extra -> NotValid (msg $$ extra)
          Nothing -> NotValid (quotes (ppr ty) <+> text "is not a data type")
      where
        msg = quotes (ppr ty) <+> text "cannot be marshalled in a foreign call"
        mk_nt_reason tc tys
          | null tys  = text "because its data constructor is not in scope"
          | otherwise = text "because the data constructor for"
                        <+> quotes (ppr tc) <+> text "is not in scope"
        nt_fix = text "Possible fix: import the data constructor to bring it into scope"

    {-
    Note [Foreign import dynamic]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    A dynamic stub must be of the form 'FunPtr ft -> ft' where ft is any foreign
    type.  Similarly, a wrapper stub must be of the form 'ft -> IO (FunPtr ft)'.

    We use isFFIDynTy to check whether a signature is well-formed. For example,
    given a (illegal) declaration like:

    foreign import ccall "dynamic"
      foo :: FunPtr (CDouble -> IO ()) -> CInt -> IO ()

    isFFIDynTy will compare the 'FunPtr' type 'CDouble -> IO ()' with the curried
    result type 'CInt -> IO ()', and return False, as they are not equal.

    ----------------------------------------------
    These chaps do the work; they are not exported
    ----------------------------------------------
    -}

    legalFEArgTyCon :: TyCon -> Validity
    legalFEArgTyCon tc
      -- It's illegal to make foreign exports that take unboxed
      -- arguments.  The RTS API currently can't invoke such things.  --SDM 7/2000
      = boxedMarshalableTyCon tc

    legalFIResultTyCon :: DynFlags -> TyCon -> Validity
    legalFIResultTyCon dflags tc
      | tc == unitTyCon         = IsValid
      | otherwise               = marshalableTyCon dflags tc

    legalFEResultTyCon :: TyCon -> Validity
    legalFEResultTyCon tc
      | tc == unitTyCon         = IsValid
      | otherwise               = boxedMarshalableTyCon tc

    legalOutgoingTyCon :: DynFlags -> Safety -> TyCon -> Validity
    -- Checks validity of types going from Haskell -> external world
    legalOutgoingTyCon dflags _ tc
      = marshalableTyCon dflags tc

    legalFFITyCon :: TyCon -> Validity
    -- True for any TyCon that can possibly be an arg or result of an FFI call
    legalFFITyCon tc
      | isUnliftedTyCon tc = IsValid
      | tc == unitTyCon    = IsValid
      | otherwise          = boxedMarshalableTyCon tc

    marshalableTyCon :: DynFlags -> TyCon -> Validity
    marshalableTyCon dflags tc
      | isUnliftedTyCon tc
      , not (isUnboxedTupleTyCon tc || isUnboxedSumTyCon tc)
      , not (null (tyConPrimRep tc)) -- Note [Marshalling void]
      = validIfUnliftedFFITypes dflags
      | otherwise
      = boxedMarshalableTyCon tc

    boxedMarshalableTyCon :: TyCon -> Validity
    boxedMarshalableTyCon tc
       | getUnique tc `elem` [ intTyConKey, int8TyConKey, int16TyConKey
                             , int32TyConKey, int64TyConKey
                             , wordTyConKey, word8TyConKey, word16TyConKey
                             , word32TyConKey, word64TyConKey
                             , floatTyConKey, doubleTyConKey
                             , ptrTyConKey, funPtrTyConKey
                             , charTyConKey
                             , stablePtrTyConKey
                             , boolTyConKey
                             ]
      = IsValid

      | otherwise = NotValid empty

    legalFIPrimArgTyCon :: DynFlags -> TyCon -> Validity
    -- Check args of 'foreign import prim', only allow simple unlifted types.
    -- Strictly speaking it is unnecessary to ban unboxed tuples and sums here since
    -- currently they're of the wrong kind to use in function args anyway.
    legalFIPrimArgTyCon dflags tc
      | isUnliftedTyCon tc
      , not (isUnboxedTupleTyCon tc || isUnboxedSumTyCon tc)
      = validIfUnliftedFFITypes dflags
      | otherwise
      = NotValid unlifted_only

    legalFIPrimResultTyCon :: DynFlags -> TyCon -> Validity
    -- Check result type of 'foreign import prim'. Allow simple unlifted
    -- types and also unboxed tuple and sum result types.
    legalFIPrimResultTyCon dflags tc
      | isUnliftedTyCon tc
      , isUnboxedTupleTyCon tc || isUnboxedSumTyCon tc
         || not (null (tyConPrimRep tc))   -- Note [Marshalling void]
      = validIfUnliftedFFITypes dflags

      | otherwise
      = NotValid unlifted_only

    unlifted_only :: MsgDoc
    unlifted_only = text "foreign import prim only accepts simple unlifted types"

    validIfUnliftedFFITypes :: DynFlags -> Validity
    validIfUnliftedFFITypes dflags
      | xopt LangExt.UnliftedFFITypes dflags =  IsValid
      | otherwise = NotValid (text "To marshal unlifted types, use UnliftedFFITypes")

    {-
    Note [Marshalling void]
    ~~~~~~~~~~~~~~~~~~~~~~~
    We don't treat State# (whose PrimRep is VoidRep) as marshalable.
    In turn that means you can't write
            foreign import foo :: Int -> State# RealWorld

    Reason: the back end falls over with panic "primRepHint:VoidRep";
            and there is no compelling reason to permit it
    -}

    {-
    ************************************************************************
    *                                                                      *
            The "Paterson size" of a type
    *                                                                      *
    ************************************************************************
    -}

    {-
    Note [Paterson conditions on PredTypes]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We are considering whether *class* constraints terminate
    (see Note [Paterson conditions]). Precisely, the Paterson conditions
    would have us check that "the constraint has fewer constructors and variables
    (taken together and counting repetitions) than the head.".

    However, we can be a bit more refined by looking at which kind of constraint
    this actually is. There are two main tricks:

     1. It seems like it should be OK not to count the tuple type constructor
        for a PredType like (Show a, Eq a) :: Constraint, since we don't
        count the "implicit" tuple in the ThetaType itself.

        In fact, the Paterson test just checks *each component* of the top level
        ThetaType against the size bound, one at a time. By analogy, it should be
        OK to return the size of the *largest* tuple component as the size of the
        whole tuple.

     2. Once we get into an implicit parameter or equality we
        can't get back to a class constraint, so it's safe
        to say "size 0".  See Trac #4200.

    NB: we don't want to detect PredTypes in sizeType (and then call
    sizePred on them), or we might get an infinite loop if that PredType
    is irreducible. See Trac #5581.
    -}

    type TypeSize = IntWithInf

    sizeType :: Type -> TypeSize
    -- Size of a type: the number of variables and constructors
    -- Ignore kinds altogether
    sizeType = go
      where
        go ty | Just exp_ty <- tcView ty = go exp_ty
        go (TyVarTy {})              = 1
        go (TyConApp tc tys)
          | isTypeFamilyTyCon tc     = infinity  -- Type-family applications can
                                               -- expand to any arbitrary size
          | otherwise                = sizeTypes (filterOutInvisibleTypes tc tys) + 1
        go (LitTy {})                = 1
        go (FunTy arg res)           = go arg + go res + 1
        go (AppTy fun arg)           = go fun + go arg
        go (ForAllTy (TvBndr tv vis) ty)
            | isVisibleArgFlag vis   = go (tyVarKind tv) + go ty + 1
            | otherwise              = go ty + 1
        go (CastTy ty _)             = go ty
        go (CoercionTy {})           = 0

    sizeTypes :: [Type] -> TypeSize
    sizeTypes tys = sum (map sizeType tys)