    {-# LANGUAGE CPP                 #-}
    {-# LANGUAGE ScopedTypeVariables #-}
    {-# LANGUAGE TupleSections       #-}
    {-# LANGUAGE BlockArguments      #-}

    {-# OPTIONS_GHC -Wno-incomplete-uni-patterns   #-}
    {-# OPTIONS_GHC -Wno-incomplete-record-updates #-}

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

    -- | Type subsumption and unification
    module GHC.Tc.Utils.Unify (
      -- Full-blown subsumption
      tcWrapResult, tcWrapResultO, tcWrapResultMono,
      tcSkolemise, tcSkolemiseScoped, tcSkolemiseET,
      tcSubType, tcSubTypeSigma, tcSubTypePat,
      tcSubMult,
      checkConstraints, checkTvConstraints,
      buildImplicationFor, buildTvImplication, emitResidualTvConstraint,

      -- Various unifications
      unifyType, unifyKind, unifyExpectedType,
      uType, promoteTcType,
      swapOverTyVars, canSolveByUnification,

      --------------------------------
      -- Holes
      tcInfer,
      matchExpectedListTy,
      matchExpectedTyConApp,
      matchExpectedAppTy,
      matchExpectedFunTys,
      matchExpectedFunKind,
      matchActualFunTySigma, matchActualFunTysRho,

      checkTyVarEq, checkTyFamEq, checkTypeEq

      ) where

    #include "HsVersions.h"

    import GHC.Prelude

    import GHC.Hs
    import GHC.Core.TyCo.Rep
    import GHC.Core.TyCo.Ppr( debugPprType )
    import GHC.Tc.Utils.TcMType
    import GHC.Tc.Utils.Monad
    import GHC.Tc.Utils.TcType
    import GHC.Tc.Utils.Env
    import GHC.Core.Type
    import GHC.Core.Coercion
    import GHC.Core.Multiplicity
    import GHC.Tc.Types.Evidence
    import GHC.Tc.Types.Constraint
    import GHC.Tc.Types.Origin
    import GHC.Types.Name( isSystemName )
    import GHC.Tc.Utils.Instantiate
    import GHC.Core.TyCon
    import GHC.Builtin.Types
    import GHC.Types.Var as Var
    import GHC.Types.Var.Set
    import GHC.Types.Var.Env
    import GHC.Utils.Error
    import GHC.Driver.Session
    import GHC.Types.Basic
    import GHC.Data.Bag
    import GHC.Utils.Misc
    import GHC.Utils.Outputable as Outputable
    import GHC.Utils.Panic

    import GHC.Exts      ( inline )
    import Control.Monad
    import Control.Arrow ( second )
    import qualified Data.Semigroup as S ( (<>) )

    {- *********************************************************************
    *                                                                      *
                  matchActualFunTys
    *                                                                      *
    ********************************************************************* -}

    -- | matchActualFunTySigma does looks for just one function arrow
    --   returning an uninstantiated sigma-type
    matchActualFunTySigma
      :: SDoc -- See Note [Herald for matchExpectedFunTys]
      -> Maybe SDoc                    -- The thing with type TcSigmaType
      -> (Arity, [Scaled TcSigmaType]) -- Total number of value args in the call, and
                                       -- types of values args to which function has
                                       --   been applied already (reversed)
                                       -- Both are used only for error messages)
      -> TcRhoType                     -- Type to analyse: a TcRhoType
      -> TcM (HsWrapper, Scaled TcSigmaType, TcSigmaType)
    -- The /argument/ is a RhoType
    -- The /result/   is an (uninstantiated) SigmaType
    --
    -- See Note [matchActualFunTy error handling] for the first three arguments

    -- If   (wrap, arg_ty, res_ty) = matchActualFunTySigma ... fun_ty
    -- then wrap :: fun_ty ~> (arg_ty -> res_ty)
    -- and NB: res_ty is an (uninstantiated) SigmaType

    matchActualFunTySigma :: SDoc
    -> Maybe SDoc
    -> (Int, [Scaled Type])
    -> Type
    -> TcM (HsWrapper, Scaled Type, Type)
    matchActualFunTySigma SDoc
    herald Maybe SDoc
    mb_thing (Int, [Scaled Type])
    err_info Type
    fun_ty
      = ASSERT2( isRhoTy fun_ty, ppr fun_ty )
        Type -> TcM (HsWrapper, Scaled Type, Type)
    go Type
    fun_ty
      where
        -- Does not allocate unnecessary meta variables: if the input already is
        -- a function, we just take it apart.  Not only is this efficient,
        -- it's important for higher rank: the argument might be of form
        --              (forall a. ty) -> other
        -- If allocated (fresh-meta-var1 -> fresh-meta-var2) and unified, we'd
        -- hide the forall inside a meta-variable
        go :: TcRhoType   -- The type we're processing, perhaps after
                          -- expanding any type synonym
           -> TcM (HsWrapper, Scaled TcSigmaType, TcSigmaType)
        go :: Type -> TcM (HsWrapper, Scaled Type, Type)
    go Type
    ty | Just Type
    ty' <- Type -> Maybe Type
    tcView Type
    ty = Type -> TcM (HsWrapper, Scaled Type, Type)
    go Type
    ty'

        go (FunTy { ft_af :: Type -> AnonArgFlag
    ft_af = AnonArgFlag
    af, ft_mult :: Type -> Type
    ft_mult = Type
    w, ft_arg :: Type -> Type
    ft_arg = Type
    arg_ty, ft_res :: Type -> Type
    ft_res = Type
    res_ty })
          = ASSERT( af == VisArg )
            (HsWrapper, Scaled Type, Type)
    -> TcM (HsWrapper, Scaled Type, Type)
    forall (m :: * -> *) a. Monad m => a -> m a
    return (HsWrapper
    idHsWrapper, Type -> Type -> Scaled Type
    forall a. Type -> a -> Scaled a
    Scaled Type
    w Type
    arg_ty, Type
    res_ty)

        go ty :: Type
    ty@(TyVarTy TcTyVar
    tv)
          | TcTyVar -> Bool
    isMetaTyVar TcTyVar
    tv
          = do { MetaDetails
    cts <- TcTyVar -> TcM MetaDetails
    readMetaTyVar TcTyVar
    tv
               ; case MetaDetails
    cts of
                   Indirect Type
    ty' -> Type -> TcM (HsWrapper, Scaled Type, Type)
    go Type
    ty'
                   MetaDetails
    Flexi        -> Type -> TcM (HsWrapper, Scaled Type, Type)
    defer Type
    ty }

           -- In all other cases we bale out into ordinary unification
           -- However unlike the meta-tyvar case, we are sure that the
           -- number of arguments doesn't match arity of the original
           -- type, so we can add a bit more context to the error message
           -- (cf #7869).
           --
           -- It is not always an error, because specialized type may have
           -- different arity, for example:
           --
           -- > f1 = f2 'a'
           -- > f2 :: Monad m => m Bool
           -- > f2 = undefined
           --
           -- But in that case we add specialized type into error context
           -- anyway, because it may be useful. See also #9605.
        go Type
    ty = (TidyEnv -> TcM (TidyEnv, SDoc))
    -> TcM (HsWrapper, Scaled Type, Type)
    -> TcM (HsWrapper, Scaled Type, Type)
    forall a. (TidyEnv -> TcM (TidyEnv, SDoc)) -> TcM a -> TcM a
    addErrCtxtM (Type -> TidyEnv -> TcM (TidyEnv, SDoc)
    mk_ctxt Type
    ty) (Type -> TcM (HsWrapper, Scaled Type, Type)
    defer Type
    ty)

        ------------
        defer :: Type -> TcM (HsWrapper, Scaled Type, Type)
    defer Type
    fun_ty
          = do { Type
    arg_ty <- TcM Type
    newOpenFlexiTyVarTy
               ; Type
    res_ty <- TcM Type
    newOpenFlexiTyVarTy
               ; Type
    mult <- Type -> TcM Type
    newFlexiTyVarTy Type
    multiplicityTy
               ; let unif_fun_ty :: Type
    unif_fun_ty = Type -> Type -> Type -> Type
    mkVisFunTy Type
    mult Type
    arg_ty Type
    res_ty
               ; Coercion
    co <- Maybe SDoc -> Type -> Type -> TcM Coercion
    unifyType Maybe SDoc
    mb_thing Type
    fun_ty Type
    unif_fun_ty
               ; (HsWrapper, Scaled Type, Type)
    -> TcM (HsWrapper, Scaled Type, Type)
    forall (m :: * -> *) a. Monad m => a -> m a
    return (Coercion -> HsWrapper
    mkWpCastN Coercion
    co, Type -> Type -> Scaled Type
    forall a. Type -> a -> Scaled a
    Scaled Type
    mult Type
    arg_ty, Type
    res_ty) }

        ------------
        mk_ctxt :: TcType -> TidyEnv -> TcM (TidyEnv, SDoc)
        mk_ctxt :: Type -> TidyEnv -> TcM (TidyEnv, SDoc)
    mk_ctxt Type
    res_ty TidyEnv
    env = TidyEnv
    -> SDoc -> [Scaled Type] -> Type -> Int -> TcM (TidyEnv, SDoc)
    mkFunTysMsg TidyEnv
    env SDoc
    herald ([Scaled Type] -> [Scaled Type]
    forall a. [a] -> [a]
    reverse [Scaled Type]
    arg_tys_so_far)
                                         Type
    res_ty Int
    n_val_args_in_call
        (Int
    n_val_args_in_call, [Scaled Type]
    arg_tys_so_far) = (Int, [Scaled Type])
    err_info

    {- Note [matchActualFunTy error handling]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    matchActualFunTySigma is made much more complicated by the
    desire to produce good error messages. Consider the application
        f @Int x y
    In GHC.Tc.Gen.Expr.tcArgs we deal with visible type arguments,
    and then call matchActualFunTysPart for each individual value
    argument. It, in turn, must instantiate any type/dictionary args,
    before looking for an arrow type.

    But if it doesn't find an arrow type, it wants to generate a message
    like "f is applied to two arguments but its type only has one".
    To do that, it needs to know about the args that tcArgs has already
    munched up -- hence passing in n_val_args_in_call and arg_tys_so_far;
    and hence also the accumulating so_far arg to 'go'.

    This allows us (in mk_ctxt) to construct f's /instantiated/ type,
    with just the values-arg arrows, which is what we really want
    in the error message.

    Ugh!
    -}

    -- Like 'matchExpectedFunTys', but used when you have an "actual" type,
    -- for example in function application
    matchActualFunTysRho :: SDoc   -- See Note [Herald for matchExpectedFunTys]
                         -> CtOrigin
                         -> Maybe SDoc  -- the thing with type TcSigmaType
                         -> Arity
                         -> TcSigmaType
                         -> TcM (HsWrapper, [Scaled TcSigmaType], TcRhoType)
    -- If    matchActualFunTysRho n ty = (wrap, [t1,..,tn], res_ty)
    -- then  wrap : ty ~> (t1 -> ... -> tn -> res_ty)
    --       and res_ty is a RhoType
    -- NB: the returned type is top-instantiated; it's a RhoType
    matchActualFunTysRho :: SDoc
    -> CtOrigin
    -> Maybe SDoc
    -> Int
    -> Type
    -> TcM (HsWrapper, [Scaled Type], Type)
    matchActualFunTysRho SDoc
    herald CtOrigin
    ct_orig Maybe SDoc
    mb_thing Int
    n_val_args_wanted Type
    fun_ty
      = Int
    -> [Scaled Type] -> Type -> TcM (HsWrapper, [Scaled Type], Type)
    go Int
    n_val_args_wanted [] Type
    fun_ty
      where
        go :: Int
    -> [Scaled Type] -> Type -> TcM (HsWrapper, [Scaled Type], Type)
    go Int
    n [Scaled Type]
    so_far Type
    fun_ty
          | Bool -> Bool
    not (Type -> Bool
    isRhoTy Type
    fun_ty)
          = do { (HsWrapper
    wrap1, Type
    rho) <- CtOrigin -> Type -> TcM (HsWrapper, Type)
    topInstantiate CtOrigin
    ct_orig Type
    fun_ty
               ; (HsWrapper
    wrap2, [Scaled Type]
    arg_tys, Type
    res_ty) <- Int
    -> [Scaled Type] -> Type -> TcM (HsWrapper, [Scaled Type], Type)
    go Int
    n [Scaled Type]
    so_far Type
    rho
               ; (HsWrapper, [Scaled Type], Type)
    -> TcM (HsWrapper, [Scaled Type], Type)
    forall (m :: * -> *) a. Monad m => a -> m a
    return (HsWrapper
    wrap2 HsWrapper -> HsWrapper -> HsWrapper
    <.> HsWrapper
    wrap1, [Scaled Type]
    arg_tys, Type
    res_ty) }

        go Int
    0 [Scaled Type]
    _ Type
    fun_ty = (HsWrapper, [Scaled Type], Type)
    -> TcM (HsWrapper, [Scaled Type], Type)
    forall (m :: * -> *) a. Monad m => a -> m a
    return (HsWrapper
    idHsWrapper, [], Type
    fun_ty)

        go Int
    n [Scaled Type]
    so_far Type
    fun_ty
          = do { (HsWrapper
    wrap_fun1, Scaled Type
    arg_ty1, Type
    res_ty1) <- SDoc
    -> Maybe SDoc
    -> (Int, [Scaled Type])
    -> Type
    -> TcM (HsWrapper, Scaled Type, Type)
    matchActualFunTySigma
                                                     SDoc
    herald Maybe SDoc
    mb_thing
                                                     (Int
    n_val_args_wanted, [Scaled Type]
    so_far)
                                                     Type
    fun_ty
               ; (HsWrapper
    wrap_res, [Scaled Type]
    arg_tys, Type
    res_ty)   <- Int
    -> [Scaled Type] -> Type -> TcM (HsWrapper, [Scaled Type], Type)
    go (Int
    nInt -> Int -> Int
    forall a. Num a => a -> a -> a
    -Int
    1) (Scaled Type
    arg_ty1Scaled Type -> [Scaled Type] -> [Scaled Type]
    forall a. a -> [a] -> [a]
    :[Scaled Type]
    so_far) Type
    res_ty1
               ; let wrap_fun2 :: HsWrapper
    wrap_fun2 = HsWrapper -> HsWrapper -> Scaled Type -> Type -> SDoc -> HsWrapper
    mkWpFun HsWrapper
    idHsWrapper HsWrapper
    wrap_res Scaled Type
    arg_ty1 Type
    res_ty SDoc
    doc
               ; (HsWrapper, [Scaled Type], Type)
    -> TcM (HsWrapper, [Scaled Type], Type)
    forall (m :: * -> *) a. Monad m => a -> m a
    return (HsWrapper
    wrap_fun2 HsWrapper -> HsWrapper -> HsWrapper
    <.> HsWrapper
    wrap_fun1, Scaled Type
    arg_ty1Scaled Type -> [Scaled Type] -> [Scaled Type]
    forall a. a -> [a] -> [a]
    :[Scaled Type]
    arg_tys, Type
    res_ty) }
          where
            doc :: SDoc
    doc = String -> SDoc
    text String
    "When inferring the argument type of a function with type" SDoc -> SDoc -> SDoc
    <+>
                  SDoc -> SDoc
    quotes (Type -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Type
    fun_ty)

    {-
    ************************************************************************
    *                                                                      *
                 matchExpected functions
    *                                                                      *
    ************************************************************************

    Note [Herald for matchExpectedFunTys]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The 'herald' always looks like:
       "The equation(s) for 'f' have"
       "The abstraction (\x.e) takes"
       "The section (+ x) expects"
       "The function 'f' is applied to"

    This is used to construct a message of form

       The abstraction `\Just 1 -> ...' takes two arguments
       but its type `Maybe a -> a' has only one

       The equation(s) for `f' have two arguments
       but its type `Maybe a -> a' has only one

       The section `(f 3)' requires 'f' to take two arguments
       but its type `Int -> Int' has only one

       The function 'f' is applied to two arguments
       but its type `Int -> Int' has only one

    When visible type applications (e.g., `f @Int 1 2`, as in #13902) enter the
    picture, we have a choice in deciding whether to count the type applications as
    proper arguments:

       The function 'f' is applied to one visible type argument
         and two value arguments
       but its type `forall a. a -> a` has only one visible type argument
         and one value argument

    Or whether to include the type applications as part of the herald itself:

       The expression 'f @Int' is applied to two arguments
       but its type `Int -> Int` has only one

    The latter is easier to implement and is arguably easier to understand, so we
    choose to implement that option.

    Note [matchExpectedFunTys]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~
    matchExpectedFunTys checks that a sigma has the form
    of an n-ary function.  It passes the decomposed type to the
    thing_inside, and returns a wrapper to coerce between the two types

    It's used wherever a language construct must have a functional type,
    namely:
            A lambda expression
            A function definition
         An operator section

    This function must be written CPS'd because it needs to fill in the
    ExpTypes produced for arguments before it can fill in the ExpType
    passed in.

    -}

    -- Use this one when you have an "expected" type.
    -- This function skolemises at each polytype.
    matchExpectedFunTys :: forall a.
                           SDoc   -- See Note [Herald for matchExpectedFunTys]
                        -> UserTypeCtxt
                        -> Arity
                        -> ExpRhoType      -- Skolemised
                        -> ([Scaled ExpSigmaType] -> ExpRhoType -> TcM a)
                        -> TcM (HsWrapper, a)
    -- If    matchExpectedFunTys n ty = (_, wrap)
    -- then  wrap : (t1 -> ... -> tn -> ty_r) ~> ty,
    --   where [t1, ..., tn], ty_r are passed to the thing_inside
    matchExpectedFunTys :: forall a.
    SDoc
    -> UserTypeCtxt
    -> Int
    -> ExpRhoType
    -> ([Scaled ExpRhoType] -> ExpRhoType -> TcM a)
    -> TcM (HsWrapper, a)
    matchExpectedFunTys SDoc
    herald UserTypeCtxt
    ctx Int
    arity ExpRhoType
    orig_ty [Scaled ExpRhoType] -> ExpRhoType -> TcM a
    thing_inside
      = case ExpRhoType
    orig_ty of
          Check Type
    ty -> [Scaled ExpRhoType] -> Int -> Type -> TcM (HsWrapper, a)
    go [] Int
    arity Type
    ty
          ExpRhoType
    _        -> [Scaled ExpRhoType] -> Int -> ExpRhoType -> TcM (HsWrapper, a)
    defer [] Int
    arity ExpRhoType
    orig_ty
      where
        -- Skolemise any foralls /before/ the zero-arg case
        -- so that we guarantee to return a rho-type
        go :: [Scaled ExpRhoType] -> Int -> Type -> TcM (HsWrapper, a)
    go [Scaled ExpRhoType]
    acc_arg_tys Int
    n Type
    ty
          | ([TcTyVar]
    tvs, ThetaType
    theta, Type
    _) <- Type -> ([TcTyVar], ThetaType, Type)
    tcSplitSigmaTy Type
    ty
          , Bool -> Bool
    not ([TcTyVar] -> Bool
    forall (t :: * -> *) a. Foldable t => t a -> Bool
    null [TcTyVar]
    tvs Bool -> Bool -> Bool
    && ThetaType -> Bool
    forall (t :: * -> *) a. Foldable t => t a -> Bool
    null ThetaType
    theta)
          = do { (HsWrapper
    wrap_gen, (HsWrapper
    wrap_res, a
    result)) <- UserTypeCtxt
    -> Type
    -> (Type -> TcM (HsWrapper, a))
    -> TcM (HsWrapper, (HsWrapper, a))
    forall result.
    UserTypeCtxt
    -> Type -> (Type -> TcM result) -> TcM (HsWrapper, result)
    tcSkolemise UserTypeCtxt
    ctx Type
    ty ((Type -> TcM (HsWrapper, a)) -> TcM (HsWrapper, (HsWrapper, a)))
    -> (Type -> TcM (HsWrapper, a)) -> TcM (HsWrapper, (HsWrapper, a))
    forall a b. (a -> b) -> a -> b
    $ \Type
    ty' ->
                                                   [Scaled ExpRhoType] -> Int -> Type -> TcM (HsWrapper, a)
    go [Scaled ExpRhoType]
    acc_arg_tys Int
    n Type
    ty'
               ; (HsWrapper, a) -> TcM (HsWrapper, a)
    forall (m :: * -> *) a. Monad m => a -> m a
    return (HsWrapper
    wrap_gen HsWrapper -> HsWrapper -> HsWrapper
    <.> HsWrapper
    wrap_res, a
    result) }

        -- No more args; do this /before/ tcView, so
        -- that we do not unnecessarily unwrap synonyms
        go [Scaled ExpRhoType]
    acc_arg_tys Int
    0 Type
    rho_ty
          = do { a
    result <- [Scaled ExpRhoType] -> ExpRhoType -> TcM a
    thing_inside ([Scaled ExpRhoType] -> [Scaled ExpRhoType]
    forall a. [a] -> [a]
    reverse [Scaled ExpRhoType]
    acc_arg_tys) (Type -> ExpRhoType
    mkCheckExpType Type
    rho_ty)
               ; (HsWrapper, a) -> TcM (HsWrapper, a)
    forall (m :: * -> *) a. Monad m => a -> m a
    return (HsWrapper
    idHsWrapper, a
    result) }

        go [Scaled ExpRhoType]
    acc_arg_tys Int
    n Type
    ty
          | Just Type
    ty' <- Type -> Maybe Type
    tcView Type
    ty = [Scaled ExpRhoType] -> Int -> Type -> TcM (HsWrapper, a)
    go [Scaled ExpRhoType]
    acc_arg_tys Int
    n Type
    ty'

        go [Scaled ExpRhoType]
    acc_arg_tys Int
    n (FunTy { ft_mult :: Type -> Type
    ft_mult = Type
    mult, ft_af :: Type -> AnonArgFlag
    ft_af = AnonArgFlag
    af, ft_arg :: Type -> Type
    ft_arg = Type
    arg_ty, ft_res :: Type -> Type
    ft_res = Type
    res_ty })
          = ASSERT( af == VisArg )
            do { (HsWrapper
    wrap_res, a
    result) <- [Scaled ExpRhoType] -> Int -> Type -> TcM (HsWrapper, a)
    go ((Type -> ExpRhoType -> Scaled ExpRhoType
    forall a. Type -> a -> Scaled a
    Scaled Type
    mult (ExpRhoType -> Scaled ExpRhoType)
    -> ExpRhoType -> Scaled ExpRhoType
    forall a b. (a -> b) -> a -> b
    $ Type -> ExpRhoType
    mkCheckExpType Type
    arg_ty) Scaled ExpRhoType -> [Scaled ExpRhoType] -> [Scaled ExpRhoType]
    forall a. a -> [a] -> [a]
    : [Scaled ExpRhoType]
    acc_arg_tys)
                                          (Int
    nInt -> Int -> Int
    forall a. Num a => a -> a -> a
    -Int
    1) Type
    res_ty
               ; let fun_wrap :: HsWrapper
    fun_wrap = HsWrapper -> HsWrapper -> Scaled Type -> Type -> SDoc -> HsWrapper
    mkWpFun HsWrapper
    idHsWrapper HsWrapper
    wrap_res (Type -> Type -> Scaled Type
    forall a. Type -> a -> Scaled a
    Scaled Type
    mult Type
    arg_ty) Type
    res_ty SDoc
    doc
               ; (HsWrapper, a) -> TcM (HsWrapper, a)
    forall (m :: * -> *) a. Monad m => a -> m a
    return ( HsWrapper
    fun_wrap, a
    result ) }
          where
            doc :: SDoc
    doc = String -> SDoc
    text String
    "When inferring the argument type of a function with type" SDoc -> SDoc -> SDoc
    <+>
                  SDoc -> SDoc
    quotes (ExpRhoType -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr ExpRhoType
    orig_ty)

        go [Scaled ExpRhoType]
    acc_arg_tys Int
    n ty :: Type
    ty@(TyVarTy TcTyVar
    tv)
          | TcTyVar -> Bool
    isMetaTyVar TcTyVar
    tv
          = do { MetaDetails
    cts <- TcTyVar -> TcM MetaDetails
    readMetaTyVar TcTyVar
    tv
               ; case MetaDetails
    cts of
                   Indirect Type
    ty' -> [Scaled ExpRhoType] -> Int -> Type -> TcM (HsWrapper, a)
    go [Scaled ExpRhoType]
    acc_arg_tys Int
    n Type
    ty'
                   MetaDetails
    Flexi        -> [Scaled ExpRhoType] -> Int -> ExpRhoType -> TcM (HsWrapper, a)
    defer [Scaled ExpRhoType]
    acc_arg_tys Int
    n (Type -> ExpRhoType
    mkCheckExpType Type
    ty) }

           -- In all other cases we bale out into ordinary unification
           -- However unlike the meta-tyvar case, we are sure that the
           -- number of arguments doesn't match arity of the original
           -- type, so we can add a bit more context to the error message
           -- (cf #7869).
           --
           -- It is not always an error, because specialized type may have
           -- different arity, for example:
           --
           -- > f1 = f2 'a'
           -- > f2 :: Monad m => m Bool
           -- > f2 = undefined
           --
           -- But in that case we add specialized type into error context
           -- anyway, because it may be useful. See also #9605.
        go [Scaled ExpRhoType]
    acc_arg_tys Int
    n Type
    ty = (TidyEnv -> TcM (TidyEnv, SDoc))
    -> TcM (HsWrapper, a) -> TcM (HsWrapper, a)
    forall a. (TidyEnv -> TcM (TidyEnv, SDoc)) -> TcM a -> TcM a
    addErrCtxtM ([Scaled ExpRhoType] -> Type -> TidyEnv -> TcM (TidyEnv, SDoc)
    mk_ctxt [Scaled ExpRhoType]
    acc_arg_tys Type
    ty) (TcM (HsWrapper, a) -> TcM (HsWrapper, a))
    -> TcM (HsWrapper, a) -> TcM (HsWrapper, a)
    forall a b. (a -> b) -> a -> b
    $
                              [Scaled ExpRhoType] -> Int -> ExpRhoType -> TcM (HsWrapper, a)
    defer [Scaled ExpRhoType]
    acc_arg_tys Int
    n (Type -> ExpRhoType
    mkCheckExpType Type
    ty)

        ------------
        defer :: [Scaled ExpSigmaType] -> Arity -> ExpRhoType -> TcM (HsWrapper, a)
        defer :: [Scaled ExpRhoType] -> Int -> ExpRhoType -> TcM (HsWrapper, a)
    defer [Scaled ExpRhoType]
    acc_arg_tys Int
    n ExpRhoType
    fun_ty
          = do { [Scaled ExpRhoType]
    more_arg_tys <- Int
    -> IOEnv (Env TcGblEnv TcLclEnv) (Scaled ExpRhoType)
    -> IOEnv (Env TcGblEnv TcLclEnv) [Scaled ExpRhoType]
    forall (m :: * -> *) a. Applicative m => Int -> m a -> m [a]
    replicateM Int
    n (Type -> ExpRhoType -> Scaled ExpRhoType
    forall a. Type -> a -> Scaled a
    mkScaled (Type -> ExpRhoType -> Scaled ExpRhoType)
    -> TcM Type
    -> IOEnv (Env TcGblEnv TcLclEnv) (ExpRhoType -> Scaled ExpRhoType)
    forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    <$> Type -> TcM Type
    newFlexiTyVarTy Type
    multiplicityTy IOEnv (Env TcGblEnv TcLclEnv) (ExpRhoType -> Scaled ExpRhoType)
    -> IOEnv (Env TcGblEnv TcLclEnv) ExpRhoType
    -> IOEnv (Env TcGblEnv TcLclEnv) (Scaled ExpRhoType)
    forall (f :: * -> *) a b. Applicative f => f (a -> b) -> f a -> f b
    <*> IOEnv (Env TcGblEnv TcLclEnv) ExpRhoType
    newInferExpType)
               ; ExpRhoType
    res_ty       <- IOEnv (Env TcGblEnv TcLclEnv) ExpRhoType
    newInferExpType
               ; a
    result       <- [Scaled ExpRhoType] -> ExpRhoType -> TcM a
    thing_inside ([Scaled ExpRhoType] -> [Scaled ExpRhoType]
    forall a. [a] -> [a]
    reverse [Scaled ExpRhoType]
    acc_arg_tys [Scaled ExpRhoType] -> [Scaled ExpRhoType] -> [Scaled ExpRhoType]
    forall a. [a] -> [a] -> [a]
    ++ [Scaled ExpRhoType]
    more_arg_tys) ExpRhoType
    res_ty
               ; [Scaled Type]
    more_arg_tys <- (Scaled ExpRhoType -> IOEnv (Env TcGblEnv TcLclEnv) (Scaled Type))
    -> [Scaled ExpRhoType]
    -> IOEnv (Env TcGblEnv TcLclEnv) [Scaled Type]
    forall (t :: * -> *) (m :: * -> *) a b.
    (Traversable t, Monad m) =>
    (a -> m b) -> t a -> m (t b)
    mapM (\(Scaled Type
    m ExpRhoType
    t) -> Type -> Type -> Scaled Type
    forall a. Type -> a -> Scaled a
    Scaled Type
    m (Type -> Scaled Type)
    -> TcM Type -> IOEnv (Env TcGblEnv TcLclEnv) (Scaled Type)
    forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    <$> ExpRhoType -> TcM Type
    readExpType ExpRhoType
    t) [Scaled ExpRhoType]
    more_arg_tys
               ; Type
    res_ty       <- ExpRhoType -> TcM Type
    readExpType ExpRhoType
    res_ty
               ; let unif_fun_ty :: Type
    unif_fun_ty = [Scaled Type] -> Type -> Type
    mkVisFunTys [Scaled Type]
    more_arg_tys Type
    res_ty
               ; HsWrapper
    wrap <- CtOrigin -> UserTypeCtxt -> Type -> ExpRhoType -> TcM HsWrapper
    tcSubType CtOrigin
    AppOrigin UserTypeCtxt
    ctx Type
    unif_fun_ty ExpRhoType
    fun_ty
                             -- Not a good origin at all :-(
               ; (HsWrapper, a) -> TcM (HsWrapper, a)
    forall (m :: * -> *) a. Monad m => a -> m a
    return (HsWrapper
    wrap, a
    result) }

        ------------
        mk_ctxt :: [Scaled ExpSigmaType] -> TcType -> TidyEnv -> TcM (TidyEnv, SDoc)
        mk_ctxt :: [Scaled ExpRhoType] -> Type -> TidyEnv -> TcM (TidyEnv, SDoc)
    mk_ctxt [Scaled ExpRhoType]
    arg_tys Type
    res_ty TidyEnv
    env
          = TidyEnv
    -> SDoc -> [Scaled Type] -> Type -> Int -> TcM (TidyEnv, SDoc)
    mkFunTysMsg TidyEnv
    env SDoc
    herald [Scaled Type]
    arg_tys' Type
    res_ty Int
    arity
          where
            arg_tys' :: [Scaled Type]
    arg_tys' = (Scaled ExpRhoType -> Scaled Type)
    -> [Scaled ExpRhoType] -> [Scaled Type]
    forall a b. (a -> b) -> [a] -> [b]
    map (\(Scaled Type
    u ExpRhoType
    v) -> Type -> Type -> Scaled Type
    forall a. Type -> a -> Scaled a
    Scaled Type
    u (String -> ExpRhoType -> Type
    checkingExpType String
    "matchExpectedFunTys" ExpRhoType
    v)) ([Scaled ExpRhoType] -> [Scaled Type])
    -> [Scaled ExpRhoType] -> [Scaled Type]
    forall a b. (a -> b) -> a -> b
    $
                       [Scaled ExpRhoType] -> [Scaled ExpRhoType]
    forall a. [a] -> [a]
    reverse [Scaled ExpRhoType]
    arg_tys
                -- this is safe b/c we're called from "go"

    mkFunTysMsg :: TidyEnv -> SDoc -> [Scaled TcType] -> TcType -> Arity
                -> TcM (TidyEnv, SDoc)
    mkFunTysMsg :: TidyEnv
    -> SDoc -> [Scaled Type] -> Type -> Int -> TcM (TidyEnv, SDoc)
    mkFunTysMsg TidyEnv
    env SDoc
    herald [Scaled Type]
    arg_tys Type
    res_ty Int
    n_val_args_in_call
      = do { (TidyEnv
    env', Type
    fun_rho) <- TidyEnv -> Type -> TcM (TidyEnv, Type)
    zonkTidyTcType TidyEnv
    env (Type -> TcM (TidyEnv, Type)) -> Type -> TcM (TidyEnv, Type)
    forall a b. (a -> b) -> a -> b
    $
                                [Scaled Type] -> Type -> Type
    mkVisFunTys [Scaled Type]
    arg_tys Type
    res_ty

           ; let ([Scaled Type]
    all_arg_tys, Type
    _) = Type -> ([Scaled Type], Type)
    splitFunTys Type
    fun_rho
                 n_fun_args :: Int
    n_fun_args = [Scaled Type] -> Int
    forall (t :: * -> *) a. Foldable t => t a -> Int
    length [Scaled Type]
    all_arg_tys

                 msg :: SDoc
    msg | Int
    n_val_args_in_call Int -> Int -> Bool
    forall a. Ord a => a -> a -> Bool
    <= Int
    n_fun_args  -- Enough args, in the end
                     = String -> SDoc
    text String
    "In the result of a function call"
                     | Bool
    otherwise
                     = SDoc -> Int -> SDoc -> SDoc
    hang (SDoc
    full_herald SDoc -> SDoc -> SDoc
    <> SDoc
    comma)
                          Int
    2 ([SDoc] -> SDoc
    sep [ String -> SDoc
    text String
    "but its type" SDoc -> SDoc -> SDoc
    <+> SDoc -> SDoc
    quotes (Type -> SDoc
    pprType Type
    fun_rho)
                                 , if Int
    n_fun_args Int -> Int -> Bool
    forall a. Eq a => a -> a -> Bool
    == Int
    0 then String -> SDoc
    text String
    "has none"
                                   else String -> SDoc
    text String
    "has only" SDoc -> SDoc -> SDoc
    <+> Int -> SDoc
    speakN Int
    n_fun_args])

           ; (TidyEnv, SDoc) -> TcM (TidyEnv, SDoc)
    forall (m :: * -> *) a. Monad m => a -> m a
    return (TidyEnv
    env', SDoc
    msg) }
     where
      full_herald :: SDoc
    full_herald = SDoc
    herald SDoc -> SDoc -> SDoc
    <+> Int -> SDoc -> SDoc
    speakNOf Int
    n_val_args_in_call (String -> SDoc
    text String
    "value argument")

    ----------------------
    matchExpectedListTy :: TcRhoType -> TcM (TcCoercionN, TcRhoType)
    -- Special case for lists
    matchExpectedListTy :: Type -> TcM (Coercion, Type)
    matchExpectedListTy Type
    exp_ty
     = do { (Coercion
    co, [Type
    elt_ty]) <- TyCon -> Type -> TcM (Coercion, ThetaType)
    matchExpectedTyConApp TyCon
    listTyCon Type
    exp_ty
          ; (Coercion, Type) -> TcM (Coercion, Type)
    forall (m :: * -> *) a. Monad m => a -> m a
    return (Coercion
    co, Type
    elt_ty) }

    ---------------------
    matchExpectedTyConApp :: TyCon                -- T :: forall kv1 ... kvm. k1 -> ... -> kn -> *
                          -> TcRhoType            -- orig_ty
                          -> TcM (TcCoercionN,    -- T k1 k2 k3 a b c ~N orig_ty
                                  [TcSigmaType])  -- Element types, k1 k2 k3 a b c

    -- It's used for wired-in tycons, so we call checkWiredInTyCon
    -- Precondition: never called with FunTyCon
    -- Precondition: input type :: *
    -- Postcondition: (T k1 k2 k3 a b c) is well-kinded

    matchExpectedTyConApp :: TyCon -> Type -> TcM (Coercion, ThetaType)
    matchExpectedTyConApp TyCon
    tc Type
    orig_ty
      = ASSERT(not $ isFunTyCon tc) go orig_ty
      where
        go :: Type -> TcM (Coercion, ThetaType)
    go Type
    ty
           | Just Type
    ty' <- Type -> Maybe Type
    tcView Type
    ty
           = Type -> TcM (Coercion, ThetaType)
    go Type
    ty'

        go ty :: Type
    ty@(TyConApp TyCon
    tycon ThetaType
    args)
           | TyCon
    tc TyCon -> TyCon -> Bool
    forall a. Eq a => a -> a -> Bool
    == TyCon
    tycon  -- Common case
           = (Coercion, ThetaType) -> TcM (Coercion, ThetaType)
    forall (m :: * -> *) a. Monad m => a -> m a
    return (Type -> Coercion
    mkTcNomReflCo Type
    ty, ThetaType
    args)

        go (TyVarTy TcTyVar
    tv)
           | TcTyVar -> Bool
    isMetaTyVar TcTyVar
    tv
           = do { MetaDetails
    cts <- TcTyVar -> TcM MetaDetails
    readMetaTyVar TcTyVar
    tv
                ; case MetaDetails
    cts of
                    Indirect Type
    ty -> Type -> TcM (Coercion, ThetaType)
    go Type
    ty
                    MetaDetails
    Flexi       -> TcM (Coercion, ThetaType)
    defer }

        go Type
    _ = TcM (Coercion, ThetaType)
    defer

        -- If the common case does not occur, instantiate a template
        -- T k1 .. kn t1 .. tm, and unify with the original type
        -- Doing it this way ensures that the types we return are
        -- kind-compatible with T.  For example, suppose we have
        --       matchExpectedTyConApp T (f Maybe)
        -- where data T a = MkT a
        -- Then we don't want to instantiate T's data constructors with
        --    (a::*) ~ Maybe
        -- because that'll make types that are utterly ill-kinded.
        -- This happened in #7368
        defer :: TcM (Coercion, ThetaType)
    defer
          = do { (TCvSubst
    _, [TcTyVar]
    arg_tvs) <- [TcTyVar] -> TcM (TCvSubst, [TcTyVar])
    newMetaTyVars (TyCon -> [TcTyVar]
    tyConTyVars TyCon
    tc)
               ; String -> SDoc -> TcRn ()
    traceTc String
    "matchExpectedTyConApp" (TyCon -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TyCon
    tc SDoc -> SDoc -> SDoc
    $$ [TcTyVar] -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr (TyCon -> [TcTyVar]
    tyConTyVars TyCon
    tc) SDoc -> SDoc -> SDoc
    $$ [TcTyVar] -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr [TcTyVar]
    arg_tvs)
               ; let args :: ThetaType
    args = [TcTyVar] -> ThetaType
    mkTyVarTys [TcTyVar]
    arg_tvs
                     tc_template :: Type
    tc_template = TyCon -> ThetaType -> Type
    mkTyConApp TyCon
    tc ThetaType
    args
               ; Coercion
    co <- Maybe SDoc -> Type -> Type -> TcM Coercion
    unifyType Maybe SDoc
    forall a. Maybe a
    Nothing Type
    tc_template Type
    orig_ty
               ; (Coercion, ThetaType) -> TcM (Coercion, ThetaType)
    forall (m :: * -> *) a. Monad m => a -> m a
    return (Coercion
    co, ThetaType
    args) }

    ----------------------
    matchExpectedAppTy :: TcRhoType                         -- orig_ty
                       -> TcM (TcCoercion,                   -- m a ~N orig_ty
                               (TcSigmaType, TcSigmaType))  -- Returns m, a
    -- If the incoming type is a mutable type variable of kind k, then
    -- matchExpectedAppTy returns a new type variable (m: * -> k); note the *.

    matchExpectedAppTy :: Type -> TcM (Coercion, (Type, Type))
    matchExpectedAppTy Type
    orig_ty
      = Type -> TcM (Coercion, (Type, Type))
    go Type
    orig_ty
      where
        go :: Type -> TcM (Coercion, (Type, Type))
    go Type
    ty
          | Just Type
    ty' <- Type -> Maybe Type
    tcView Type
    ty = Type -> TcM (Coercion, (Type, Type))
    go Type
    ty'

          | Just (Type
    fun_ty, Type
    arg_ty) <- Type -> Maybe (Type, Type)
    tcSplitAppTy_maybe Type
    ty
          = (Coercion, (Type, Type)) -> TcM (Coercion, (Type, Type))
    forall (m :: * -> *) a. Monad m => a -> m a
    return (Type -> Coercion
    mkTcNomReflCo Type
    orig_ty, (Type
    fun_ty, Type
    arg_ty))

        go (TyVarTy TcTyVar
    tv)
          | TcTyVar -> Bool
    isMetaTyVar TcTyVar
    tv
          = do { MetaDetails
    cts <- TcTyVar -> TcM MetaDetails
    readMetaTyVar TcTyVar
    tv
               ; case MetaDetails
    cts of
                   Indirect Type
    ty -> Type -> TcM (Coercion, (Type, Type))
    go Type
    ty
                   MetaDetails
    Flexi       -> TcM (Coercion, (Type, Type))
    defer }

        go Type
    _ = TcM (Coercion, (Type, Type))
    defer

        -- Defer splitting by generating an equality constraint
        defer :: TcM (Coercion, (Type, Type))
    defer
          = do { Type
    ty1 <- Type -> TcM Type
    newFlexiTyVarTy Type
    kind1
               ; Type
    ty2 <- Type -> TcM Type
    newFlexiTyVarTy Type
    kind2
               ; Coercion
    co <- Maybe SDoc -> Type -> Type -> TcM Coercion
    unifyType Maybe SDoc
    forall a. Maybe a
    Nothing (Type -> Type -> Type
    mkAppTy Type
    ty1 Type
    ty2) Type
    orig_ty
               ; (Coercion, (Type, Type)) -> TcM (Coercion, (Type, Type))
    forall (m :: * -> *) a. Monad m => a -> m a
    return (Coercion
    co, (Type
    ty1, Type
    ty2)) }

        orig_kind :: Type
    orig_kind = HasDebugCallStack => Type -> Type
    Type -> Type
    tcTypeKind Type
    orig_ty
        kind1 :: Type
    kind1 = Type -> Type -> Type
    mkVisFunTyMany Type
    liftedTypeKind Type
    orig_kind
        kind2 :: Type
    kind2 = Type
    liftedTypeKind    -- m :: * -> k
                                  -- arg type :: *

    {-
    ************************************************************************
    *                                                                      *
                    Subsumption checking
    *                                                                      *
    ************************************************************************

    Note [Subsumption checking: tcSubType]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    All the tcSubType calls have the form
                    tcSubType actual_ty expected_ty
    which checks
                    actual_ty <= expected_ty

    That is, that a value of type actual_ty is acceptable in
    a place expecting a value of type expected_ty.  I.e. that

        actual ty   is more polymorphic than   expected_ty

    It returns a wrapper function
            co_fn :: actual_ty ~ expected_ty
    which takes an HsExpr of type actual_ty into one of type
    expected_ty.
    -}

    -----------------
    -- tcWrapResult needs both un-type-checked (for origins and error messages)
    -- and type-checked (for wrapping) expressions
    tcWrapResult :: HsExpr GhcRn -> HsExpr GhcTc -> TcSigmaType -> ExpRhoType
                 -> TcM (HsExpr GhcTc)
    tcWrapResult :: HsExpr GhcRn
    -> HsExpr GhcTc -> Type -> ExpRhoType -> TcM (HsExpr GhcTc)
    tcWrapResult HsExpr GhcRn
    rn_expr = CtOrigin
    -> HsExpr GhcRn
    -> HsExpr GhcTc
    -> Type
    -> ExpRhoType
    -> TcM (HsExpr GhcTc)
    tcWrapResultO (HsExpr GhcRn -> CtOrigin
    exprCtOrigin HsExpr GhcRn
    rn_expr) HsExpr GhcRn
    rn_expr

    tcWrapResultO :: CtOrigin -> HsExpr GhcRn -> HsExpr GhcTc -> TcSigmaType -> ExpRhoType
                   -> TcM (HsExpr GhcTc)
    tcWrapResultO :: CtOrigin
    -> HsExpr GhcRn
    -> HsExpr GhcTc
    -> Type
    -> ExpRhoType
    -> TcM (HsExpr GhcTc)
    tcWrapResultO CtOrigin
    orig HsExpr GhcRn
    rn_expr HsExpr GhcTc
    expr Type
    actual_ty ExpRhoType
    res_ty
      = do { String -> SDoc -> TcRn ()
    traceTc String
    "tcWrapResult" ([SDoc] -> SDoc
    vcat [ String -> SDoc
    text String
    "Actual:  " SDoc -> SDoc -> SDoc
    <+> Type -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Type
    actual_ty
                                          , String -> SDoc
    text String
    "Expected:" SDoc -> SDoc -> SDoc
    <+> ExpRhoType -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr ExpRhoType
    res_ty ])
           ; HsWrapper
    wrap <- CtOrigin
    -> UserTypeCtxt
    -> Maybe SDoc
    -> Type
    -> ExpRhoType
    -> TcM HsWrapper
    tcSubTypeNC CtOrigin
    orig UserTypeCtxt
    GenSigCtxt (SDoc -> Maybe SDoc
    forall a. a -> Maybe a
    Just (HsExpr GhcRn -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr HsExpr GhcRn
    rn_expr)) Type
    actual_ty ExpRhoType
    res_ty
           ; HsExpr GhcTc -> TcM (HsExpr GhcTc)
    forall (m :: * -> *) a. Monad m => a -> m a
    return (HsWrapper -> HsExpr GhcTc -> HsExpr GhcTc
    mkHsWrap HsWrapper
    wrap HsExpr GhcTc
    expr) }

    tcWrapResultMono :: HsExpr GhcRn -> HsExpr GhcTc
                     -> TcRhoType   -- Actual -- a rho-type not a sigma-type
                     -> ExpRhoType  -- Expected
                     -> TcM (HsExpr GhcTc)
    -- A version of tcWrapResult to use when the actual type is a
    -- rho-type, so nothing to instantiate; just go straight to unify.
    -- It means we don't need to pass in a CtOrigin
    tcWrapResultMono :: HsExpr GhcRn
    -> HsExpr GhcTc -> Type -> ExpRhoType -> TcM (HsExpr GhcTc)
    tcWrapResultMono HsExpr GhcRn
    rn_expr HsExpr GhcTc
    expr Type
    act_ty ExpRhoType
    res_ty
      = ASSERT2( isRhoTy act_ty, ppr act_ty $$ ppr rn_expr )
        do { Coercion
    co <- HsExpr GhcRn -> Type -> ExpRhoType -> TcM Coercion
    unifyExpectedType HsExpr GhcRn
    rn_expr Type
    act_ty ExpRhoType
    res_ty
           ; HsExpr GhcTc -> TcM (HsExpr GhcTc)
    forall (m :: * -> *) a. Monad m => a -> m a
    return (Coercion -> HsExpr GhcTc -> HsExpr GhcTc
    mkHsWrapCo Coercion
    co HsExpr GhcTc
    expr) }

    unifyExpectedType :: HsExpr GhcRn
                      -> TcRhoType   -- Actual -- a rho-type not a sigma-type
                      -> ExpRhoType  -- Expected
                      -> TcM TcCoercionN
    unifyExpectedType :: HsExpr GhcRn -> Type -> ExpRhoType -> TcM Coercion
    unifyExpectedType HsExpr GhcRn
    rn_expr Type
    act_ty ExpRhoType
    exp_ty
      = case ExpRhoType
    exp_ty of
          Infer InferResult
    inf_res -> Type -> InferResult -> TcM Coercion
    fillInferResult Type
    act_ty InferResult
    inf_res
          Check Type
    exp_ty  -> Maybe SDoc -> Type -> Type -> TcM Coercion
    unifyType (SDoc -> Maybe SDoc
    forall a. a -> Maybe a
    Just (HsExpr GhcRn -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr HsExpr GhcRn
    rn_expr)) Type
    act_ty Type
    exp_ty

    ------------------------
    tcSubTypePat :: CtOrigin -> UserTypeCtxt
                -> ExpSigmaType -> TcSigmaType -> TcM HsWrapper
    -- Used in patterns; polarity is backwards compared
    --   to tcSubType
    -- If wrap = tc_sub_type_et t1 t2
    --    => wrap :: t1 ~> t2
    tcSubTypePat :: CtOrigin -> UserTypeCtxt -> ExpRhoType -> Type -> TcM HsWrapper
    tcSubTypePat CtOrigin
    inst_orig UserTypeCtxt
    ctxt (Check Type
    ty_actual) Type
    ty_expected
      = do { DynFlags
    dflags <- IOEnv (Env TcGblEnv TcLclEnv) DynFlags
    forall (m :: * -> *). HasDynFlags m => m DynFlags
    getDynFlags
           ; DynFlags
    -> (Type -> Type -> TcM Coercion)
    -> CtOrigin
    -> UserTypeCtxt
    -> Type
    -> Type
    -> TcM HsWrapper
    tc_sub_type DynFlags
    dflags Type -> Type -> TcM Coercion
    unifyTypeET CtOrigin
    inst_orig UserTypeCtxt
    ctxt Type
    ty_actual Type
    ty_expected }

    tcSubTypePat CtOrigin
    _ UserTypeCtxt
    _ (Infer InferResult
    inf_res) Type
    ty_expected
      = do { Coercion
    co <- Type -> InferResult -> TcM Coercion
    fillInferResult Type
    ty_expected InferResult
    inf_res
                   -- In patterns we do not instantatiate

           ; HsWrapper -> TcM HsWrapper
    forall (m :: * -> *) a. Monad m => a -> m a
    return (Coercion -> HsWrapper
    mkWpCastN (Coercion -> Coercion
    mkTcSymCo Coercion
    co)) }

    ---------------
    tcSubType :: CtOrigin -> UserTypeCtxt
              -> TcSigmaType  -- Actual
              -> ExpRhoType   -- Expected
              -> TcM HsWrapper
    -- Checks that 'actual' is more polymorphic than 'expected'
    tcSubType :: CtOrigin -> UserTypeCtxt -> Type -> ExpRhoType -> TcM HsWrapper
    tcSubType CtOrigin
    orig UserTypeCtxt
    ctxt Type
    ty_actual ExpRhoType
    ty_expected
      = Type -> ExpRhoType -> TcM HsWrapper -> TcM HsWrapper
    forall a. Type -> ExpRhoType -> TcM a -> TcM a
    addSubTypeCtxt Type
    ty_actual ExpRhoType
    ty_expected (TcM HsWrapper -> TcM HsWrapper) -> TcM HsWrapper -> TcM HsWrapper
    forall a b. (a -> b) -> a -> b
    $
        do { String -> SDoc -> TcRn ()
    traceTc String
    "tcSubType" ([SDoc] -> SDoc
    vcat [UserTypeCtxt -> SDoc
    pprUserTypeCtxt UserTypeCtxt
    ctxt, Type -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Type
    ty_actual, ExpRhoType -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr ExpRhoType
    ty_expected])
           ; CtOrigin
    -> UserTypeCtxt
    -> Maybe SDoc
    -> Type
    -> ExpRhoType
    -> TcM HsWrapper
    tcSubTypeNC CtOrigin
    orig UserTypeCtxt
    ctxt Maybe SDoc
    forall a. Maybe a
    Nothing Type
    ty_actual ExpRhoType
    ty_expected }

    tcSubTypeNC :: CtOrigin       -- Used when instantiating
                -> UserTypeCtxt   -- Used when skolemising
                -> Maybe SDoc     -- The expression that has type 'actual' (if known)
                -> TcSigmaType            -- Actual type
                -> ExpRhoType             -- Expected type
                -> TcM HsWrapper
    tcSubTypeNC :: CtOrigin
    -> UserTypeCtxt
    -> Maybe SDoc
    -> Type
    -> ExpRhoType
    -> TcM HsWrapper
    tcSubTypeNC CtOrigin
    inst_orig UserTypeCtxt
    ctxt Maybe SDoc
    m_thing Type
    ty_actual ExpRhoType
    res_ty
      = case ExpRhoType
    res_ty of
          Check Type
    ty_expected -> do { DynFlags
    dflags <- IOEnv (Env TcGblEnv TcLclEnv) DynFlags
    forall (m :: * -> *). HasDynFlags m => m DynFlags
    getDynFlags
                                  ; DynFlags
    -> (Type -> Type -> TcM Coercion)
    -> CtOrigin
    -> UserTypeCtxt
    -> Type
    -> Type
    -> TcM HsWrapper
    tc_sub_type DynFlags
    dflags (Maybe SDoc -> Type -> Type -> TcM Coercion
    unifyType Maybe SDoc
    m_thing) CtOrigin
    inst_orig UserTypeCtxt
    ctxt
                                                Type
    ty_actual Type
    ty_expected }

          Infer InferResult
    inf_res -> do { (HsWrapper
    wrap, Type
    rho) <- CtOrigin -> Type -> TcM (HsWrapper, Type)
    topInstantiate CtOrigin
    inst_orig Type
    ty_actual
                                       -- See Note [Instantiation of InferResult]
                              ; Coercion
    co <- Type -> InferResult -> TcM Coercion
    fillInferResult Type
    rho InferResult
    inf_res
                              ; HsWrapper -> TcM HsWrapper
    forall (m :: * -> *) a. Monad m => a -> m a
    return (Coercion -> HsWrapper
    mkWpCastN Coercion
    co HsWrapper -> HsWrapper -> HsWrapper
    <.> HsWrapper
    wrap) }

    {- Note [Instantiation of InferResult]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We now always instantiate before filling in InferResult, so that
    the result is a TcRhoType: see #17173 for discussion.

    For example:

    1. Consider
        f x = (*)
       We want to instantiate the type of (*) before returning, else we
       will infer the type
         f :: forall {a}. a -> forall b. Num b => b -> b -> b
       This is surely confusing for users.

       And worse, the monomorphism restriction won't work properly. The MR is
       dealt with in simplifyInfer, and simplifyInfer has no way of
       instantiating. This could perhaps be worked around, but it may be
       hard to know even when instantiation should happen.

    2. Another reason.  Consider
           f :: (?x :: Int) => a -> a
           g y = let ?x = 3::Int in f
       Here want to instantiate f's type so that the ?x::Int constraint
       gets discharged by the enclosing implicit-parameter binding.

    3. Suppose one defines plus = (+). If we instantiate lazily, we will
       infer plus :: forall a. Num a => a -> a -> a. However, the monomorphism
       restriction compels us to infer
          plus :: Integer -> Integer -> Integer
       (or similar monotype). Indeed, the only way to know whether to apply
       the monomorphism restriction at all is to instantiate

    There is one place where we don't want to instantiate eagerly,
    namely in GHC.Tc.Module.tcRnExpr, which implements GHCi's :type
    command. See Note [Implementing :type] in GHC.Tc.Module.
    -}

    ---------------
    tcSubTypeSigma :: UserTypeCtxt -> TcSigmaType -> TcSigmaType -> TcM HsWrapper
    -- External entry point, but no ExpTypes on either side
    -- Checks that actual <= expected
    -- Returns HsWrapper :: actual ~ expected
    tcSubTypeSigma :: UserTypeCtxt -> Type -> Type -> TcM HsWrapper
    tcSubTypeSigma UserTypeCtxt
    ctxt Type
    ty_actual Type
    ty_expected
      = do { DynFlags
    dflags <- IOEnv (Env TcGblEnv TcLclEnv) DynFlags
    forall (m :: * -> *). HasDynFlags m => m DynFlags
    getDynFlags
           ; DynFlags
    -> (Type -> Type -> TcM Coercion)
    -> CtOrigin
    -> UserTypeCtxt
    -> Type
    -> Type
    -> TcM HsWrapper
    tc_sub_type DynFlags
    dflags (Maybe SDoc -> Type -> Type -> TcM Coercion
    unifyType Maybe SDoc
    forall a. Maybe a
    Nothing) CtOrigin
    eq_orig UserTypeCtxt
    ctxt Type
    ty_actual Type
    ty_expected }
      where
        eq_orig :: CtOrigin
    eq_orig = TypeEqOrigin { uo_actual :: Type
    uo_actual   = Type
    ty_actual
                               , uo_expected :: Type
    uo_expected = Type
    ty_expected
                               , uo_thing :: Maybe SDoc
    uo_thing    = Maybe SDoc
    forall a. Maybe a
    Nothing
                               , uo_visible :: Bool
    uo_visible  = Bool
    True }

    ---------------
    tc_sub_type :: DynFlags
                -> (TcType -> TcType -> TcM TcCoercionN)  -- How to unify
                -> CtOrigin       -- Used when instantiating
                -> UserTypeCtxt   -- Used when skolemising
                -> TcSigmaType    -- Actual; a sigma-type
                -> TcSigmaType    -- Expected; also a sigma-type
                -> TcM HsWrapper
    -- Checks that actual_ty is more polymorphic than expected_ty
    -- If wrap = tc_sub_type t1 t2
    --    => wrap :: t1 ~> t2
    tc_sub_type :: DynFlags
    -> (Type -> Type -> TcM Coercion)
    -> CtOrigin
    -> UserTypeCtxt
    -> Type
    -> Type
    -> TcM HsWrapper
    tc_sub_type DynFlags
    dflags Type -> Type -> TcM Coercion
    unify CtOrigin
    inst_orig UserTypeCtxt
    ctxt Type
    ty_actual Type
    ty_expected
      | Type -> Bool
    definitely_poly Type
    ty_expected      -- See Note [Don't skolemise unnecessarily]
      , Bool -> Bool
    not (Type -> Bool
    possibly_poly Type
    ty_actual)
      = do { String -> SDoc -> TcRn ()
    traceTc String
    "tc_sub_type (drop to equality)" (SDoc -> TcRn ()) -> SDoc -> TcRn ()
    forall a b. (a -> b) -> a -> b
    $
             [SDoc] -> SDoc
    vcat [ String -> SDoc
    text String
    "ty_actual   =" SDoc -> SDoc -> SDoc
    <+> Type -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Type
    ty_actual
                  , String -> SDoc
    text String
    "ty_expected =" SDoc -> SDoc -> SDoc
    <+> Type -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Type
    ty_expected ]
           ; Coercion -> HsWrapper
    mkWpCastN (Coercion -> HsWrapper) -> TcM Coercion -> TcM HsWrapper
    forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    <$>
             Type -> Type -> TcM Coercion
    unify Type
    ty_actual Type
    ty_expected }

      | Bool
    otherwise   -- This is the general case
      = do { String -> SDoc -> TcRn ()
    traceTc String
    "tc_sub_type (general case)" (SDoc -> TcRn ()) -> SDoc -> TcRn ()
    forall a b. (a -> b) -> a -> b
    $
             [SDoc] -> SDoc
    vcat [ String -> SDoc
    text String
    "ty_actual   =" SDoc -> SDoc -> SDoc
    <+> Type -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Type
    ty_actual
                  , String -> SDoc
    text String
    "ty_expected =" SDoc -> SDoc -> SDoc
    <+> Type -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Type
    ty_expected ]

           ; (HsWrapper
    sk_wrap, HsWrapper
    inner_wrap)
               <- UserTypeCtxt
    -> Type -> (Type -> TcM HsWrapper) -> TcM (HsWrapper, HsWrapper)
    forall result.
    UserTypeCtxt
    -> Type -> (Type -> TcM result) -> TcM (HsWrapper, result)
    tcSkolemise UserTypeCtxt
    ctxt Type
    ty_expected ((Type -> TcM HsWrapper) -> TcM (HsWrapper, HsWrapper))
    -> (Type -> TcM HsWrapper) -> TcM (HsWrapper, HsWrapper)
    forall a b. (a -> b) -> a -> b
    $ \ Type
    sk_rho ->
                  do { (HsWrapper
    wrap, Type
    rho_a) <- CtOrigin -> Type -> TcM (HsWrapper, Type)
    topInstantiate CtOrigin
    inst_orig Type
    ty_actual
                     ; Coercion
    cow           <- Type -> Type -> TcM Coercion
    unify Type
    rho_a Type
    sk_rho
                     ; HsWrapper -> TcM HsWrapper
    forall (m :: * -> *) a. Monad m => a -> m a
    return (Coercion -> HsWrapper
    mkWpCastN Coercion
    cow HsWrapper -> HsWrapper -> HsWrapper
    <.> HsWrapper
    wrap) }

           ; HsWrapper -> TcM HsWrapper
    forall (m :: * -> *) a. Monad m => a -> m a
    return (HsWrapper
    sk_wrap HsWrapper -> HsWrapper -> HsWrapper
    <.> HsWrapper
    inner_wrap) }
      where
        possibly_poly :: Type -> Bool
    possibly_poly Type
    ty = Bool -> Bool
    not (Type -> Bool
    isRhoTy Type
    ty)

        definitely_poly :: Type -> Bool
    definitely_poly Type
    ty
          | ([TcTyVar]
    tvs, ThetaType
    theta, Type
    tau) <- Type -> ([TcTyVar], ThetaType, Type)
    tcSplitSigmaTy Type
    ty
          , (TcTyVar
    tv:[TcTyVar]
    _) <- [TcTyVar]
    tvs
          , ThetaType -> Bool
    forall (t :: * -> *) a. Foldable t => t a -> Bool
    null ThetaType
    theta
          , DynFlags -> TcTyVar -> Type -> CheckTyEqResult
    checkTyVarEq DynFlags
    dflags TcTyVar
    tv Type
    tau CheckTyEqResult -> CheckTyEqProblem -> Bool
    `cterHasProblem` CheckTyEqProblem
    cteInsolubleOccurs
          = Bool
    True
          | Bool
    otherwise
          = Bool
    False

    ------------------------
    addSubTypeCtxt :: TcType -> ExpType -> TcM a -> TcM a
    addSubTypeCtxt :: forall a. Type -> ExpRhoType -> TcM a -> TcM a
    addSubTypeCtxt Type
    ty_actual ExpRhoType
    ty_expected TcM a
    thing_inside
     | Type -> Bool
    isRhoTy Type
    ty_actual        -- If there is no polymorphism involved, the
     , ExpRhoType -> Bool
    isRhoExpTy ExpRhoType
    ty_expected   -- TypeEqOrigin stuff (added by the _NC functions)
     = TcM a
    thing_inside             -- gives enough context by itself
     | Bool
    otherwise
     = (TidyEnv -> TcM (TidyEnv, SDoc)) -> TcM a -> TcM a
    forall a. (TidyEnv -> TcM (TidyEnv, SDoc)) -> TcM a -> TcM a
    addErrCtxtM TidyEnv -> TcM (TidyEnv, SDoc)
    mk_msg TcM a
    thing_inside
      where
        mk_msg :: TidyEnv -> TcM (TidyEnv, SDoc)
    mk_msg TidyEnv
    tidy_env
          = do { (TidyEnv
    tidy_env, Type
    ty_actual)   <- TidyEnv -> Type -> TcM (TidyEnv, Type)
    zonkTidyTcType TidyEnv
    tidy_env Type
    ty_actual
                       -- might not be filled if we're debugging. ugh.
               ; Maybe Type
    mb_ty_expected          <- ExpRhoType -> TcM (Maybe Type)
    readExpType_maybe ExpRhoType
    ty_expected
               ; (TidyEnv
    tidy_env, ExpRhoType
    ty_expected) <- case Maybe Type
    mb_ty_expected of
                                              Just Type
    ty -> (Type -> ExpRhoType) -> (TidyEnv, Type) -> (TidyEnv, ExpRhoType)
    forall (a :: * -> * -> *) b c d.
    Arrow a =>
    a b c -> a (d, b) (d, c)
    second Type -> ExpRhoType
    mkCheckExpType ((TidyEnv, Type) -> (TidyEnv, ExpRhoType))
    -> TcM (TidyEnv, Type)
    -> IOEnv (Env TcGblEnv TcLclEnv) (TidyEnv, ExpRhoType)
    forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    <$>
                                                         TidyEnv -> Type -> TcM (TidyEnv, Type)
    zonkTidyTcType TidyEnv
    tidy_env Type
    ty
                                              Maybe Type
    Nothing -> (TidyEnv, ExpRhoType)
    -> IOEnv (Env TcGblEnv TcLclEnv) (TidyEnv, ExpRhoType)
    forall (m :: * -> *) a. Monad m => a -> m a
    return (TidyEnv
    tidy_env, ExpRhoType
    ty_expected)
               ; Type
    ty_expected             <- ExpRhoType -> TcM Type
    readExpType ExpRhoType
    ty_expected
               ; (TidyEnv
    tidy_env, Type
    ty_expected) <- TidyEnv -> Type -> TcM (TidyEnv, Type)
    zonkTidyTcType TidyEnv
    tidy_env Type
    ty_expected
               ; let msg :: SDoc
    msg = [SDoc] -> SDoc
    vcat [ SDoc -> Int -> SDoc -> SDoc
    hang (String -> SDoc
    text String
    "When checking that:")
                                     Int
    4 (Type -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Type
    ty_actual)
                                , Int -> SDoc -> SDoc
    nest Int
    2 (SDoc -> Int -> SDoc -> SDoc
    hang (String -> SDoc
    text String
    "is more polymorphic than:")
                                             Int
    2 (Type -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Type
    ty_expected)) ]
               ; (TidyEnv, SDoc) -> TcM (TidyEnv, SDoc)
    forall (m :: * -> *) a. Monad m => a -> m a
    return (TidyEnv
    tidy_env, SDoc
    msg) }

    {- Note [Don't skolemise unnecessarily]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Suppose we are trying to solve
        (Char->Char) <= (forall a. a->a)
    We could skolemise the 'forall a', and then complain
    that (Char ~ a) is insoluble; but that's a pretty obscure
    error.  It's better to say that
        (Char->Char) ~ (forall a. a->a)
    fails.

    So roughly:
     * if the ty_expected has an outermost forall
          (i.e. skolemisation is the next thing we'd do)
     * and the ty_actual has no top-level polymorphism (but looking deeply)
    then we can revert to simple equality.  But we need to be careful.
    These examples are all fine:

     * (Char->Char) <= (forall a. Char -> Char)
          ty_expected isn't really polymorphic

     * (Char->Char) <= (forall a. (a~Char) => a -> a)
          ty_expected isn't really polymorphic

     * (Char->Char) <= (forall a. F [a] Char -> Char)
                       where type instance F [x] t = t
         ty_expected isn't really polymorphic

    If we prematurely go to equality we'll reject a program we should
    accept (e.g. #13752).  So the test (which is only to improve
    error message) is very conservative:
     * ty_actual is /definitely/ monomorphic
     * ty_expected is /definitely/ polymorphic

    Note [Settting the argument context]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider we are doing the ambiguity check for the (bogus)
      f :: (forall a b. C b => a -> a) -> Int

    We'll call
       tcSubType ((forall a b. C b => a->a) -> Int )
                 ((forall a b. C b => a->a) -> Int )

    with a UserTypeCtxt of (FunSigCtxt "f").  Then we'll do the co/contra thing
    on the argument type of the (->) -- and at that point we want to switch
    to a UserTypeCtxt of GenSigCtxt.  Why?

    * Error messages.  If we stick with FunSigCtxt we get errors like
         * Could not deduce: C b
           from the context: C b0
            bound by the type signature for:
                f :: forall a b. C b => a->a
      But of course f does not have that type signature!
      Example tests: T10508, T7220a, Simple14

    * Implications. We may decide to build an implication for the whole
      ambiguity check, but we don't need one for each level within it,
      and GHC.Tc.Utils.Unify.alwaysBuildImplication checks the UserTypeCtxt.
      See Note [When to build an implication]

    Note [Wrapper returned from tcSubMult]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    There is no notion of multiplicity coercion in Core, therefore the wrapper
    returned by tcSubMult (and derived functions such as tcCheckUsage and
    checkManyPattern) is quite unlike any other wrapper: it checks whether the
    coercion produced by the constraint solver is trivial, producing a type error
    is it is not. This is implemented via the WpMultCoercion wrapper, as desugared
    by GHC.HsToCore.Binds.dsHsWrapper, which does the reflexivity check.

    This wrapper needs to be placed in the term; otherwise, checking of the
    eventual coercion won't be triggered during desugaring. But it can be put
    anywhere, since it doesn't affect the desugared code.

    Why do we check this in the desugarer? It's a convenient place, since it's
    right after all the constraints are solved. We need the constraints to be
    solved to check whether they are trivial or not. Plus there is precedent for
    type errors during desuraging (such as the levity polymorphism
    restriction). An alternative would be to have a kind of constraint which can
    only produce trivial evidence, then this check would happen in the constraint
    solver.
    -}

    tcSubMult :: CtOrigin -> Mult -> Mult -> TcM HsWrapper
    tcSubMult :: CtOrigin -> Type -> Type -> TcM HsWrapper
    tcSubMult CtOrigin
    origin Type
    w_actual Type
    w_expected
      | Just (Type
    w1, Type
    w2) <- Type -> Maybe (Type, Type)
    isMultMul Type
    w_actual =
      do { HsWrapper
    w1 <- CtOrigin -> Type -> Type -> TcM HsWrapper
    tcSubMult CtOrigin
    origin Type
    w1 Type
    w_expected
         ; HsWrapper
    w2 <- CtOrigin -> Type -> Type -> TcM HsWrapper
    tcSubMult CtOrigin
    origin Type
    w2 Type
    w_expected
         ; HsWrapper -> TcM HsWrapper
    forall (m :: * -> *) a. Monad m => a -> m a
    return (HsWrapper
    w1 HsWrapper -> HsWrapper -> HsWrapper
    <.> HsWrapper
    w2) }
      -- Currently, we consider p*q and sup p q to be equal.  Therefore, p*q <= r is
      -- equivalent to p <= r and q <= r.  For other cases, we approximate p <= q by p
      -- ~ q.  This is not complete, but it's sound. See also Note [Overapproximating
      -- multiplicities] in Multiplicity.
    tcSubMult CtOrigin
    origin Type
    w_actual Type
    w_expected =
      case Type -> Type -> IsSubmult
    submult Type
    w_actual Type
    w_expected of
        IsSubmult
    Submult -> HsWrapper -> TcM HsWrapper
    forall (m :: * -> *) a. Monad m => a -> m a
    return HsWrapper
    WpHole
        IsSubmult
    Unknown -> CtOrigin -> Type -> Type -> TcM HsWrapper
    tcEqMult CtOrigin
    origin Type
    w_actual Type
    w_expected

    tcEqMult :: CtOrigin -> Mult -> Mult -> TcM HsWrapper
    tcEqMult :: CtOrigin -> Type -> Type -> TcM HsWrapper
    tcEqMult CtOrigin
    origin Type
    w_actual Type
    w_expected = do
      {
      -- Note that here we do not call to `submult`, so we check
      -- for strict equality.
      ; Coercion
    coercion <- TypeOrKind -> CtOrigin -> Type -> Type -> TcM Coercion
    uType TypeOrKind
    TypeLevel CtOrigin
    origin Type
    w_actual Type
    w_expected
      ; HsWrapper -> TcM HsWrapper
    forall (m :: * -> *) a. Monad m => a -> m a
    return (HsWrapper -> TcM HsWrapper) -> HsWrapper -> TcM HsWrapper
    forall a b. (a -> b) -> a -> b
    $ if Coercion -> Bool
    isReflCo Coercion
    coercion then HsWrapper
    WpHole else Coercion -> HsWrapper
    WpMultCoercion Coercion
    coercion }

    {- *********************************************************************
    *                                                                      *
                        Generalisation
    *                                                                      *
    ********************************************************************* -}

    {- Note [Skolemisation]
    ~~~~~~~~~~~~~~~~~~~~~~~
    tcSkolemise takes "expected type" and strip off quantifiers to expose the
    type underneath, binding the new skolems for the 'thing_inside'
    The returned 'HsWrapper' has type (specific_ty -> expected_ty).

    Note that for a nested type like
       forall a. Eq a => forall b. Ord b => blah
    we still only build one implication constraint
       forall a b. (Eq a, Ord b) => <constraints>
    This is just an optimisation, but it's why we use topSkolemise to
    build the pieces from all the layers, before making a single call
    to checkConstraints.

    tcSkolemiseScoped is very similar, but differs in two ways:

    * It deals specially with just the outer forall, bringing those type
      variables into lexical scope.  To my surprise, I found that doing
      this unconditionally in tcSkolemise (i.e. doing it even if we don't
      need to bring the variables into lexical scope, which is harmless)
      caused a non-trivial (1%-ish) perf hit on the compiler.

    * It always calls checkConstraints, even if there are no skolem
      variables at all.  Reason: there might be nested deferred errors
      that must not be allowed to float to top level.
      See Note [When to build an implication] below.
    -}

    tcSkolemise, tcSkolemiseScoped
        :: UserTypeCtxt -> TcSigmaType
        -> (TcType -> TcM result)
        -> TcM (HsWrapper, result)
            -- ^ The wrapper has type: spec_ty ~> expected_ty
    -- See Note [Skolemisation] for the differences between
    -- tcSkolemiseScoped and tcSkolemise

    tcSkolemiseScoped :: forall result.
    UserTypeCtxt
    -> Type -> (Type -> TcM result) -> TcM (HsWrapper, result)
    tcSkolemiseScoped UserTypeCtxt
    ctxt Type
    expected_ty Type -> TcM result
    thing_inside
      = do { (HsWrapper
    wrap, [(Name, TcTyVar)]
    tv_prs, [TcTyVar]
    given, Type
    rho_ty) <- Type -> TcM (HsWrapper, [(Name, TcTyVar)], [TcTyVar], Type)
    topSkolemise Type
    expected_ty
           ; let skol_tvs :: [TcTyVar]
    skol_tvs  = ((Name, TcTyVar) -> TcTyVar) -> [(Name, TcTyVar)] -> [TcTyVar]
    forall a b. (a -> b) -> [a] -> [b]
    map (Name, TcTyVar) -> TcTyVar
    forall a b. (a, b) -> b
    snd [(Name, TcTyVar)]
    tv_prs
                 skol_info :: SkolemInfo
    skol_info = UserTypeCtxt -> Type -> [(Name, TcTyVar)] -> SkolemInfo
    SigSkol UserTypeCtxt
    ctxt Type
    expected_ty [(Name, TcTyVar)]
    tv_prs

           ; (TcEvBinds
    ev_binds, result
    res)
                 <- SkolemInfo
    -> [TcTyVar] -> [TcTyVar] -> TcM result -> TcM (TcEvBinds, result)
    forall result.
    SkolemInfo
    -> [TcTyVar] -> [TcTyVar] -> TcM result -> TcM (TcEvBinds, result)
    checkConstraints SkolemInfo
    skol_info [TcTyVar]
    skol_tvs [TcTyVar]
    given (TcM result -> TcM (TcEvBinds, result))
    -> TcM result -> TcM (TcEvBinds, result)
    forall a b. (a -> b) -> a -> b
    $
                    [(Name, TcTyVar)] -> TcM result -> TcM result
    forall r. [(Name, TcTyVar)] -> TcM r -> TcM r
    tcExtendNameTyVarEnv [(Name, TcTyVar)]
    tv_prs               (TcM result -> TcM result) -> TcM result -> TcM result
    forall a b. (a -> b) -> a -> b
    $
                    Type -> TcM result
    thing_inside Type
    rho_ty

           ; (HsWrapper, result) -> TcM (HsWrapper, result)
    forall (m :: * -> *) a. Monad m => a -> m a
    return (HsWrapper
    wrap HsWrapper -> HsWrapper -> HsWrapper
    <.> TcEvBinds -> HsWrapper
    mkWpLet TcEvBinds
    ev_binds, result
    res) }

    tcSkolemise :: forall result.
    UserTypeCtxt
    -> Type -> (Type -> TcM result) -> TcM (HsWrapper, result)
    tcSkolemise UserTypeCtxt
    ctxt Type
    expected_ty Type -> TcM result
    thing_inside
      | Type -> Bool
    isRhoTy Type
    expected_ty  -- Short cut for common case
      = do { result
    res <- Type -> TcM result
    thing_inside Type
    expected_ty
           ; (HsWrapper, result) -> TcM (HsWrapper, result)
    forall (m :: * -> *) a. Monad m => a -> m a
    return (HsWrapper
    idHsWrapper, result
    res) }
      | Bool
    otherwise
      = do  { (HsWrapper
    wrap, [(Name, TcTyVar)]
    tv_prs, [TcTyVar]
    given, Type
    rho_ty) <- Type -> TcM (HsWrapper, [(Name, TcTyVar)], [TcTyVar], Type)
    topSkolemise Type
    expected_ty

            ; let skol_tvs :: [TcTyVar]
    skol_tvs  = ((Name, TcTyVar) -> TcTyVar) -> [(Name, TcTyVar)] -> [TcTyVar]
    forall a b. (a -> b) -> [a] -> [b]
    map (Name, TcTyVar) -> TcTyVar
    forall a b. (a, b) -> b
    snd [(Name, TcTyVar)]
    tv_prs
                  skol_info :: SkolemInfo
    skol_info = UserTypeCtxt -> Type -> [(Name, TcTyVar)] -> SkolemInfo
    SigSkol UserTypeCtxt
    ctxt Type
    expected_ty [(Name, TcTyVar)]
    tv_prs

            ; (TcEvBinds
    ev_binds, result
    result)
                  <- SkolemInfo
    -> [TcTyVar] -> [TcTyVar] -> TcM result -> TcM (TcEvBinds, result)
    forall result.
    SkolemInfo
    -> [TcTyVar] -> [TcTyVar] -> TcM result -> TcM (TcEvBinds, result)
    checkConstraints SkolemInfo
    skol_info [TcTyVar]
    skol_tvs [TcTyVar]
    given (TcM result -> TcM (TcEvBinds, result))
    -> TcM result -> TcM (TcEvBinds, result)
    forall a b. (a -> b) -> a -> b
    $
                     Type -> TcM result
    thing_inside Type
    rho_ty

            ; (HsWrapper, result) -> TcM (HsWrapper, result)
    forall (m :: * -> *) a. Monad m => a -> m a
    return (HsWrapper
    wrap HsWrapper -> HsWrapper -> HsWrapper
    <.> TcEvBinds -> HsWrapper
    mkWpLet TcEvBinds
    ev_binds, result
    result) }
              -- The ev_binds returned by checkConstraints is very
              -- often empty, in which case mkWpLet is a no-op

    -- | Variant of 'tcSkolemise' that takes an ExpType
    tcSkolemiseET :: UserTypeCtxt -> ExpSigmaType
                  -> (ExpRhoType -> TcM result)
                  -> TcM (HsWrapper, result)
    tcSkolemiseET :: forall result.
    UserTypeCtxt
    -> ExpRhoType
    -> (ExpRhoType -> TcM result)
    -> TcM (HsWrapper, result)
    tcSkolemiseET UserTypeCtxt
    _ et :: ExpRhoType
    et@(Infer {}) ExpRhoType -> TcM result
    thing_inside
      = (HsWrapper
    idHsWrapper, ) (result -> (HsWrapper, result))
    -> TcM result -> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, result)
    forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    <$> ExpRhoType -> TcM result
    thing_inside ExpRhoType
    et
    tcSkolemiseET UserTypeCtxt
    ctxt (Check Type
    ty) ExpRhoType -> TcM result
    thing_inside
      = UserTypeCtxt
    -> Type
    -> (Type -> TcM result)
    -> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, result)
    forall result.
    UserTypeCtxt
    -> Type -> (Type -> TcM result) -> TcM (HsWrapper, result)
    tcSkolemise UserTypeCtxt
    ctxt Type
    ty ((Type -> TcM result)
     -> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, result))
    -> (Type -> TcM result)
    -> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, result)
    forall a b. (a -> b) -> a -> b
    $ \Type
    rho_ty ->
        ExpRhoType -> TcM result
    thing_inside (Type -> ExpRhoType
    mkCheckExpType Type
    rho_ty)

    checkConstraints :: SkolemInfo
                     -> [TcTyVar]           -- Skolems
                     -> [EvVar]             -- Given
                     -> TcM result
                     -> TcM (TcEvBinds, result)

    checkConstraints :: forall result.
    SkolemInfo
    -> [TcTyVar] -> [TcTyVar] -> TcM result -> TcM (TcEvBinds, result)
    checkConstraints SkolemInfo
    skol_info [TcTyVar]
    skol_tvs [TcTyVar]
    given TcM result
    thing_inside
      = do { Bool
    implication_needed <- SkolemInfo -> [TcTyVar] -> [TcTyVar] -> TcM Bool
    implicationNeeded SkolemInfo
    skol_info [TcTyVar]
    skol_tvs [TcTyVar]
    given

           ; if Bool
    implication_needed
             then do { (TcLevel
    tclvl, WantedConstraints
    wanted, result
    result) <- TcM result -> TcM (TcLevel, WantedConstraints, result)
    forall a. TcM a -> TcM (TcLevel, WantedConstraints, a)
    pushLevelAndCaptureConstraints TcM result
    thing_inside
                     ; (Bag Implication
    implics, TcEvBinds
    ev_binds) <- TcLevel
    -> SkolemInfo
    -> [TcTyVar]
    -> [TcTyVar]
    -> WantedConstraints
    -> TcM (Bag Implication, TcEvBinds)
    buildImplicationFor TcLevel
    tclvl SkolemInfo
    skol_info [TcTyVar]
    skol_tvs [TcTyVar]
    given WantedConstraints
    wanted
                     ; String -> SDoc -> TcRn ()
    traceTc String
    "checkConstraints" (TcLevel -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TcLevel
    tclvl SDoc -> SDoc -> SDoc
    $$ [TcTyVar] -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr [TcTyVar]
    skol_tvs)
                     ; Bag Implication -> TcRn ()
    emitImplications Bag Implication
    implics
                     ; (TcEvBinds, result) -> TcM (TcEvBinds, result)
    forall (m :: * -> *) a. Monad m => a -> m a
    return (TcEvBinds
    ev_binds, result
    result) }

             else -- Fast path.  We check every function argument with tcCheckPolyExpr,
                  -- which uses tcSkolemise and hence checkConstraints.
                  -- So this fast path is well-exercised
                  do { result
    res <- TcM result
    thing_inside
                     ; (TcEvBinds, result) -> TcM (TcEvBinds, result)
    forall (m :: * -> *) a. Monad m => a -> m a
    return (TcEvBinds
    emptyTcEvBinds, result
    res) } }

    checkTvConstraints :: SkolemInfo
                       -> [TcTyVar]          -- Skolem tyvars
                       -> TcM result
                       -> TcM result

    checkTvConstraints :: forall result. SkolemInfo -> [TcTyVar] -> TcM result -> TcM result
    checkTvConstraints SkolemInfo
    skol_info [TcTyVar]
    skol_tvs TcM result
    thing_inside
      = do { (TcLevel
    tclvl, WantedConstraints
    wanted, result
    result) <- TcM result -> TcM (TcLevel, WantedConstraints, result)
    forall a. TcM a -> TcM (TcLevel, WantedConstraints, a)
    pushLevelAndCaptureConstraints TcM result
    thing_inside
           ; SkolemInfo -> [TcTyVar] -> TcLevel -> WantedConstraints -> TcRn ()
    emitResidualTvConstraint SkolemInfo
    skol_info [TcTyVar]
    skol_tvs TcLevel
    tclvl WantedConstraints
    wanted
           ; result -> TcM result
    forall (m :: * -> *) a. Monad m => a -> m a
    return result
    result }

    emitResidualTvConstraint :: SkolemInfo -> [TcTyVar]
                             -> TcLevel -> WantedConstraints -> TcM ()
    emitResidualTvConstraint :: SkolemInfo -> [TcTyVar] -> TcLevel -> WantedConstraints -> TcRn ()
    emitResidualTvConstraint SkolemInfo
    skol_info [TcTyVar]
    skol_tvs TcLevel
    tclvl WantedConstraints
    wanted
      | Bool -> Bool
    not (WantedConstraints -> Bool
    isEmptyWC WantedConstraints
    wanted) Bool -> Bool -> Bool
    ||
        SkolemInfo -> Bool
    checkTelescopeSkol SkolemInfo
    skol_info
      = -- checkTelescopeSkol: in this case, /always/ emit this implication
        -- even if 'wanted' is empty. We need the implication so that we check
        -- for a bad telescope. See Note [Skolem escape and forall-types] in
        -- GHC.Tc.Gen.HsType
        do { Implication
    implic <- SkolemInfo
    -> [TcTyVar] -> TcLevel -> WantedConstraints -> TcM Implication
    buildTvImplication SkolemInfo
    skol_info [TcTyVar]
    skol_tvs TcLevel
    tclvl WantedConstraints
    wanted
           ; Implication -> TcRn ()
    emitImplication Implication
    implic }

      | Bool
    otherwise  -- Empty 'wanted', emit nothing
      = () -> TcRn ()
    forall (m :: * -> *) a. Monad m => a -> m a
    return ()

    buildTvImplication :: SkolemInfo -> [TcTyVar]
                       -> TcLevel -> WantedConstraints -> TcM Implication
    buildTvImplication :: SkolemInfo
    -> [TcTyVar] -> TcLevel -> WantedConstraints -> TcM Implication
    buildTvImplication SkolemInfo
    skol_info [TcTyVar]
    skol_tvs TcLevel
    tclvl WantedConstraints
    wanted
      = do { EvBindsVar
    ev_binds <- TcM EvBindsVar
    newNoTcEvBinds  -- Used for equalities only, so all the constraints
                                         -- are solved by filling in coercion holes, not
                                         -- by creating a value-level evidence binding
           ; Implication
    implic   <- TcM Implication
    newImplication

           ; Implication -> TcM Implication
    forall (m :: * -> *) a. Monad m => a -> m a
    return (Implication
    implic { ic_tclvl :: TcLevel
    ic_tclvl     = TcLevel
    tclvl
                            , ic_skols :: [TcTyVar]
    ic_skols     = [TcTyVar]
    skol_tvs
                            , ic_given_eqs :: HasGivenEqs
    ic_given_eqs = HasGivenEqs
    NoGivenEqs
                            , ic_wanted :: WantedConstraints
    ic_wanted    = WantedConstraints
    wanted
                            , ic_binds :: EvBindsVar
    ic_binds     = EvBindsVar
    ev_binds
                            , ic_info :: SkolemInfo
    ic_info      = SkolemInfo
    skol_info }) }

    implicationNeeded :: SkolemInfo -> [TcTyVar] -> [EvVar] -> TcM Bool
    -- See Note [When to build an implication]
    implicationNeeded :: SkolemInfo -> [TcTyVar] -> [TcTyVar] -> TcM Bool
    implicationNeeded SkolemInfo
    skol_info [TcTyVar]
    skol_tvs [TcTyVar]
    given
      | [TcTyVar] -> Bool
    forall (t :: * -> *) a. Foldable t => t a -> Bool
    null [TcTyVar]
    skol_tvs
      , [TcTyVar] -> Bool
    forall (t :: * -> *) a. Foldable t => t a -> Bool
    null [TcTyVar]
    given
      , Bool -> Bool
    not (SkolemInfo -> Bool
    alwaysBuildImplication SkolemInfo
    skol_info)
      = -- Empty skolems and givens
        do { TcLevel
    tc_lvl <- TcM TcLevel
    getTcLevel
           ; if Bool -> Bool
    not (TcLevel -> Bool
    isTopTcLevel TcLevel
    tc_lvl)  -- No implication needed if we are
             then Bool -> TcM Bool
    forall (m :: * -> *) a. Monad m => a -> m a
    return Bool
    False             -- already inside an implication
             else
        do { DynFlags
    dflags <- IOEnv (Env TcGblEnv TcLclEnv) DynFlags
    forall (m :: * -> *). HasDynFlags m => m DynFlags
    getDynFlags       -- If any deferral can happen,
                                         -- we must build an implication
           ; Bool -> TcM Bool
    forall (m :: * -> *) a. Monad m => a -> m a
    return (GeneralFlag -> DynFlags -> Bool
    gopt GeneralFlag
    Opt_DeferTypeErrors DynFlags
    dflags Bool -> Bool -> Bool
    ||
                     GeneralFlag -> DynFlags -> Bool
    gopt GeneralFlag
    Opt_DeferTypedHoles DynFlags
    dflags Bool -> Bool -> Bool
    ||
                     GeneralFlag -> DynFlags -> Bool
    gopt GeneralFlag
    Opt_DeferOutOfScopeVariables DynFlags
    dflags) } }

      | Bool
    otherwise     -- Non-empty skolems or givens
      = Bool -> TcM Bool
    forall (m :: * -> *) a. Monad m => a -> m a
    return Bool
    True   -- Definitely need an implication

    alwaysBuildImplication :: SkolemInfo -> Bool
    -- See Note [When to build an implication]
    alwaysBuildImplication :: SkolemInfo -> Bool
    alwaysBuildImplication SkolemInfo
    _ = Bool
    False

    {-  Commmented out for now while I figure out about error messages.
        See #14185

    alwaysBuildImplication (SigSkol ctxt _ _)
      = case ctxt of
          FunSigCtxt {} -> True  -- RHS of a binding with a signature
          _             -> False
    alwaysBuildImplication (RuleSkol {})      = True
    alwaysBuildImplication (InstSkol {})      = True
    alwaysBuildImplication (FamInstSkol {})   = True
    alwaysBuildImplication _                  = False
    -}

    buildImplicationFor :: TcLevel -> SkolemInfo -> [TcTyVar]
                       -> [EvVar] -> WantedConstraints
                       -> TcM (Bag Implication, TcEvBinds)
    buildImplicationFor :: TcLevel
    -> SkolemInfo
    -> [TcTyVar]
    -> [TcTyVar]
    -> WantedConstraints
    -> TcM (Bag Implication, TcEvBinds)
    buildImplicationFor TcLevel
    tclvl SkolemInfo
    skol_info [TcTyVar]
    skol_tvs [TcTyVar]
    given WantedConstraints
    wanted
      | WantedConstraints -> Bool
    isEmptyWC WantedConstraints
    wanted Bool -> Bool -> Bool
    && [TcTyVar] -> Bool
    forall (t :: * -> *) a. Foldable t => t a -> Bool
    null [TcTyVar]
    given
                 -- Optimisation : if there are no wanteds, and no givens
                 -- don't generate an implication at all.
                 -- Reason for the (null given): we don't want to lose
                 -- the "inaccessible alternative" error check
      = (Bag Implication, TcEvBinds) -> TcM (Bag Implication, TcEvBinds)
    forall (m :: * -> *) a. Monad m => a -> m a
    return (Bag Implication
    forall a. Bag a
    emptyBag, TcEvBinds
    emptyTcEvBinds)

      | Bool
    otherwise
      = ASSERT2( all (isSkolemTyVar <||> isTyVarTyVar) skol_tvs, ppr skol_tvs )
          -- Why allow TyVarTvs? Because implicitly declared kind variables in
          -- non-CUSK type declarations are TyVarTvs, and we need to bring them
          -- into scope as a skolem in an implication. This is OK, though,
          -- because TyVarTvs will always remain tyvars, even after unification.
        do { EvBindsVar
    ev_binds_var <- TcM EvBindsVar
    newTcEvBinds
           ; Implication
    implic <- TcM Implication
    newImplication
           ; let implic' :: Implication
    implic' = Implication
    implic { ic_tclvl :: TcLevel
    ic_tclvl  = TcLevel
    tclvl
                                  , ic_skols :: [TcTyVar]
    ic_skols  = [TcTyVar]
    skol_tvs
                                  , ic_given :: [TcTyVar]
    ic_given  = [TcTyVar]
    given
                                  , ic_wanted :: WantedConstraints
    ic_wanted = WantedConstraints
    wanted
                                  , ic_binds :: EvBindsVar
    ic_binds  = EvBindsVar
    ev_binds_var
                                  , ic_info :: SkolemInfo
    ic_info   = SkolemInfo
    skol_info }

           ; (Bag Implication, TcEvBinds) -> TcM (Bag Implication, TcEvBinds)
    forall (m :: * -> *) a. Monad m => a -> m a
    return (Implication -> Bag Implication
    forall a. a -> Bag a
    unitBag Implication
    implic', EvBindsVar -> TcEvBinds
    TcEvBinds EvBindsVar
    ev_binds_var) }

    {- Note [When to build an implication]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Suppose we have some 'skolems' and some 'givens', and we are
    considering whether to wrap the constraints in their scope into an
    implication.  We must /always/ so if either 'skolems' or 'givens' are
    non-empty.  But what if both are empty?  You might think we could
    always drop the implication.  Other things being equal, the fewer
    implications the better.  Less clutter and overhead.  But we must
    take care:

    * If we have an unsolved [W] g :: a ~# b, and -fdefer-type-errors,
      we'll make a /term-level/ evidence binding for 'g = error "blah"'.
      We must have an EvBindsVar those bindings!, otherwise they end up as
      top-level unlifted bindings, which are verboten. This only matters
      at top level, so we check for that
      See also Note [Deferred errors for coercion holes] in GHC.Tc.Errors.
      cf #14149 for an example of what goes wrong.

    * If you have
         f :: Int;  f = f_blah
         g :: Bool; g = g_blah
      If we don't build an implication for f or g (no tyvars, no givens),
      the constraints for f_blah and g_blah are solved together.  And that
      can yield /very/ confusing error messages, because we can get
          [W] C Int b1    -- from f_blah
          [W] C Int b2    -- from g_blan
      and fundpes can yield [D] b1 ~ b2, even though the two functions have
      literally nothing to do with each other.  #14185 is an example.
      Building an implication keeps them separate.
    -}

    {-
    ************************************************************************
    *                                                                      *
                    Boxy unification
    *                                                                      *
    ************************************************************************

    The exported functions are all defined as versions of some
    non-exported generic functions.
    -}

    unifyType :: Maybe SDoc  -- ^ If present, the thing that has type ty1
              -> TcTauType -> TcTauType    -- ty1, ty2
              -> TcM TcCoercionN           -- :: ty1 ~# ty2
    -- Actual and expected types
    -- Returns a coercion : ty1 ~ ty2
    unifyType :: Maybe SDoc -> Type -> Type -> TcM Coercion
    unifyType Maybe SDoc
    thing Type
    ty1 Type
    ty2
      = TypeOrKind -> CtOrigin -> Type -> Type -> TcM Coercion
    uType TypeOrKind
    TypeLevel CtOrigin
    origin Type
    ty1 Type
    ty2
      where
        origin :: CtOrigin
    origin = TypeEqOrigin { uo_actual :: Type
    uo_actual   = Type
    ty1
                              , uo_expected :: Type
    uo_expected = Type
    ty2
                              , uo_thing :: Maybe SDoc
    uo_thing    = SDoc -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr (SDoc -> SDoc) -> Maybe SDoc -> Maybe SDoc
    forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    <$> Maybe SDoc
    thing
                              , uo_visible :: Bool
    uo_visible  = Bool
    True }

    unifyTypeET :: TcTauType -> TcTauType -> TcM CoercionN
    -- Like unifyType, but swap expected and actual in error messages
    -- This is used when typechecking patterns
    unifyTypeET :: Type -> Type -> TcM Coercion
    unifyTypeET Type
    ty1 Type
    ty2
      = TypeOrKind -> CtOrigin -> Type -> Type -> TcM Coercion
    uType TypeOrKind
    TypeLevel CtOrigin
    origin Type
    ty1 Type
    ty2
      where
        origin :: CtOrigin
    origin = TypeEqOrigin { uo_actual :: Type
    uo_actual   = Type
    ty2   -- NB swapped
                              , uo_expected :: Type
    uo_expected = Type
    ty1   -- NB swapped
                              , uo_thing :: Maybe SDoc
    uo_thing    = Maybe SDoc
    forall a. Maybe a
    Nothing
                              , uo_visible :: Bool
    uo_visible  = Bool
    True }

    unifyKind :: Maybe SDoc -> TcKind -> TcKind -> TcM CoercionN
    unifyKind :: Maybe SDoc -> Type -> Type -> TcM Coercion
    unifyKind Maybe SDoc
    mb_thing Type
    ty1 Type
    ty2
      = TypeOrKind -> CtOrigin -> Type -> Type -> TcM Coercion
    uType TypeOrKind
    KindLevel CtOrigin
    origin Type
    ty1 Type
    ty2
      where
        origin :: CtOrigin
    origin = TypeEqOrigin { uo_actual :: Type
    uo_actual   = Type
    ty1
                              , uo_expected :: Type
    uo_expected = Type
    ty2
                              , uo_thing :: Maybe SDoc
    uo_thing    = Maybe SDoc
    mb_thing
                              , uo_visible :: Bool
    uo_visible  = Bool
    True }

    {-
    %************************************************************************
    %*                                                                      *
                     uType and friends
    %*                                                                      *
    %************************************************************************

    uType is the heart of the unifier.
    -}

    uType, uType_defer
      :: TypeOrKind
      -> CtOrigin
      -> TcType    -- ty1 is the *actual* type
      -> TcType    -- ty2 is the *expected* type
      -> TcM CoercionN

    --------------
    -- It is always safe to defer unification to the main constraint solver
    -- See Note [Deferred unification]
    uType_defer :: TypeOrKind -> CtOrigin -> Type -> Type -> TcM Coercion
    uType_defer TypeOrKind
    t_or_k CtOrigin
    origin Type
    ty1 Type
    ty2
      = do { Coercion
    co <- CtOrigin -> TypeOrKind -> Role -> Type -> Type -> TcM Coercion
    emitWantedEq CtOrigin
    origin TypeOrKind
    t_or_k Role
    Nominal Type
    ty1 Type
    ty2

           -- Error trace only
           -- NB. do *not* call mkErrInfo unless tracing is on,
           --     because it is hugely expensive (#5631)
           ; DumpFlag -> TcRn () -> TcRn ()
    forall gbl lcl. DumpFlag -> TcRnIf gbl lcl () -> TcRnIf gbl lcl ()
    whenDOptM DumpFlag
    Opt_D_dump_tc_trace (TcRn () -> TcRn ()) -> TcRn () -> TcRn ()
    forall a b. (a -> b) -> a -> b
    $ do
                { [ErrCtxt]
    ctxt <- TcM [ErrCtxt]
    getErrCtxt
                ; SDoc
    doc <- TidyEnv -> [ErrCtxt] -> TcM SDoc
    mkErrInfo TidyEnv
    emptyTidyEnv [ErrCtxt]
    ctxt
                ; String -> SDoc -> TcRn ()
    traceTc String
    "utype_defer" ([SDoc] -> SDoc
    vcat [ Type -> SDoc
    debugPprType Type
    ty1
                                              , Type -> SDoc
    debugPprType Type
    ty2
                                              , CtOrigin -> SDoc
    pprCtOrigin CtOrigin
    origin
                                              , SDoc
    doc])
                ; String -> SDoc -> TcRn ()
    traceTc String
    "utype_defer2" (Coercion -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Coercion
    co)
                }
           ; Coercion -> TcM Coercion
    forall (m :: * -> *) a. Monad m => a -> m a
    return Coercion
    co }

    --------------
    uType :: TypeOrKind -> CtOrigin -> Type -> Type -> TcM Coercion
    uType TypeOrKind
    t_or_k CtOrigin
    origin Type
    orig_ty1 Type
    orig_ty2
      = do { TcLevel
    tclvl <- TcM TcLevel
    getTcLevel
           ; String -> SDoc -> TcRn ()
    traceTc String
    "u_tys" (SDoc -> TcRn ()) -> SDoc -> TcRn ()
    forall a b. (a -> b) -> a -> b
    $ [SDoc] -> SDoc
    vcat
                  [ String -> SDoc
    text String
    "tclvl" SDoc -> SDoc -> SDoc
    <+> TcLevel -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TcLevel
    tclvl
                  , [SDoc] -> SDoc
    sep [ Type -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Type
    orig_ty1, String -> SDoc
    text String
    "~", Type -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Type
    orig_ty2]
                  , CtOrigin -> SDoc
    pprCtOrigin CtOrigin
    origin]
           ; Coercion
    co <- Type -> Type -> TcM Coercion
    go Type
    orig_ty1 Type
    orig_ty2
           ; if Coercion -> Bool
    isReflCo Coercion
    co
                then String -> SDoc -> TcRn ()
    traceTc String
    "u_tys yields no coercion" SDoc
    Outputable.empty
                else String -> SDoc -> TcRn ()
    traceTc String
    "u_tys yields coercion:" (Coercion -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Coercion
    co)
           ; Coercion -> TcM Coercion
    forall (m :: * -> *) a. Monad m => a -> m a
    return Coercion
    co }
      where
        go :: TcType -> TcType -> TcM CoercionN
            -- The arguments to 'go' are always semantically identical
            -- to orig_ty{1,2} except for looking through type synonyms

         -- Unwrap casts before looking for variables. This way, we can easily
         -- recognize (t |> co) ~ (t |> co), which is nice. Previously, we
         -- didn't do it this way, and then the unification above was deferred.
        go :: Type -> Type -> TcM Coercion
    go (CastTy Type
    t1 Coercion
    co1) Type
    t2
          = do { Coercion
    co_tys <- TypeOrKind -> CtOrigin -> Type -> Type -> TcM Coercion
    uType TypeOrKind
    t_or_k CtOrigin
    origin Type
    t1 Type
    t2
               ; Coercion -> TcM Coercion
    forall (m :: * -> *) a. Monad m => a -> m a
    return (Role -> Type -> Coercion -> Coercion -> Coercion
    mkCoherenceLeftCo Role
    Nominal Type
    t1 Coercion
    co1 Coercion
    co_tys) }

        go Type
    t1 (CastTy Type
    t2 Coercion
    co2)
          = do { Coercion
    co_tys <- TypeOrKind -> CtOrigin -> Type -> Type -> TcM Coercion
    uType TypeOrKind
    t_or_k CtOrigin
    origin Type
    t1 Type
    t2
               ; Coercion -> TcM Coercion
    forall (m :: * -> *) a. Monad m => a -> m a
    return (Role -> Type -> Coercion -> Coercion -> Coercion
    mkCoherenceRightCo Role
    Nominal Type
    t2 Coercion
    co2 Coercion
    co_tys) }

            -- Variables; go for uUnfilledVar
            -- Note that we pass in *original* (before synonym expansion),
            -- so that type variables tend to get filled in with
            -- the most informative version of the type
        go (TyVarTy TcTyVar
    tv1) Type
    ty2
          = do { Maybe Type
    lookup_res <- TcTyVar -> TcM (Maybe Type)
    isFilledMetaTyVar_maybe TcTyVar
    tv1
               ; case Maybe Type
    lookup_res of
                   Just Type
    ty1 -> do { String -> SDoc -> TcRn ()
    traceTc String
    "found filled tyvar" (TcTyVar -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TcTyVar
    tv1 SDoc -> SDoc -> SDoc
    <+> String -> SDoc
    text String
    ":->" SDoc -> SDoc -> SDoc
    <+> Type -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Type
    ty1)
                                    ; Type -> Type -> TcM Coercion
    go Type
    ty1 Type
    ty2 }
                   Maybe Type
    Nothing  -> CtOrigin
    -> TypeOrKind -> SwapFlag -> TcTyVar -> Type -> TcM Coercion
    uUnfilledVar CtOrigin
    origin TypeOrKind
    t_or_k SwapFlag
    NotSwapped TcTyVar
    tv1 Type
    ty2 }
        go Type
    ty1 (TyVarTy TcTyVar
    tv2)
          = do { Maybe Type
    lookup_res <- TcTyVar -> TcM (Maybe Type)
    isFilledMetaTyVar_maybe TcTyVar
    tv2
               ; case Maybe Type
    lookup_res of
                   Just Type
    ty2 -> do { String -> SDoc -> TcRn ()
    traceTc String
    "found filled tyvar" (TcTyVar -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TcTyVar
    tv2 SDoc -> SDoc -> SDoc
    <+> String -> SDoc
    text String
    ":->" SDoc -> SDoc -> SDoc
    <+> Type -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Type
    ty2)
                                  ; Type -> Type -> TcM Coercion
    go Type
    ty1 Type
    ty2 }
                   Maybe Type
    Nothing  -> CtOrigin
    -> TypeOrKind -> SwapFlag -> TcTyVar -> Type -> TcM Coercion
    uUnfilledVar CtOrigin
    origin TypeOrKind
    t_or_k SwapFlag
    IsSwapped TcTyVar
    tv2 Type
    ty1 }

          -- See Note [Expanding synonyms during unification]
        go ty1 :: Type
    ty1@(TyConApp TyCon
    tc1 []) (TyConApp TyCon
    tc2 [])
          | TyCon
    tc1 TyCon -> TyCon -> Bool
    forall a. Eq a => a -> a -> Bool
    == TyCon
    tc2
          = Coercion -> TcM Coercion
    forall (m :: * -> *) a. Monad m => a -> m a
    return (Coercion -> TcM Coercion) -> Coercion -> TcM Coercion
    forall a b. (a -> b) -> a -> b
    $ Type -> Coercion
    mkNomReflCo Type
    ty1

            -- See Note [Expanding synonyms during unification]
            --
            -- Also NB that we recurse to 'go' so that we don't push a
            -- new item on the origin stack. As a result if we have
            --   type Foo = Int
            -- and we try to unify  Foo ~ Bool
            -- we'll end up saying "can't match Foo with Bool"
            -- rather than "can't match "Int with Bool".  See #4535.
        go Type
    ty1 Type
    ty2
          | Just Type
    ty1' <- Type -> Maybe Type
    tcView Type
    ty1 = Type -> Type -> TcM Coercion
    go Type
    ty1' Type
    ty2
          | Just Type
    ty2' <- Type -> Maybe Type
    tcView Type
    ty2 = Type -> Type -> TcM Coercion
    go Type
    ty1  Type
    ty2'

        -- Functions (t1 -> t2) just check the two parts
        -- Do not attempt (c => t); just defer
        go (FunTy { ft_af :: Type -> AnonArgFlag
    ft_af = AnonArgFlag
    VisArg, ft_mult :: Type -> Type
    ft_mult = Type
    w1, ft_arg :: Type -> Type
    ft_arg = Type
    arg1, ft_res :: Type -> Type
    ft_res = Type
    res1 })
           (FunTy { ft_af :: Type -> AnonArgFlag
    ft_af = AnonArgFlag
    VisArg, ft_mult :: Type -> Type
    ft_mult = Type
    w2, ft_arg :: Type -> Type
    ft_arg = Type
    arg2, ft_res :: Type -> Type
    ft_res = Type
    res2 })
          = do { Coercion
    co_l <- TypeOrKind -> CtOrigin -> Type -> Type -> TcM Coercion
    uType TypeOrKind
    t_or_k CtOrigin
    origin Type
    arg1 Type
    arg2
               ; Coercion
    co_r <- TypeOrKind -> CtOrigin -> Type -> Type -> TcM Coercion
    uType TypeOrKind
    t_or_k CtOrigin
    origin Type
    res1 Type
    res2
               ; Coercion
    co_w <- TypeOrKind -> CtOrigin -> Type -> Type -> TcM Coercion
    uType TypeOrKind
    t_or_k CtOrigin
    origin Type
    w1 Type
    w2
               ; Coercion -> TcM Coercion
    forall (m :: * -> *) a. Monad m => a -> m a
    return (Coercion -> TcM Coercion) -> Coercion -> TcM Coercion
    forall a b. (a -> b) -> a -> b
    $ Role -> Coercion -> Coercion -> Coercion -> Coercion
    mkFunCo Role
    Nominal Coercion
    co_w Coercion
    co_l Coercion
    co_r }

            -- Always defer if a type synonym family (type function)
            -- is involved.  (Data families behave rigidly.)
        go ty1 :: Type
    ty1@(TyConApp TyCon
    tc1 ThetaType
    _) Type
    ty2
          | TyCon -> Bool
    isTypeFamilyTyCon TyCon
    tc1 = Type -> Type -> TcM Coercion
    defer Type
    ty1 Type
    ty2
        go Type
    ty1 ty2 :: Type
    ty2@(TyConApp TyCon
    tc2 ThetaType
    _)
          | TyCon -> Bool
    isTypeFamilyTyCon TyCon
    tc2 = Type -> Type -> TcM Coercion
    defer Type
    ty1 Type
    ty2

        go (TyConApp TyCon
    tc1 ThetaType
    tys1) (TyConApp TyCon
    tc2 ThetaType
    tys2)
          -- See Note [Mismatched type lists and application decomposition]
          | TyCon
    tc1 TyCon -> TyCon -> Bool
    forall a. Eq a => a -> a -> Bool
    == TyCon
    tc2, ThetaType -> ThetaType -> Bool
    forall a b. [a] -> [b] -> Bool
    equalLength ThetaType
    tys1 ThetaType
    tys2
          = ASSERT2( isGenerativeTyCon tc1 Nominal, ppr tc1 )
            do { [Coercion]
    cos <- (CtOrigin -> Type -> Type -> TcM Coercion)
    -> [CtOrigin]
    -> ThetaType
    -> ThetaType
    -> IOEnv (Env TcGblEnv TcLclEnv) [Coercion]
    forall (m :: * -> *) a b c d.
    Monad m =>
    (a -> b -> c -> m d) -> [a] -> [b] -> [c] -> m [d]
    zipWith3M (TypeOrKind -> CtOrigin -> Type -> Type -> TcM Coercion
    uType TypeOrKind
    t_or_k) [CtOrigin]
    origins' ThetaType
    tys1 ThetaType
    tys2
               ; Coercion -> TcM Coercion
    forall (m :: * -> *) a. Monad m => a -> m a
    return (Coercion -> TcM Coercion) -> Coercion -> TcM Coercion
    forall a b. (a -> b) -> a -> b
    $ HasDebugCallStack => Role -> TyCon -> [Coercion] -> Coercion
    Role -> TyCon -> [Coercion] -> Coercion
    mkTyConAppCo Role
    Nominal TyCon
    tc1 [Coercion]
    cos }
          where
            origins' :: [CtOrigin]
    origins' = (Bool -> CtOrigin) -> [Bool] -> [CtOrigin]
    forall a b. (a -> b) -> [a] -> [b]
    map (\Bool
    is_vis -> if Bool
    is_vis then CtOrigin
    origin else CtOrigin -> CtOrigin
    toInvisibleOrigin CtOrigin
    origin)
                           (TyCon -> [Bool]
    tcTyConVisibilities TyCon
    tc1)

        go (LitTy TyLit
    m) ty :: Type
    ty@(LitTy TyLit
    n)
          | TyLit
    m TyLit -> TyLit -> Bool
    forall a. Eq a => a -> a -> Bool
    == TyLit
    n
          = Coercion -> TcM Coercion
    forall (m :: * -> *) a. Monad m => a -> m a
    return (Coercion -> TcM Coercion) -> Coercion -> TcM Coercion
    forall a b. (a -> b) -> a -> b
    $ Type -> Coercion
    mkNomReflCo Type
    ty

            -- See Note [Care with type applications]
            -- Do not decompose FunTy against App;
            -- it's often a type error, so leave it for the constraint solver
        go (AppTy Type
    s1 Type
    t1) (AppTy Type
    s2 Type
    t2)
          = Bool -> Type -> Type -> Type -> Type -> TcM Coercion
    go_app (Type -> Bool
    isNextArgVisible Type
    s1) Type
    s1 Type
    t1 Type
    s2 Type
    t2

        go (AppTy Type
    s1 Type
    t1) (TyConApp TyCon
    tc2 ThetaType
    ts2)
          | Just (ThetaType
    ts2', Type
    t2') <- ThetaType -> Maybe (ThetaType, Type)
    forall a. [a] -> Maybe ([a], a)
    snocView ThetaType
    ts2
          = ASSERT( not (mustBeSaturated tc2) )
            Bool -> Type -> Type -> Type -> Type -> TcM Coercion
    go_app (TyCon -> ThetaType -> Bool
    isNextTyConArgVisible TyCon
    tc2 ThetaType
    ts2') Type
    s1 Type
    t1 (TyCon -> ThetaType -> Type
    TyConApp TyCon
    tc2 ThetaType
    ts2') Type
    t2'

        go (TyConApp TyCon
    tc1 ThetaType
    ts1) (AppTy Type
    s2 Type
    t2)
          | Just (ThetaType
    ts1', Type
    t1') <- ThetaType -> Maybe (ThetaType, Type)
    forall a. [a] -> Maybe ([a], a)
    snocView ThetaType
    ts1
          = ASSERT( not (mustBeSaturated tc1) )
            Bool -> Type -> Type -> Type -> Type -> TcM Coercion
    go_app (TyCon -> ThetaType -> Bool
    isNextTyConArgVisible TyCon
    tc1 ThetaType
    ts1') (TyCon -> ThetaType -> Type
    TyConApp TyCon
    tc1 ThetaType
    ts1') Type
    t1' Type
    s2 Type
    t2

        go (CoercionTy Coercion
    co1) (CoercionTy Coercion
    co2)
          = do { let ty1 :: Type
    ty1 = Coercion -> Type
    coercionType Coercion
    co1
                     ty2 :: Type
    ty2 = Coercion -> Type
    coercionType Coercion
    co2
               ; Coercion
    kco <- TypeOrKind -> CtOrigin -> Type -> Type -> TcM Coercion
    uType TypeOrKind
    KindLevel
                              (Type -> Type -> CtOrigin -> Maybe TypeOrKind -> CtOrigin
    KindEqOrigin Type
    orig_ty1 Type
    orig_ty2 CtOrigin
    origin
                                            (TypeOrKind -> Maybe TypeOrKind
    forall a. a -> Maybe a
    Just TypeOrKind
    t_or_k))
                              Type
    ty1 Type
    ty2
               ; Coercion -> TcM Coercion
    forall (m :: * -> *) a. Monad m => a -> m a
    return (Coercion -> TcM Coercion) -> Coercion -> TcM Coercion
    forall a b. (a -> b) -> a -> b
    $ Role -> Coercion -> Coercion -> Coercion -> Coercion
    mkProofIrrelCo Role
    Nominal Coercion
    kco Coercion
    co1 Coercion
    co2 }

            -- Anything else fails
            -- E.g. unifying for-all types, which is relative unusual
        go Type
    ty1 Type
    ty2 = Type -> Type -> TcM Coercion
    defer Type
    ty1 Type
    ty2

        ------------------
        defer :: Type -> Type -> TcM Coercion
    defer Type
    ty1 Type
    ty2   -- See Note [Check for equality before deferring]
          | Type
    ty1 HasDebugCallStack => Type -> Type -> Bool
    Type -> Type -> Bool
    `tcEqType` Type
    ty2 = Coercion -> TcM Coercion
    forall (m :: * -> *) a. Monad m => a -> m a
    return (Type -> Coercion
    mkNomReflCo Type
    ty1)
          | Bool
    otherwise          = TypeOrKind -> CtOrigin -> Type -> Type -> TcM Coercion
    uType_defer TypeOrKind
    t_or_k CtOrigin
    origin Type
    ty1 Type
    ty2

        ------------------
        go_app :: Bool -> Type -> Type -> Type -> Type -> TcM Coercion
    go_app Bool
    vis Type
    s1 Type
    t1 Type
    s2 Type
    t2
          = do { Coercion
    co_s <- TypeOrKind -> CtOrigin -> Type -> Type -> TcM Coercion
    uType TypeOrKind
    t_or_k CtOrigin
    origin Type
    s1 Type
    s2
               ; let arg_origin :: CtOrigin
    arg_origin
                       | Bool
    vis       = CtOrigin
    origin
                       | Bool
    otherwise = CtOrigin -> CtOrigin
    toInvisibleOrigin CtOrigin
    origin
               ; Coercion
    co_t <- TypeOrKind -> CtOrigin -> Type -> Type -> TcM Coercion
    uType TypeOrKind
    t_or_k CtOrigin
    arg_origin Type
    t1 Type
    t2
               ; Coercion -> TcM Coercion
    forall (m :: * -> *) a. Monad m => a -> m a
    return (Coercion -> TcM Coercion) -> Coercion -> TcM Coercion
    forall a b. (a -> b) -> a -> b
    $ Coercion -> Coercion -> Coercion
    mkAppCo Coercion
    co_s Coercion
    co_t }

    {- Note [Check for equality before deferring]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Particularly in ambiguity checks we can get equalities like (ty ~ ty).
    If ty involves a type function we may defer, which isn't very sensible.
    An egregious example of this was in test T9872a, which has a type signature
           Proxy :: Proxy (Solutions Cubes)
    Doing the ambiguity check on this signature generates the equality
       Solutions Cubes ~ Solutions Cubes
    and currently the constraint solver normalises both sides at vast cost.
    This little short-cut in 'defer' helps quite a bit.

    Note [Care with type applications]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Note: type applications need a bit of care!
    They can match FunTy and TyConApp, so use splitAppTy_maybe
    NB: we've already dealt with type variables and Notes,
    so if one type is an App the other one jolly well better be too

    Note [Mismatched type lists and application decomposition]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    When we find two TyConApps, you might think that the argument lists
    are guaranteed equal length.  But they aren't. Consider matching
            w (T x) ~ Foo (T x y)
    We do match (w ~ Foo) first, but in some circumstances we simply create
    a deferred constraint; and then go ahead and match (T x ~ T x y).
    This came up in #3950.

    So either
       (a) either we must check for identical argument kinds
           when decomposing applications,

       (b) or we must be prepared for ill-kinded unification sub-problems

    Currently we adopt (b) since it seems more robust -- no need to maintain
    a global invariant.

    Note [Expanding synonyms during unification]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We expand synonyms during unification, but:
     * We expand *after* the variable case so that we tend to unify
       variables with un-expanded type synonym. This just makes it
       more likely that the inferred types will mention type synonyms
       understandable to the user

     * Similarly, we expand *after* the CastTy case, just in case the
       CastTy wraps a variable.

     * We expand *before* the TyConApp case.  For example, if we have
          type Phantom a = Int
       and are unifying
          Phantom Int ~ Phantom Char
       it is *wrong* to unify Int and Char.

     * The problem case immediately above can happen only with arguments
       to the tycon. So we check for nullary tycons *before* expanding.
       This is particularly helpful when checking (* ~ *), because * is
       now a type synonym.

    Note [Deferred unification]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We may encounter a unification ty1 ~ ty2 that cannot be performed syntactically,
    and yet its consistency is undetermined. Previously, there was no way to still
    make it consistent. So a mismatch error was issued.

    Now these unifications are deferred until constraint simplification, where type
    family instances and given equations may (or may not) establish the consistency.
    Deferred unifications are of the form
                    F ... ~ ...
    or              x ~ ...
    where F is a type function and x is a type variable.
    E.g.
            id :: x ~ y => x -> y
            id e = e

    involves the unification x = y. It is deferred until we bring into account the
    context x ~ y to establish that it holds.

    If available, we defer original types (rather than those where closed type
    synonyms have already been expanded via tcCoreView).  This is, as usual, to
    improve error messages.

    ************************************************************************
    *                                                                      *
                     uUnfilledVar and friends
    *                                                                      *
    ************************************************************************

    @uunfilledVar@ is called when at least one of the types being unified is a
    variable.  It does {\em not} assume that the variable is a fixed point
    of the substitution; rather, notice that @uVar@ (defined below) nips
    back into @uTys@ if it turns out that the variable is already bound.
    -}

    ----------
    uUnfilledVar :: CtOrigin
                 -> TypeOrKind
                 -> SwapFlag
                 -> TcTyVar        -- Tyvar 1: not necessarily a meta-tyvar
                                   --    definitely not a /filled/ meta-tyvar
                 -> TcTauType      -- Type 2
                 -> TcM Coercion
    -- "Unfilled" means that the variable is definitely not a filled-in meta tyvar
    --            It might be a skolem, or untouchable, or meta

    uUnfilledVar :: CtOrigin
    -> TypeOrKind -> SwapFlag -> TcTyVar -> Type -> TcM Coercion
    uUnfilledVar CtOrigin
    origin TypeOrKind
    t_or_k SwapFlag
    swapped TcTyVar
    tv1 Type
    ty2
      = do { Type
    ty2 <- Type -> TcM Type
    zonkTcType Type
    ty2
                 -- Zonk to expose things to the
                 -- occurs check, and so that if ty2
                 -- looks like a type variable then it
                 -- /is/ a type variable
           ; CtOrigin
    -> TypeOrKind -> SwapFlag -> TcTyVar -> Type -> TcM Coercion
    uUnfilledVar1 CtOrigin
    origin TypeOrKind
    t_or_k SwapFlag
    swapped TcTyVar
    tv1 Type
    ty2 }

    ----------
    uUnfilledVar1 :: CtOrigin
                  -> TypeOrKind
                  -> SwapFlag
                  -> TcTyVar        -- Tyvar 1: not necessarily a meta-tyvar
                                    --    definitely not a /filled/ meta-tyvar
                  -> TcTauType      -- Type 2, zonked
                  -> TcM Coercion
    uUnfilledVar1 :: CtOrigin
    -> TypeOrKind -> SwapFlag -> TcTyVar -> Type -> TcM Coercion
    uUnfilledVar1 CtOrigin
    origin TypeOrKind
    t_or_k SwapFlag
    swapped TcTyVar
    tv1 Type
    ty2
      | Just TcTyVar
    tv2 <- Type -> Maybe TcTyVar
    tcGetTyVar_maybe Type
    ty2
      = TcTyVar -> TcM Coercion
    go TcTyVar
    tv2

      | Bool
    otherwise
      = CtOrigin
    -> TypeOrKind -> SwapFlag -> TcTyVar -> Type -> TcM Coercion
    uUnfilledVar2 CtOrigin
    origin TypeOrKind
    t_or_k SwapFlag
    swapped TcTyVar
    tv1 Type
    ty2

      where
        -- 'go' handles the case where both are
        -- tyvars so we might want to swap
        -- E.g. maybe tv2 is a meta-tyvar and tv1 is not
        go :: TcTyVar -> TcM Coercion
    go TcTyVar
    tv2 | TcTyVar
    tv1 TcTyVar -> TcTyVar -> Bool
    forall a. Eq a => a -> a -> Bool
    == TcTyVar
    tv2  -- Same type variable => no-op
               = Coercion -> TcM Coercion
    forall (m :: * -> *) a. Monad m => a -> m a
    return (Type -> Coercion
    mkNomReflCo (TcTyVar -> Type
    mkTyVarTy TcTyVar
    tv1))

               | Bool -> TcTyVar -> TcTyVar -> Bool
    swapOverTyVars Bool
    False TcTyVar
    tv1 TcTyVar
    tv2   -- Distinct type variables
                   -- Swap meta tyvar to the left if poss
               = do { TcTyVar
    tv1 <- TcTyVar -> TcM TcTyVar
    zonkTyCoVarKind TcTyVar
    tv1
                         -- We must zonk tv1's kind because that might
                         -- not have happened yet, and it's an invariant of
                         -- uUnfilledTyVar2 that ty2 is fully zonked
                         -- Omitting this caused #16902
                    ; CtOrigin
    -> TypeOrKind -> SwapFlag -> TcTyVar -> Type -> TcM Coercion
    uUnfilledVar2 CtOrigin
    origin TypeOrKind
    t_or_k (SwapFlag -> SwapFlag
    flipSwap SwapFlag
    swapped)
                               TcTyVar
    tv2 (TcTyVar -> Type
    mkTyVarTy TcTyVar
    tv1) }

               | Bool
    otherwise
               = CtOrigin
    -> TypeOrKind -> SwapFlag -> TcTyVar -> Type -> TcM Coercion
    uUnfilledVar2 CtOrigin
    origin TypeOrKind
    t_or_k SwapFlag
    swapped TcTyVar
    tv1 Type
    ty2

    ----------
    uUnfilledVar2 :: CtOrigin
                  -> TypeOrKind
                  -> SwapFlag
                  -> TcTyVar        -- Tyvar 1: not necessarily a meta-tyvar
                                    --    definitely not a /filled/ meta-tyvar
                  -> TcTauType      -- Type 2, zonked
                  -> TcM Coercion
    uUnfilledVar2 :: CtOrigin
    -> TypeOrKind -> SwapFlag -> TcTyVar -> Type -> TcM Coercion
    uUnfilledVar2 CtOrigin
    origin TypeOrKind
    t_or_k SwapFlag
    swapped TcTyVar
    tv1 Type
    ty2
      = do { DynFlags
    dflags  <- IOEnv (Env TcGblEnv TcLclEnv) DynFlags
    forall (m :: * -> *). HasDynFlags m => m DynFlags
    getDynFlags
           ; TcLevel
    cur_lvl <- TcM TcLevel
    getTcLevel
           ; DynFlags -> TcLevel -> TcM Coercion
    go DynFlags
    dflags TcLevel
    cur_lvl }
      where
        go :: DynFlags -> TcLevel -> TcM Coercion
    go DynFlags
    dflags TcLevel
    cur_lvl
          | TcLevel -> TcTyVar -> Bool
    isTouchableMetaTyVar TcLevel
    cur_lvl TcTyVar
    tv1
               -- See Note [Unification preconditions], (UNTOUCHABLE) wrinkles
          , MetaInfo -> Type -> Bool
    canSolveByUnification (TcTyVar -> MetaInfo
    metaTyVarInfo TcTyVar
    tv1) Type
    ty2
          , CheckTyEqResult -> Bool
    cterHasNoProblem (DynFlags -> TcTyVar -> Type -> CheckTyEqResult
    checkTyVarEq DynFlags
    dflags TcTyVar
    tv1 Type
    ty2)
               -- See Note [Prevent unification with type families]
          = do { Coercion
    co_k <- TypeOrKind -> CtOrigin -> Type -> Type -> TcM Coercion
    uType TypeOrKind
    KindLevel CtOrigin
    kind_origin (HasDebugCallStack => Type -> Type
    Type -> Type
    tcTypeKind Type
    ty2) (TcTyVar -> Type
    tyVarKind TcTyVar
    tv1)
               ; String -> SDoc -> TcRn ()
    traceTc String
    "uUnfilledVar2 ok" (SDoc -> TcRn ()) -> SDoc -> TcRn ()
    forall a b. (a -> b) -> a -> b
    $
                 [SDoc] -> SDoc
    vcat [ TcTyVar -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TcTyVar
    tv1 SDoc -> SDoc -> SDoc
    <+> SDoc
    dcolon SDoc -> SDoc -> SDoc
    <+> Type -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr (TcTyVar -> Type
    tyVarKind TcTyVar
    tv1)
                      , Type -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Type
    ty2 SDoc -> SDoc -> SDoc
    <+> SDoc
    dcolon SDoc -> SDoc -> SDoc
    <+> Type -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr (HasDebugCallStack => Type -> Type
    Type -> Type
    tcTypeKind  Type
    ty2)
                      , Bool -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr (Coercion -> Bool
    isTcReflCo Coercion
    co_k), Coercion -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Coercion
    co_k ]

               ; if Coercion -> Bool
    isTcReflCo Coercion
    co_k
                   -- Only proceed if the kinds match
                   -- NB: tv1 should still be unfilled, despite the kind unification
                   --     because tv1 is not free in ty2 (or, hence, in its kind)
                 then do { TcTyVar -> Type -> TcRn ()
    writeMetaTyVar TcTyVar
    tv1 Type
    ty2
                         ; Coercion -> TcM Coercion
    forall (m :: * -> *) a. Monad m => a -> m a
    return (Type -> Coercion
    mkTcNomReflCo Type
    ty2) }

                 else TcM Coercion
    defer } -- This cannot be solved now.  See GHC.Tc.Solver.Canonical
                              -- Note [Equalities with incompatible kinds]

          | Bool
    otherwise
          = do { String -> SDoc -> TcRn ()
    traceTc String
    "uUnfilledVar2 not ok" (TcTyVar -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TcTyVar
    tv1 SDoc -> SDoc -> SDoc
    $$ Type -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Type
    ty2)
                   -- Occurs check or an untouchable: just defer
                   -- NB: occurs check isn't necessarily fatal:
                   --     eg tv1 occurred in type family parameter
                ; TcM Coercion
    defer }

        ty1 :: Type
    ty1 = TcTyVar -> Type
    mkTyVarTy TcTyVar
    tv1
        kind_origin :: CtOrigin
    kind_origin = Type -> Type -> CtOrigin -> Maybe TypeOrKind -> CtOrigin
    KindEqOrigin Type
    ty1 Type
    ty2 CtOrigin
    origin (TypeOrKind -> Maybe TypeOrKind
    forall a. a -> Maybe a
    Just TypeOrKind
    t_or_k)

        defer :: TcM Coercion
    defer = SwapFlag
    -> (Type -> Type -> TcM Coercion) -> Type -> Type -> TcM Coercion
    forall a b. SwapFlag -> (a -> a -> b) -> a -> a -> b
    unSwap SwapFlag
    swapped (TypeOrKind -> CtOrigin -> Type -> Type -> TcM Coercion
    uType_defer TypeOrKind
    t_or_k CtOrigin
    origin) Type
    ty1 Type
    ty2

    canSolveByUnification :: MetaInfo -> TcType -> Bool
    -- See Note [Unification preconditions, (TYVAR-TV)]
    canSolveByUnification :: MetaInfo -> Type -> Bool
    canSolveByUnification MetaInfo
    info Type
    xi
      = case MetaInfo
    info of
          MetaInfo
    CycleBreakerTv -> Bool
    False
          MetaInfo
    TyVarTv -> case Type -> Maybe TcTyVar
    tcGetTyVar_maybe Type
    xi of
                       Maybe TcTyVar
    Nothing -> Bool
    False
                       Just TcTyVar
    tv -> case TcTyVar -> TcTyVarDetails
    tcTyVarDetails TcTyVar
    tv of
                                     MetaTv { mtv_info :: TcTyVarDetails -> MetaInfo
    mtv_info = MetaInfo
    info }
                                                -> case MetaInfo
    info of
                                                     MetaInfo
    TyVarTv -> Bool
    True
                                                     MetaInfo
    _       -> Bool
    False
                                     SkolemTv {} -> Bool
    True
                                     TcTyVarDetails
    RuntimeUnk  -> Bool
    True
          MetaInfo
    _ -> Bool
    True

    swapOverTyVars :: Bool -> TcTyVar -> TcTyVar -> Bool
    swapOverTyVars :: Bool -> TcTyVar -> TcTyVar -> Bool
    swapOverTyVars Bool
    is_given TcTyVar
    tv1 TcTyVar
    tv2
      -- See Note [Unification variables on the left]
      | Bool -> Bool
    not Bool
    is_given, Int
    pri1 Int -> Int -> Bool
    forall a. Eq a => a -> a -> Bool
    == Int
    0, Int
    pri2 Int -> Int -> Bool
    forall a. Ord a => a -> a -> Bool
    > Int
    0 = Bool
    True
      | Bool -> Bool
    not Bool
    is_given, Int
    pri2 Int -> Int -> Bool
    forall a. Eq a => a -> a -> Bool
    == Int
    0, Int
    pri1 Int -> Int -> Bool
    forall a. Ord a => a -> a -> Bool
    > Int
    0 = Bool
    False

      -- Level comparison: see Note [TyVar/TyVar orientation]
      | TcLevel
    lvl1 TcLevel -> TcLevel -> Bool
    `strictlyDeeperThan` TcLevel
    lvl2 = Bool
    False
      | TcLevel
    lvl2 TcLevel -> TcLevel -> Bool
    `strictlyDeeperThan` TcLevel
    lvl1 = Bool
    True

      -- Priority: see Note [TyVar/TyVar orientation]
      | Int
    pri1 Int -> Int -> Bool
    forall a. Ord a => a -> a -> Bool
    > Int
    pri2 = Bool
    False
      | Int
    pri2 Int -> Int -> Bool
    forall a. Ord a => a -> a -> Bool
    > Int
    pri1 = Bool
    True

      -- Names: see Note [TyVar/TyVar orientation]
      | Name -> Bool
    isSystemName Name
    tv2_name, Bool -> Bool
    not (Name -> Bool
    isSystemName Name
    tv1_name) = Bool
    True

      | Bool
    otherwise = Bool
    False

      where
        lvl1 :: TcLevel
    lvl1 = TcTyVar -> TcLevel
    tcTyVarLevel TcTyVar
    tv1
        lvl2 :: TcLevel
    lvl2 = TcTyVar -> TcLevel
    tcTyVarLevel TcTyVar
    tv2
        pri1 :: Int
    pri1 = TcTyVar -> Int
    lhsPriority TcTyVar
    tv1
        pri2 :: Int
    pri2 = TcTyVar -> Int
    lhsPriority TcTyVar
    tv2
        tv1_name :: Name
    tv1_name = TcTyVar -> Name
    Var.varName TcTyVar
    tv1
        tv2_name :: Name
    tv2_name = TcTyVar -> Name
    Var.varName TcTyVar
    tv2

    lhsPriority :: TcTyVar -> Int
    -- Higher => more important to be on the LHS
    --        => more likely to be eliminated
    -- See Note [TyVar/TyVar orientation]
    lhsPriority :: TcTyVar -> Int
    lhsPriority TcTyVar
    tv
      = ASSERT2( isTyVar tv, ppr tv)
        case TcTyVar -> TcTyVarDetails
    tcTyVarDetails TcTyVar
    tv of
          TcTyVarDetails
    RuntimeUnk  -> Int
    0
          SkolemTv {} -> Int
    0
          MetaTv { mtv_info :: TcTyVarDetails -> MetaInfo
    mtv_info = MetaInfo
    info } -> case MetaInfo
    info of
                                         MetaInfo
    CycleBreakerTv -> Int
    0
                                         MetaInfo
    TyVarTv        -> Int
    1
                                         MetaInfo
    TauTv          -> Int
    2
                                         MetaInfo
    RuntimeUnkTv   -> Int
    3

    {- Note [Unification preconditions]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Question: given a homogeneous equality (alpha ~# ty), when is it OK to
    unify alpha := ty?

    This note only applied to /homogeneous/ equalities, in which both
    sides have the same kind.

    There are three reasons not to unify:

    1. (SKOL-ESC) Skolem-escape
       Consider the constraint
            forall[2] a[2]. alpha[1] ~ Maybe a[2]
       If we unify alpha := Maybe a, the skolem 'a' may escape its scope.
       The level alpha[1] says that alpha may be used outside this constraint,
       where 'a' is not in scope at all.  So we must not unify.

       Bottom line: when looking at a constraint alpha[n] := ty, do not unify
       if any free variable of 'ty' has level deeper (greater) than n

    2. (UNTOUCHABLE) Untouchable unification variables
       Consider the constraint
            forall[2] a[2]. b[1] ~ Int => alpha[1] ~ Int
       There is no (SKOL-ESC) problem with unifying alpha := Int, but it might
       not be the principal solution. Perhaps the "right" solution is alpha := b.
       We simply can't tell.  See "OutsideIn(X): modular type inference with local
       assumptions", section 2.2.  We say that alpha[1] is "untouchable" inside
       this implication.

       Bottom line: at amibient level 'l', when looking at a constraint
       alpha[n] ~ ty, do not unify alpha := ty if there are any given equalities
       between levels 'n' and 'l'.

       Exactly what is a "given equality" for the purpose of (UNTOUCHABLE)?
       Answer: see Note [Tracking Given equalities] in GHC.Tc.Solver.Monad

    3. (TYVAR-TV) Unifying TyVarTvs and CycleBreakerTvs
       This precondition looks at the MetaInfo of the unification variable:

       * TyVarTv: When considering alpha{tyv} ~ ty, if alpha{tyv} is a
         TyVarTv it can only unify with a type variable, not with a
         structured type.  So if 'ty' is a structured type, such as (Maybe x),
         don't unify.

       * CycleBreakerTv: never unified, except by restoreTyVarCycles.

    Needless to say, all three have wrinkles:

    * (SKOL-ESC) Promotion.  Given alpha[n] ~ ty, what if beta[k] is free
      in 'ty', where beta is a unification variable, and k>n?  'beta'
      stands for a monotype, and since it is part of a level-n type
      (equal to alpha[n]), we must /promote/ beta to level n.  Just make
      up a fresh gamma[n], and unify beta[k] := gamma[n].

    * (TYVAR-TV) Unification variables.  Suppose alpha[tyv,n] is a level-n
      TyVarTv (see Note [TyVarTv] in GHC.Tc.Types.TcMType)? Now
      consider alpha[tyv,n] ~ Bool.  We don't want to unify because that
      would break the TyVarTv invariant.

      What about alpha[tyv,n] ~ beta[tau,n], where beta is an ordinary
      TauTv?  Again, don't unify, because beta might later be unified
      with, say Bool.  (If levels permit, we reverse the orientation here;
      see Note [TyVar/TyVar orientation].)

    * (UNTOUCHABLE) Untouchability.  When considering (alpha[n] ~ ty), how
      do we know whether there are any given equalities between level n
      and the ambient level?  We answer in two ways:

      * In the eager unifier, we only unify if l=n.  If not, alpha may be
        untouchable, and defer to the constraint solver.  This check is
        made in GHC.Tc.Utils.uUnifilledVar2, in the guard
        isTouchableMetaTyVar.

      * In the constraint solver, we track where Given equalities occur
        and use that to guard unification in GHC.Tc.Solver.Canonical.unifyTest
        More details in Note [Tracking Given equalities] in GHC.Tc.Solver.InertSet

        Historical note: in the olden days (pre 2021) the constraint solver
        also used to unify only if l=n.  Equalities were "floated" out of the
        implication in a separate step, so that they would become touchable.
        But the float/don't-float question turned out to be very delicate,
        as you can see if you look at the long series of Notes associated with
        GHC.Tc.Solver.floatEqualities, around Nov 2020.  It's much easier
        to unify in-place, with no floating.

    Note [TyVar/TyVar orientation]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Given (a ~ b), should we orient the CEqCan as (a~b) or (b~a)?
    This is a surprisingly tricky question! This is invariant (TyEq:TV).

    The question is answered by swapOverTyVars, which is used
      - in the eager unifier, in GHC.Tc.Utils.Unify.uUnfilledVar1
      - in the constraint solver, in GHC.Tc.Solver.Canonical.canEqCanLHS2

    First note: only swap if you have to!
       See Note [Avoid unnecessary swaps]

    So we look for a positive reason to swap, using a three-step test:

    * Level comparison. If 'a' has deeper level than 'b',
      put 'a' on the left.  See Note [Deeper level on the left]

    * Priority.  If the levels are the same, look at what kind of
      type variable it is, using 'lhsPriority'.

      Generally speaking we always try to put a MetaTv on the left
      in preference to SkolemTv or RuntimeUnkTv:
         a) Because the MetaTv may be touchable and can be unified
         b) Even if it's not touchable, GHC.Tc.Solver.floatEqualities
            looks for meta tyvars on the left

      Tie-breaking rules for MetaTvs:
      - CycleBreakerTv: This is essentially a stand-in for another type;
           it's untouchable and should have the same priority as a skolem: 0.

      - TyVarTv: These can unify only with another tyvar, but we can't unify
           a TyVarTv with a TauTv, because then the TyVarTv could (transitively)
           get a non-tyvar type. So give these a low priority: 1.

      - TauTv: This is the common case; we want these on the left so that they
           can be written to: 2.

      - RuntimeUnkTv: These aren't really meta-variables used in type inference,
           but just a convenience in the implementation of the GHCi debugger.
           Eagerly write to these: 3. See Note [RuntimeUnkTv] in
           GHC.Runtime.Heap.Inspect.

    * Names. If the level and priority comparisons are all
      equal, try to eliminate a TyVar with a System Name in
      favour of ones with a Name derived from a user type signature

    * Age.  At one point in the past we tried to break any remaining
      ties by eliminating the younger type variable, based on their
      Uniques.  See Note [Eliminate younger unification variables]
      (which also explains why we don't do this any more)

    Note [Unification variables on the left]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    For wanteds, but not givens, swap (skolem ~ meta-tv) regardless of
    level, so that the unification variable is on the left.

    * We /don't/ want this for Givens because if we ave
        [G] a[2] ~ alpha[1]
        [W] Bool ~ a[2]
      we want to rewrite the wanted to Bool ~ alpha[1],
      so we can float the constraint and solve it.

    * But for Wanteds putting the unification variable on
      the left means an easier job when floating, and when
      reporting errors -- just fewer cases to consider.

      In particular, we get better skolem-escape messages:
      see #18114

    Note [Deeper level on the left]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The most important thing is that we want to put tyvars with
    the deepest level on the left.  The reason to do so differs for
    Wanteds and Givens, but either way, deepest wins!  Simple.

    * Wanteds.  Putting the deepest variable on the left maximise the
      chances that it's a touchable meta-tyvar which can be solved.

    * Givens. Suppose we have something like
         forall a[2]. b[1] ~ a[2] => beta[1] ~ a[2]

      If we orient the Given a[2] on the left, we'll rewrite the Wanted to
      (beta[1] ~ b[1]), and that can float out of the implication.
      Otherwise it can't.  By putting the deepest variable on the left
      we maximise our changes of eliminating skolem capture.

      See also GHC.Tc.Solver.Monad Note [Let-bound skolems] for another reason
      to orient with the deepest skolem on the left.

      IMPORTANT NOTE: this test does a level-number comparison on
      skolems, so it's important that skolems have (accurate) level
      numbers.

    See #15009 for an further analysis of why "deepest on the left"
    is a good plan.

    Note [Avoid unnecessary swaps]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    If we swap without actually improving matters, we can get an infinite loop.
    Consider
        work item:  a ~ b
       inert item:  b ~ c
    We canonicalise the work-item to (a ~ c).  If we then swap it before
    adding to the inert set, we'll add (c ~ a), and therefore kick out the
    inert guy, so we get
       new work item:  b ~ c
       inert item:     c ~ a
    And now the cycle just repeats

    Historical Note [Eliminate younger unification variables]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Given a choice of unifying
         alpha := beta   or   beta := alpha
    we try, if possible, to eliminate the "younger" one, as determined
    by `ltUnique`.  Reason: the younger one is less likely to appear free in
    an existing inert constraint, and hence we are less likely to be forced
    into kicking out and rewriting inert constraints.

    This is a performance optimisation only.  It turns out to fix
    #14723 all by itself, but clearly not reliably so!

    It's simple to implement (see nicer_to_update_tv2 in swapOverTyVars).
    But, to my surprise, it didn't seem to make any significant difference
    to the compiler's performance, so I didn't take it any further.  Still
    it seemed too nice to discard altogether, so I'm leaving these
    notes.  SLPJ Jan 18.

    Note [Prevent unification with type families]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We prevent unification with type families because of an uneasy compromise.
    It's perfectly sound to unify with type families, and it even improves the
    error messages in the testsuite. It also modestly improves performance, at
    least in some cases. But it's disastrous for test case perf/compiler/T3064.
    Here is the problem: Suppose we have (F ty) where we also have [G] F ty ~ a.
    What do we do? Do we reduce F? Or do we use the given? Hard to know what's
    best. GHC reduces. This is a disaster for T3064, where the type's size
    spirals out of control during reduction. If we prevent
    unification with type families, then the solver happens to use the equality
    before expanding the type family.

    It would be lovely in the future to revisit this problem and remove this
    extra, unnecessary check. But we retain it for now as it seems to work
    better in practice.

    Revisited in Nov '20, along with removing flattening variables. Problem
    is still present, and the solution is still the same.

    Note [Refactoring hazard: metaTyVarUpdateOK]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    I (Richard E.) have a sad story about refactoring this code, retained here
    to prevent others (or a future me!) from falling into the same traps.

    It all started with #11407, which was caused by the fact that the TyVarTy
    case of defer_me didn't look in the kind. But it seemed reasonable to
    simply remove the defer_me check instead.

    It referred to two Notes (since removed) that were out of date, and the
    fast_check code in occurCheckExpand seemed to do just about the same thing as
    defer_me. The one piece that defer_me did that wasn't repeated by
    occurCheckExpand was the type-family check. (See Note [Prevent unification
    with type families].) So I checked the result of occurCheckExpand for any
    type family occurrences and deferred if there were any. This was done
    in commit e9bf7bb5cc9fb3f87dd05111aa23da76b86a8967 .

    This approach turned out not to be performant, because the expanded
    type was bigger than the original type, and tyConsOfType (needed to
    see if there are any type family occurrences) looks through type
    synonyms. So it then struck me that we could dispense with the
    defer_me check entirely. This simplified the code nicely, and it cut
    the allocations in T5030 by half. But, as documented in Note [Prevent
    unification with type families], this destroyed performance in
    T3064. Regardless, I missed this regression and the change was
    committed as 3f5d1a13f112f34d992f6b74656d64d95a3f506d .

    Bottom lines:
     * defer_me is back, but now fixed w.r.t. #11407.
     * Tread carefully before you start to refactor here. There can be
       lots of hard-to-predict consequences.

    Note [Type synonyms and the occur check]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Generally speaking we try to update a variable with type synonyms not
    expanded, which improves later error messages, unless looking
    inside a type synonym may help resolve a spurious occurs check
    error. Consider:
              type A a = ()

              f :: (A a -> a -> ()) -> ()
              f = \ _ -> ()

              x :: ()
              x = f (\ x p -> p x)

    We will eventually get a constraint of the form t ~ A t. The ok function above will
    properly expand the type (A t) to just (), which is ok to be unified with t. If we had
    unified with the original type A t, we would lead the type checker into an infinite loop.

    Hence, if the occurs check fails for a type synonym application, then (and *only* then),
    the ok function expands the synonym to detect opportunities for occurs check success using
    the underlying definition of the type synonym.

    The same applies later on in the constraint interaction code; see GHC.Tc.Solver.Interact,
    function @occ_check_ok@.

    Note [Non-TcTyVars in GHC.Tc.Utils.Unify]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Because the same code is now shared between unifying types and unifying
    kinds, we sometimes will see proper TyVars floating around the unifier.
    Example (from test case polykinds/PolyKinds12):

        type family Apply (f :: k1 -> k2) (x :: k1) :: k2
        type instance Apply g y = g y

    When checking the instance declaration, we first *kind-check* the LHS
    and RHS, discovering that the instance really should be

        type instance Apply k3 k4 (g :: k3 -> k4) (y :: k3) = g y

    During this kind-checking, all the tyvars will be TcTyVars. Then, however,
    as a second pass, we desugar the RHS (which is done in functions prefixed
    with "tc" in GHC.Tc.TyCl"). By this time, all the kind-vars are proper
    TyVars, not TcTyVars, get some kind unification must happen.

    Thus, we always check if a TyVar is a TcTyVar before asking if it's a
    meta-tyvar.

    This used to not be necessary for type-checking (that is, before * :: *)
    because expressions get desugared via an algorithm separate from
    type-checking (with wrappers, etc.). Types get desugared very differently,
    causing this wibble in behavior seen here.
    -}

    -- | Breaks apart a function kind into its pieces.
    matchExpectedFunKind
      :: Outputable fun
      => fun             -- ^ type, only for errors
      -> Arity           -- ^ n: number of desired arrows
      -> TcKind          -- ^ fun_ kind
      -> TcM Coercion    -- ^ co :: fun_kind ~ (arg1 -> ... -> argn -> res)

    matchExpectedFunKind :: forall fun. Outputable fun => fun -> Int -> Type -> TcM Coercion
    matchExpectedFunKind fun
    hs_ty Int
    n Type
    k = Int -> Type -> TcM Coercion
    go Int
    n Type
    k
      where
        go :: Int -> Type -> TcM Coercion
    go Int
    0 Type
    k = Coercion -> TcM Coercion
    forall (m :: * -> *) a. Monad m => a -> m a
    return (Type -> Coercion
    mkNomReflCo Type
    k)

        go Int
    n Type
    k | Just Type
    k' <- Type -> Maybe Type
    tcView Type
    k = Int -> Type -> TcM Coercion
    go Int
    n Type
    k'

        go Int
    n k :: Type
    k@(TyVarTy TcTyVar
    kvar)
          | TcTyVar -> Bool
    isMetaTyVar TcTyVar
    kvar
          = do { MetaDetails
    maybe_kind <- TcTyVar -> TcM MetaDetails
    readMetaTyVar TcTyVar
    kvar
               ; case MetaDetails
    maybe_kind of
                    Indirect Type
    fun_kind -> Int -> Type -> TcM Coercion
    go Int
    n Type
    fun_kind
                    MetaDetails
    Flexi ->             Int -> Type -> TcM Coercion
    defer Int
    n Type
    k }

        go Int
    n (FunTy { ft_mult :: Type -> Type
    ft_mult = Type
    w, ft_arg :: Type -> Type
    ft_arg = Type
    arg, ft_res :: Type -> Type
    ft_res = Type
    res })
          = do { Coercion
    co <- Int -> Type -> TcM Coercion
    go (Int
    nInt -> Int -> Int
    forall a. Num a => a -> a -> a
    -Int
    1) Type
    res
               ; Coercion -> TcM Coercion
    forall (m :: * -> *) a. Monad m => a -> m a
    return (Role -> Coercion -> Coercion -> Coercion -> Coercion
    mkTcFunCo Role
    Nominal (Type -> Coercion
    mkTcNomReflCo Type
    w) (Type -> Coercion
    mkTcNomReflCo Type
    arg) Coercion
    co) }

        go Int
    n Type
    other
         = Int -> Type -> TcM Coercion
    defer Int
    n Type
    other

        defer :: Int -> Type -> TcM Coercion
    defer Int
    n Type
    k
          = do { ThetaType
    arg_kinds <- Int -> TcM ThetaType
    newMetaKindVars Int
    n
               ; Type
    res_kind  <- TcM Type
    newMetaKindVar
               ; let new_fun :: Type
    new_fun = ThetaType -> Type -> Type
    mkVisFunTysMany ThetaType
    arg_kinds Type
    res_kind
                     origin :: CtOrigin
    origin  = TypeEqOrigin { uo_actual :: Type
    uo_actual   = Type
    k
                                            , uo_expected :: Type
    uo_expected = Type
    new_fun
                                            , uo_thing :: Maybe SDoc
    uo_thing    = SDoc -> Maybe SDoc
    forall a. a -> Maybe a
    Just (fun -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr fun
    hs_ty)
                                            , uo_visible :: Bool
    uo_visible  = Bool
    True
                                            }
               ; TypeOrKind -> CtOrigin -> Type -> Type -> TcM Coercion
    uType TypeOrKind
    KindLevel CtOrigin
    origin Type
    k Type
    new_fun }

    {- *********************************************************************
    *                                                                      *
                     Equality invariant checking
    *                                                                      *
    ********************************************************************* -}

    {-  Note [Checking for foralls]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Unless we have -XImpredicativeTypes (which is a totally unsupported
    feature), we do not want to unify
        alpha ~ (forall a. a->a) -> Int
    So we look for foralls hidden inside the type, and it's convenient
    to do that at the same time as the occurs check (which looks for
    occurrences of alpha).

    However, it's not just a question of looking for foralls /anywhere/!
    Consider
       (alpha :: forall k. k->*)  ~  (beta :: forall k. k->*)
    This is legal; e.g. dependent/should_compile/T11635.

    We don't want to reject it because of the forall in beta's kind, but
    (see Note [Occurrence checking: look inside kinds] in GHC.Core.Type)
    we do need to look in beta's kind.  So we carry a flag saying if a
    'forall' is OK, and switch the flag on when stepping inside a kind.

    Why is it OK?  Why does it not count as impredicative polymorphism?
    The reason foralls are bad is because we reply on "seeing" foralls
    when doing implicit instantiation.  But the forall inside the kind is
    fine.  We'll generate a kind equality constraint
      (forall k. k->*) ~ (forall k. k->*)
    to check that the kinds of lhs and rhs are compatible.  If alpha's
    kind had instead been
      (alpha :: kappa)
    then this kind equality would rightly complain about unifying kappa
    with (forall k. k->*)

    -}

    ----------------
    {-# NOINLINE checkTyVarEq #-}  -- checkTyVarEq becomes big after the `inline` fires
    checkTyVarEq :: DynFlags -> TcTyVar -> TcType -> CheckTyEqResult
    checkTyVarEq :: DynFlags -> TcTyVar -> Type -> CheckTyEqResult
    checkTyVarEq DynFlags
    dflags TcTyVar
    tv Type
    ty
      = (DynFlags -> CanEqLHS -> Type -> CheckTyEqResult)
    -> DynFlags -> CanEqLHS -> Type -> CheckTyEqResult
    forall a. a -> a
    inline DynFlags -> CanEqLHS -> Type -> CheckTyEqResult
    checkTypeEq DynFlags
    dflags (TcTyVar -> CanEqLHS
    TyVarLHS TcTyVar
    tv) Type
    ty
        -- inline checkTypeEq so that the `case`s over the CanEqLHS get blasted away

    {-# NOINLINE checkTyFamEq #-}  -- checkTyFamEq becomes big after the `inline` fires
    checkTyFamEq :: DynFlags
                 -> TyCon     -- type function
                 -> [TcType]  -- args, exactly saturated
                 -> TcType    -- RHS
                 -> CheckTyEqResult   -- always drops cteTypeFamily
    checkTyFamEq :: DynFlags -> TyCon -> ThetaType -> Type -> CheckTyEqResult
    checkTyFamEq DynFlags
    dflags TyCon
    fun_tc ThetaType
    fun_args Type
    ty
      = (DynFlags -> CanEqLHS -> Type -> CheckTyEqResult)
    -> DynFlags -> CanEqLHS -> Type -> CheckTyEqResult
    forall a. a -> a
    inline DynFlags -> CanEqLHS -> Type -> CheckTyEqResult
    checkTypeEq DynFlags
    dflags (TyCon -> ThetaType -> CanEqLHS
    TyFamLHS TyCon
    fun_tc ThetaType
    fun_args) Type
    ty
        CheckTyEqResult -> CheckTyEqProblem -> CheckTyEqResult
    `cterRemoveProblem` CheckTyEqProblem
    cteTypeFamily
        -- inline checkTypeEq so that the `case`s over the CanEqLHS get blasted away

    checkTypeEq :: DynFlags -> CanEqLHS -> TcType -> CheckTyEqResult
    -- If cteHasNoProblem (checkTypeEq dflags lhs rhs), then lhs ~ rhs
    -- is a canonical CEqCan.
    --
    -- In particular, this looks for:
    --   (a) a forall type (forall a. blah)
    --   (b) a predicate type (c => ty)
    --   (c) a type family; see Note [Prevent unification with type families]
    --   (d) a blocking coercion hole
    --   (e) an occurrence of the LHS (occurs check)
    --
    -- Note that an occurs-check does not mean "definite error".  For example
    --   type family F a
    --   type instance F Int = Int
    -- consider
    --   b0 ~ F b0
    -- This is perfectly reasonable, if we later get b0 ~ Int.  But we
    -- certainly can't unify b0 := F b0
    --
    -- For (a), (b), and (c) we check only the top level of the type, NOT
    -- inside the kinds of variables it mentions.  For (d) we look deeply
    -- in coercions when the LHS is a tyvar (but skip coercions for type family
    -- LHSs), and for (e) see Note [CEqCan occurs check] in GHC.Tc.Types.Constraint.
    --
    -- checkTypeEq is called from
    --    * checkTyFamEq, checkTyVarEq (which inline it to specialise away the
    --      case-analysis on 'lhs')
    --    * checkEqCanLHSFinish, which does not know the form of 'lhs'
    checkTypeEq :: DynFlags -> CanEqLHS -> Type -> CheckTyEqResult
    checkTypeEq DynFlags
    dflags CanEqLHS
    lhs Type
    ty
      = Type -> CheckTyEqResult
    go Type
    ty
      where
        impredicative :: CheckTyEqResult
    impredicative    = CheckTyEqProblem -> CheckTyEqResult
    cteProblem CheckTyEqProblem
    cteImpredicative
        type_family :: CheckTyEqResult
    type_family      = CheckTyEqProblem -> CheckTyEqResult
    cteProblem CheckTyEqProblem
    cteTypeFamily
        hole_blocker :: CheckTyEqResult
    hole_blocker     = CheckTyEqProblem -> CheckTyEqResult
    cteProblem CheckTyEqProblem
    cteHoleBlocker
        insoluble_occurs :: CheckTyEqResult
    insoluble_occurs = CheckTyEqProblem -> CheckTyEqResult
    cteProblem CheckTyEqProblem
    cteInsolubleOccurs
        soluble_occurs :: CheckTyEqResult
    soluble_occurs   = CheckTyEqProblem -> CheckTyEqResult
    cteProblem CheckTyEqProblem
    cteSolubleOccurs

        -- The GHCi runtime debugger does its type-matching with
        -- unification variables that can unify with a polytype
        -- or a TyCon that would usually be disallowed by bad_tc
        -- See Note [RuntimeUnkTv] in GHC.Runtime.Heap.Inspect
        ghci_tv :: Bool
    ghci_tv
          | TyVarLHS TcTyVar
    tv <- CanEqLHS
    lhs
          , MetaTv { mtv_info :: TcTyVarDetails -> MetaInfo
    mtv_info = MetaInfo
    RuntimeUnkTv } <- TcTyVar -> TcTyVarDetails
    tcTyVarDetails TcTyVar
    tv
          = Bool
    True

          | Bool
    otherwise
          = Bool
    False

        go :: TcType -> CheckTyEqResult
        go :: Type -> CheckTyEqResult
    go (TyVarTy TcTyVar
    tv')           = TcTyVar -> CheckTyEqResult
    go_tv TcTyVar
    tv'
        go (TyConApp TyCon
    tc ThetaType
    tys)       = TyCon -> ThetaType -> CheckTyEqResult
    go_tc TyCon
    tc ThetaType
    tys
        go (LitTy {})              = CheckTyEqResult
    cteOK
        go (FunTy {ft_af :: Type -> AnonArgFlag
    ft_af = AnonArgFlag
    af, ft_mult :: Type -> Type
    ft_mult = Type
    w, ft_arg :: Type -> Type
    ft_arg = Type
    a, ft_res :: Type -> Type
    ft_res = Type
    r})
                                   = Type -> CheckTyEqResult
    go Type
    w CheckTyEqResult -> CheckTyEqResult -> CheckTyEqResult
    forall a. Semigroup a => a -> a -> a
    S.<> Type -> CheckTyEqResult
    go Type
    a CheckTyEqResult -> CheckTyEqResult -> CheckTyEqResult
    forall a. Semigroup a => a -> a -> a
    S.<> Type -> CheckTyEqResult
    go Type
    r CheckTyEqResult -> CheckTyEqResult -> CheckTyEqResult
    forall a. Semigroup a => a -> a -> a
    S.<>
                                     if Bool -> Bool
    not Bool
    ghci_tv Bool -> Bool -> Bool
    && AnonArgFlag
    af AnonArgFlag -> AnonArgFlag -> Bool
    forall a. Eq a => a -> a -> Bool
    == AnonArgFlag
    InvisArg
                                       then CheckTyEqResult
    impredicative
                                       else CheckTyEqResult
    cteOK
        go (AppTy Type
    fun Type
    arg) = Type -> CheckTyEqResult
    go Type
    fun CheckTyEqResult -> CheckTyEqResult -> CheckTyEqResult
    forall a. Semigroup a => a -> a -> a
    S.<> Type -> CheckTyEqResult
    go Type
    arg
        go (CastTy Type
    ty Coercion
    co)  = Type -> CheckTyEqResult
    go Type
    ty  CheckTyEqResult -> CheckTyEqResult -> CheckTyEqResult
    forall a. Semigroup a => a -> a -> a
    S.<> Coercion -> CheckTyEqResult
    go_co Coercion
    co
        go (CoercionTy Coercion
    co) = Coercion -> CheckTyEqResult
    go_co Coercion
    co
        go (ForAllTy (Bndr TcTyVar
    tv' ArgFlag
    _) Type
    ty) = (case CanEqLHS
    lhs of
          TyVarLHS TcTyVar
    tv | TcTyVar
    tv TcTyVar -> TcTyVar -> Bool
    forall a. Eq a => a -> a -> Bool
    == TcTyVar
    tv' -> Type -> CheckTyEqResult
    go_occ (TcTyVar -> Type
    tyVarKind TcTyVar
    tv') CheckTyEqResult -> CheckTyEqResult -> CheckTyEqResult
    forall a. Semigroup a => a -> a -> a
    S.<> CheckTyEqResult -> CheckTyEqResult
    cterClearOccursCheck (Type -> CheckTyEqResult
    go Type
    ty)
                      | Bool
    otherwise -> Type -> CheckTyEqResult
    go_occ (TcTyVar -> Type
    tyVarKind TcTyVar
    tv') CheckTyEqResult -> CheckTyEqResult -> CheckTyEqResult
    forall a. Semigroup a => a -> a -> a
    S.<> Type -> CheckTyEqResult
    go Type
    ty
          CanEqLHS
    _                       -> Type -> CheckTyEqResult
    go Type
    ty)
          CheckTyEqResult -> CheckTyEqResult -> CheckTyEqResult
    forall a. Semigroup a => a -> a -> a
    S.<>
          if Bool
    ghci_tv then CheckTyEqResult
    cteOK else CheckTyEqResult
    impredicative

        go_tv :: TcTyVar -> CheckTyEqResult
          -- this slightly peculiar way of defining this means
          -- we don't have to evaluate this `case` at every variable
          -- occurrence
        go_tv :: TcTyVar -> CheckTyEqResult
    go_tv = case CanEqLHS
    lhs of
          TyVarLHS TcTyVar
    tv -> \ TcTyVar
    tv' -> Type -> CheckTyEqResult
    go_occ (TcTyVar -> Type
    tyVarKind TcTyVar
    tv') CheckTyEqResult -> CheckTyEqResult -> CheckTyEqResult
    forall a. Semigroup a => a -> a -> a
    S.<>
                                  if TcTyVar
    tv TcTyVar -> TcTyVar -> Bool
    forall a. Eq a => a -> a -> Bool
    == TcTyVar
    tv' then CheckTyEqResult
    insoluble_occurs else CheckTyEqResult
    cteOK
          TyFamLHS {} -> \ TcTyVar
    _tv' -> CheckTyEqResult
    cteOK
               -- See Note [Occurrence checking: look inside kinds] in GHC.Core.Type

         -- For kinds, we only do an occurs check; we do not worry
         -- about type families or foralls
         -- See Note [Checking for foralls]
        go_occ :: Type -> CheckTyEqResult
    go_occ Type
    k = CheckTyEqResult -> CheckTyEqResult
    cterFromKind (CheckTyEqResult -> CheckTyEqResult)
    -> CheckTyEqResult -> CheckTyEqResult
    forall a b. (a -> b) -> a -> b
    $ Type -> CheckTyEqResult
    go Type
    k

        go_tc :: TyCon -> [TcType] -> CheckTyEqResult
          -- this slightly peculiar way of defining this means
          -- we don't have to evaluate this `case` at every tyconapp
        go_tc :: TyCon -> ThetaType -> CheckTyEqResult
    go_tc = case CanEqLHS
    lhs of
          TyVarLHS {} -> \ TyCon
    tc ThetaType
    tys -> TyCon -> CheckTyEqResult
    check_tc TyCon
    tc CheckTyEqResult -> CheckTyEqResult -> CheckTyEqResult
    forall a. Semigroup a => a -> a -> a
    S.<> TyCon -> ThetaType -> CheckTyEqResult
    go_tc_args TyCon
    tc ThetaType
    tys
          TyFamLHS TyCon
    fam_tc ThetaType
    fam_args -> \ TyCon
    tc ThetaType
    tys ->
            if TyCon -> ThetaType -> TyCon -> ThetaType -> Bool
    tcEqTyConApps TyCon
    fam_tc ThetaType
    fam_args TyCon
    tc ThetaType
    tys
              then CheckTyEqResult
    insoluble_occurs
              else TyCon -> CheckTyEqResult
    check_tc TyCon
    tc CheckTyEqResult -> CheckTyEqResult -> CheckTyEqResult
    forall a. Semigroup a => a -> a -> a
    S.<> TyCon -> ThetaType -> CheckTyEqResult
    go_tc_args TyCon
    tc ThetaType
    tys

          -- just look at arguments, not the tycon itself
        go_tc_args :: TyCon -> [TcType] -> CheckTyEqResult
        go_tc_args :: TyCon -> ThetaType -> CheckTyEqResult
    go_tc_args TyCon
    tc ThetaType
    tys | TyCon -> Role -> Bool
    isGenerativeTyCon TyCon
    tc Role
    Nominal = (Type -> CheckTyEqResult) -> ThetaType -> CheckTyEqResult
    forall (t :: * -> *) m a.
    (Foldable t, Monoid m) =>
    (a -> m) -> t a -> m
    foldMap Type -> CheckTyEqResult
    go ThetaType
    tys
                          | Bool
    otherwise
                          = let (ThetaType
    tf_args, ThetaType
    non_tf_args) = Int -> ThetaType -> (ThetaType, ThetaType)
    forall a. Int -> [a] -> ([a], [a])
    splitAt (TyCon -> Int
    tyConArity TyCon
    tc) ThetaType
    tys in
                            CheckTyEqResult -> CheckTyEqResult
    cterSetOccursCheckSoluble ((Type -> CheckTyEqResult) -> ThetaType -> CheckTyEqResult
    forall (t :: * -> *) m a.
    (Foldable t, Monoid m) =>
    (a -> m) -> t a -> m
    foldMap Type -> CheckTyEqResult
    go ThetaType
    tf_args) CheckTyEqResult -> CheckTyEqResult -> CheckTyEqResult
    forall a. Semigroup a => a -> a -> a
    S.<> (Type -> CheckTyEqResult) -> ThetaType -> CheckTyEqResult
    forall (t :: * -> *) m a.
    (Foldable t, Monoid m) =>
    (a -> m) -> t a -> m
    foldMap Type -> CheckTyEqResult
    go ThetaType
    non_tf_args

         -- no bother about impredicativity in coercions, as they're
         -- inferred
        go_co :: Coercion -> CheckTyEqResult
    go_co Coercion
    co | TyVarLHS TcTyVar
    tv <- CanEqLHS
    lhs
                 , TcTyVar
    tv TcTyVar -> VarSet -> Bool
    `elemVarSet` Coercion -> VarSet
    tyCoVarsOfCo Coercion
    co
                 = CheckTyEqResult
    soluble_occurs CheckTyEqResult -> CheckTyEqResult -> CheckTyEqResult
    forall a. Semigroup a => a -> a -> a
    S.<> CheckTyEqResult
    maybe_hole_blocker

            -- Don't check coercions for type families; see commentary at top of function
                 | Bool
    otherwise
                 = CheckTyEqResult
    maybe_hole_blocker
          where
            -- See GHC.Tc.Solver.Canonical Note [Equalities with incompatible kinds]
            -- Wrinkle (2) about this case in general, Wrinkle (4b) about the check for
            -- deferred type errors
            maybe_hole_blocker :: CheckTyEqResult
    maybe_hole_blocker | Bool -> Bool
    not (GeneralFlag -> DynFlags -> Bool
    gopt GeneralFlag
    Opt_DeferTypeErrors DynFlags
    dflags)
                               , Coercion -> Bool
    hasCoercionHoleCo Coercion
    co
                               = CheckTyEqResult
    hole_blocker

                               | Bool
    otherwise
                               = CheckTyEqResult
    cteOK

        check_tc :: TyCon -> CheckTyEqResult
        check_tc :: TyCon -> CheckTyEqResult
    check_tc
          | Bool
    ghci_tv   = \ TyCon
    _tc -> CheckTyEqResult
    cteOK
          | Bool
    otherwise = \ TyCon
    tc  -> (if TyCon -> Bool
    isTauTyCon TyCon
    tc then CheckTyEqResult
    cteOK else CheckTyEqResult
    impredicative) CheckTyEqResult -> CheckTyEqResult -> CheckTyEqResult
    forall a. Semigroup a => a -> a -> a
    S.<>
                                 (if TyCon -> Bool
    isFamFreeTyCon TyCon
    tc then CheckTyEqResult
    cteOK else CheckTyEqResult
    type_family)