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

    Type subsumption and unification
    -}

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

    module TcUnify (
      -- Full-blown subsumption
      tcWrapResult, tcWrapResultO, tcSkolemise, tcSkolemiseET,
      tcSubTypeHR, tcSubType, tcSubTypeO, tcSubType_NC, tcSubTypeDS, tcSubTypeDS_O,
      tcSubTypeDS_NC, tcSubTypeDS_NC_O, tcSubTypeET, tcSubTypeET_NC,
      checkConstraints, buildImplicationFor,

      -- Various unifications
      unifyType_, unifyType, unifyTheta, unifyKind, noThing,
      uType, unifyExpType,
      swapOverTyVars, canSolveByUnification,

      --------------------------------
      -- Holes
      tcInfer,
      matchExpectedListTy,
      matchExpectedPArrTy,
      matchExpectedTyConApp,
      matchExpectedAppTy,
      matchExpectedFunTys,
      matchActualFunTys, matchActualFunTysPart,
      matchExpectedFunKind,

      wrapFunResCoercion

      ) where

    #include "HsVersions.h"

    import HsSyn
    import TyCoRep
    import TcMType
    import TcRnMonad
    import TcType
    import Type
    import Coercion
    import TcEvidence
    import Name ( isSystemName )
    import Inst
    import TyCon
    import TysWiredIn
    import Var
    import VarSet
    import VarEnv
    import ErrUtils
    import DynFlags
    import BasicTypes
    import Name   ( Name )
    import Bag
    import Util
    import Outputable
    import FastString

    import Control.Monad
    import Control.Arrow ( second )

    {-
    ************************************************************************
    *                                                                      *
                 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

    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.
    matchExpectedFunTys :: SDoc   -- See Note [Herald for matchExpectedFunTys]
                        -> Arity
                        -> ExpRhoType  -- deeply skolemised
                        -> ([ExpSigmaType] -> ExpRhoType -> TcM a)
                              -- must fill in these ExpTypes here
                        -> TcM (a, HsWrapper)
    -- If    matchExpectedFunTys n ty = (_, wrap)
    -- then  wrap : (t1 -> ... -> tn -> ty_r) "->" ty,
    --   where [t1, ..., tn], ty_r are passed to the thing_inside
    matchExpectedFunTys herald arity orig_ty thing_inside
      = case orig_ty of
          Check ty -> go [] arity ty
          _        -> defer [] arity orig_ty
      where
        go acc_arg_tys 0 ty
          = do { result <- thing_inside (reverse acc_arg_tys) (mkCheckExpType ty)
               ; return (result, idHsWrapper) }

        go acc_arg_tys n ty
          | Just ty' <- coreView ty = go acc_arg_tys n ty'

        go acc_arg_tys n (ForAllTy (Anon arg_ty) res_ty)
          = ASSERT( not (isPredTy arg_ty) )
            do { (result, wrap_res) <- go (mkCheckExpType arg_ty : acc_arg_tys)
                                          (n-1) res_ty
               ; return ( result
                        , mkWpFun idHsWrapper wrap_res arg_ty res_ty ) }

        go acc_arg_tys n ty@(TyVarTy tv)
          | ASSERT( isTcTyVar tv) isMetaTyVar tv
          = do { cts <- readMetaTyVar tv
               ; case cts of
                   Indirect ty' -> go acc_arg_tys n ty'
                   Flexi        -> defer acc_arg_tys n (mkCheckExpType 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 Trac #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 Trac #9605.
        go acc_arg_tys n ty = addErrCtxtM mk_ctxt $
                              defer acc_arg_tys n (mkCheckExpType ty)

        ------------
        defer acc_arg_tys n fun_ty
          = do { more_arg_tys <- replicateM n newOpenInferExpType
               ; res_ty       <- newOpenInferExpType
               ; result       <- thing_inside (reverse acc_arg_tys ++ more_arg_tys) res_ty
               ; more_arg_tys <- mapM readExpType more_arg_tys
               ; res_ty       <- readExpType res_ty
               ; let unif_fun_ty = mkFunTys more_arg_tys res_ty
               ; wrap <- tcSubTypeDS GenSigCtxt noThing unif_fun_ty fun_ty
               ; return (result, wrap) }

        ------------
        mk_ctxt :: TidyEnv -> TcM (TidyEnv, MsgDoc)
        mk_ctxt env = do { (env', ty) <- zonkTidyTcType env orig_tc_ty
                         ; let (args, _) = tcSplitFunTys ty
                               n_actual = length args
                               (env'', orig_ty') = tidyOpenType env' orig_tc_ty
                         ; return ( env''
                                  , mk_fun_tys_msg orig_ty' ty n_actual arity herald) }
          where
            orig_tc_ty = checkingExpType "matchExpectedFunTys" orig_ty
                -- this is safe b/c we're called from "go"

    -- Like 'matchExpectedFunTys', but used when you have an "actual" type,
    -- for example in function application
    matchActualFunTys :: Outputable a
                      => SDoc   -- See Note [Herald for matchExpectedFunTys]
                      -> CtOrigin
                      -> Maybe a   -- the thing with type TcSigmaType
                      -> Arity
                      -> TcSigmaType
                      -> TcM (HsWrapper, [TcSigmaType], TcSigmaType)
    -- If    matchActualFunTys n ty = (wrap, [t1,..,tn], ty_r)
    -- then  wrap : ty "->" (t1 -> ... -> tn -> ty_r)
    matchActualFunTys herald ct_orig mb_thing arity ty
      = matchActualFunTysPart herald ct_orig mb_thing arity ty [] arity

    -- | Variant of 'matchActualFunTys' that works when supplied only part
    -- (that is, to the right of some arrows) of the full function type
    matchActualFunTysPart :: Outputable a
                          => SDoc -- See Note [Herald for matchExpectedFunTys]
                          -> CtOrigin
                          -> Maybe a  -- the thing with type TcSigmaType
                          -> Arity
                          -> TcSigmaType
                          -> [TcSigmaType] -- reversed args. See (*) below.
                          -> Arity   -- overall arity of the function, for errs
                          -> TcM (HsWrapper, [TcSigmaType], TcSigmaType)
    matchActualFunTysPart herald ct_orig mb_thing arity orig_ty
                          orig_old_args full_arity
      = go arity orig_old_args orig_ty
    -- 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

    -- (*) Sometimes it's necessary to call matchActualFunTys with only part
    -- (that is, to the right of some arrows) of the type of the function in
    -- question. (See TcExpr.tcArgs.) This argument is the reversed list of
    -- arguments already seen (that is, not part of the TcSigmaType passed
    -- in elsewhere).

      where
        -- This function has a bizarre mechanic: it accumulates arguments on
        -- the way down and also builds an argument list on the way up. Why:
        -- 1. The returns args list and the accumulated args list might be different.
        --    The accumulated args include all the arg types for the function,
        --    including those from before this function was called. The returned
        --    list should include only those arguments produced by this call of
        --    matchActualFunTys
        --
        -- 2. The HsWrapper can be built only on the way up. It seems (more)
        --    bizarre to build the HsWrapper but not the arg_tys.
        --
        -- Refactoring is welcome.
        go :: Arity
           -> [TcSigmaType] -- accumulator of arguments (reversed)
           -> TcSigmaType   -- the remainder of the type as we're processing
           -> TcM (HsWrapper, [TcSigmaType], TcSigmaType)
        go 0 _ ty = return (idHsWrapper, [], ty)

        go n acc_args ty
          | not (null tvs && null theta)
          = do { (wrap1, rho) <- topInstantiate ct_orig ty
               ; (wrap2, arg_tys, res_ty) <- go n acc_args rho
               ; return (wrap2 <.> wrap1, arg_tys, res_ty) }
          where
            (tvs, theta, _) = tcSplitSigmaTy ty

        go n acc_args ty
          | Just ty' <- coreView ty = go n acc_args ty'

        go n acc_args (ForAllTy (Anon arg_ty) res_ty)
          = ASSERT( not (isPredTy arg_ty) )
            do { (wrap_res, tys, ty_r) <- go (n-1) (arg_ty : acc_args) res_ty
               ; return ( mkWpFun idHsWrapper wrap_res arg_ty ty_r
                        , arg_ty : tys, ty_r ) }

        go n acc_args ty@(TyVarTy tv)
          | ASSERT( isTcTyVar tv) isMetaTyVar tv
          = do { cts <- readMetaTyVar tv
               ; case cts of
                   Indirect ty' -> go n acc_args ty'
                   Flexi        -> defer n 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 Trac #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 Trac #9605.
        go n acc_args ty = addErrCtxtM (mk_ctxt (reverse acc_args) ty) $
                           defer n ty

        ------------
        defer n fun_ty
          = do { arg_tys <- replicateM n newOpenFlexiTyVarTy
               ; res_ty  <- newOpenFlexiTyVarTy
               ; let unif_fun_ty = mkFunTys arg_tys res_ty
               ; co <- unifyType mb_thing fun_ty unif_fun_ty
               ; return (mkWpCastN co, arg_tys, res_ty) }

        ------------
        mk_ctxt :: [TcSigmaType] -> TcSigmaType -> TidyEnv -> TcM (TidyEnv, MsgDoc)
        mk_ctxt arg_tys res_ty env
          = do { let ty = mkFunTys arg_tys res_ty
               ; (env1, zonked) <- zonkTidyTcType env ty
                       -- zonking might change # of args
               ; let (zonked_args, _) = tcSplitFunTys zonked
                     n_actual         = length zonked_args
                     (env2, unzonked) = tidyOpenType env1 ty
               ; return ( env2
                        , mk_fun_tys_msg unzonked zonked n_actual full_arity herald) }

    mk_fun_tys_msg :: TcType  -- the full type passed in (unzonked)
                   -> TcType  -- the full type passed in (zonked)
                   -> Arity   -- the # of args found
                   -> Arity   -- the # of args wanted
                   -> SDoc    -- overall herald
                   -> SDoc
    mk_fun_tys_msg full_ty ty n_args full_arity herald
      = herald <+> speakNOf full_arity (text "argument") <> comma $$
        if n_args == full_arity
          then text "its type is" <+> quotes (pprType full_ty) <>
               comma $$
               text "it is specialized to" <+> quotes (pprType ty)
          else sep [text "but its type" <+> quotes (pprType ty),
                    if n_args == 0 then text "has none"
                    else text "has only" <+> speakN n_args]

    ----------------------
    matchExpectedListTy :: TcRhoType -> TcM (TcCoercionN, TcRhoType)
    -- Special case for lists
    matchExpectedListTy exp_ty
     = do { (co, [elt_ty]) <- matchExpectedTyConApp listTyCon exp_ty
          ; return (co, elt_ty) }

    ----------------------
    matchExpectedPArrTy :: TcRhoType -> TcM (TcCoercionN, TcRhoType)
    -- Special case for parrs
    matchExpectedPArrTy exp_ty
      = do { (co, [elt_ty]) <- matchExpectedTyConApp parrTyCon exp_ty
           ; return (co, 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 tc orig_ty
      = go orig_ty
      where
        go ty
           | Just ty' <- coreView ty
           = go ty'

        go ty@(TyConApp tycon args)
           | tc == tycon  -- Common case
           = return (mkTcNomReflCo ty, args)

        go (TyVarTy tv)
           | ASSERT( isTcTyVar tv) isMetaTyVar tv
           = do { cts <- readMetaTyVar tv
                ; case cts of
                    Indirect ty -> go ty
                    Flexi       -> defer }

        go _ = 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 instantate T's data constructors with
        --    (a::*) ~ Maybe
        -- because that'll make types that are utterly ill-kinded.
        -- This happened in Trac #7368
        defer
          = do { (_subst, args) <- tcInstBinders (tyConBinders tc)
               ; co <- unifyType noThing (mkTyConApp tc args) orig_ty
               ; return (co, 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 orig_ty
      = go orig_ty
      where
        go ty
          | Just ty' <- coreView ty = go ty'

          | Just (fun_ty, arg_ty) <- tcSplitAppTy_maybe ty
          = return (mkTcNomReflCo orig_ty, (fun_ty, arg_ty))

        go (TyVarTy tv)
          | ASSERT( isTcTyVar tv) isMetaTyVar tv
          = do { cts <- readMetaTyVar tv
               ; case cts of
                   Indirect ty -> go ty
                   Flexi       -> defer }

        go _ = defer

        -- Defer splitting by generating an equality constraint
        defer
          = do { ty1 <- newFlexiTyVarTy kind1
               ; ty2 <- newFlexiTyVarTy kind2
               ; co <- unifyType noThing (mkAppTy ty1 ty2) orig_ty
               ; return (co, (ty1, ty2)) }

        orig_kind = typeKind orig_ty
        kind1 = mkFunTy liftedTypeKind orig_kind
        kind2 = 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 coercion function
            co_fn :: actual_ty ~ expected_ty
    which takes an HsExpr of type actual_ty into one of type
    expected_ty.

    These functions do not actually check for subsumption. They check if
    expected_ty is an appropriate annotation to use for something of type
    actual_ty. This difference matters when thinking about visible type
    application. For example,

       forall a. a -> forall b. b -> b
          DOES NOT SUBSUME
       forall a b. a -> b -> b

    because the type arguments appear in a different order. (Neither does
    it work the other way around.) BUT, these types are appropriate annotations
    for one another. Because the user directs annotations, it's OK if some
    arguments shuffle around -- after all, it's what the user wants.
    Bottom line: none of this changes with visible type application.

    There are a number of wrinkles (below).

    Notice that Wrinkle 1 and 2 both require eta-expansion, which technically
    may increase termination.  We just put up with this, in exchange for getting
    more predictable type inference.

    Wrinkle 1: Note [Deep skolemisation]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We want   (forall a. Int -> a -> a)  <=  (Int -> forall a. a->a)
    (see section 4.6 of "Practical type inference for higher rank types")
    So we must deeply-skolemise the RHS before we instantiate the LHS.

    That is why tc_sub_type starts with a call to tcSkolemise (which does the
    deep skolemisation), and then calls the DS variant (which assumes
    that expected_ty is deeply skolemised)

    Wrinkle 2: Note [Co/contra-variance of subsumption checking]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider  g :: (Int -> Int) -> Int
      f1 :: (forall a. a -> a) -> Int
      f1 = g

      f2 :: (forall a. a -> a) -> Int
      f2 x = g x
    f2 will typecheck, and it would be odd/fragile if f1 did not.
    But f1 will only typecheck if we have that
        (Int->Int) -> Int  <=  (forall a. a->a) -> Int
    And that is only true if we do the full co/contravariant thing
    in the subsumption check.  That happens in the FunTy case of
    tcSubTypeDS_NC_O, and is the sole reason for the WpFun form of
    HsWrapper.

    Another powerful reason for doing this co/contra stuff is visible
    in Trac #9569, involving instantiation of constraint variables,
    and again involving eta-expansion.

    Wrinkle 3: Note [Higher rank types]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider tc150:
      f y = \ (x::forall a. a->a). blah
    The following happens:
    * We will infer the type of the RHS, ie with a res_ty = alpha.
    * Then the lambda will split  alpha := beta -> gamma.
    * And then we'll check tcSubType IsSwapped beta (forall a. a->a)

    So it's important that we unify beta := forall a. a->a, rather than
    skolemising the type.
    -}

    -- | Call this variant when you are in a higher-rank situation and
    -- you know the right-hand type is deeply skolemised.
    tcSubTypeHR :: Outputable a
                => CtOrigin    -- ^ of the actual type
                -> Maybe a     -- ^ If present, it has type ty_actual
                -> TcSigmaType -> ExpRhoType -> TcM HsWrapper
    tcSubTypeHR orig = tcSubTypeDS_NC_O orig GenSigCtxt

    tcSubType :: Outputable a
              => UserTypeCtxt -> Maybe a  -- ^ If present, it has type ty_actual
              -> TcSigmaType -> ExpSigmaType -> TcM HsWrapper
    -- Checks that actual <= expected
    -- Returns HsWrapper :: actual ~ expected
    tcSubType ctxt maybe_thing ty_actual ty_expected
      = tcSubTypeO origin ctxt ty_actual ty_expected
      where
        origin = TypeEqOrigin { uo_actual   = ty_actual
                              , uo_expected = ty_expected
                              , uo_thing    = mkErrorThing <$> maybe_thing }

    -- | This is like 'tcSubType' but accepts an 'ExpType' as the /actual/ type.
    -- You probably want this only when looking at patterns, never expressions.
    tcSubTypeET :: CtOrigin -> ExpSigmaType -> TcSigmaType -> TcM HsWrapper
    tcSubTypeET orig ty_actual ty_expected
      = uExpTypeX orig ty_expected ty_actual
                  (return . mkWpCastN . mkTcSymCo)
                  (\ty_a -> tcSubTypeO orig GenSigCtxt ty_a
                                        (mkCheckExpType ty_expected))

    -- | This is like 'tcSubType' but accepts an 'ExpType' as the /actual/ type.
    -- You probably want this only when looking at patterns, never expressions.
    -- Does not add context.
    tcSubTypeET_NC :: UserTypeCtxt -> ExpSigmaType -> TcSigmaType -> TcM HsWrapper
    tcSubTypeET_NC _ ty_actual@(Infer {}) ty_expected
      = mkWpCastN . mkTcSymCo <$> unifyExpType noThing ty_expected ty_actual
    tcSubTypeET_NC ctxt (Check ty_actual) ty_expected
      = tc_sub_type orig orig ctxt ty_actual ty_expected'
      where
        ty_expected' = mkCheckExpType ty_expected
        orig = TypeEqOrigin { uo_actual   = ty_actual
                            , uo_expected = ty_expected'
                            , uo_thing    = Nothing }

    tcSubTypeO :: CtOrigin      -- ^ of the actual type
               -> UserTypeCtxt  -- ^ of the expected type
               -> TcSigmaType
               -> ExpSigmaType
               -> TcM HsWrapper
    tcSubTypeO origin ctxt ty_actual ty_expected
      = addSubTypeCtxt ty_actual ty_expected $
        do { traceTc "tcSubType" (vcat [ pprCtOrigin origin
                                       , pprUserTypeCtxt ctxt
                                       , ppr ty_actual
                                       , ppr ty_expected ])
           ; tc_sub_type eq_orig origin ctxt ty_actual ty_expected }
      where
        eq_orig | TypeEqOrigin {} <- origin = origin
                | otherwise
                = TypeEqOrigin { uo_actual   = ty_actual
                               , uo_expected = ty_expected
                               , uo_thing    = Nothing }

    tcSubTypeDS :: Outputable a => UserTypeCtxt -> Maybe a  -- ^ has type ty_actual
                -> TcSigmaType -> ExpRhoType -> TcM HsWrapper
    -- Just like tcSubType, but with the additional precondition that
    -- ty_expected is deeply skolemised (hence "DS")
    tcSubTypeDS ctxt m_expr ty_actual ty_expected
      = addSubTypeCtxt ty_actual ty_expected $
        tcSubTypeDS_NC ctxt m_expr ty_actual ty_expected

    -- | Like 'tcSubTypeDS', but takes a 'CtOrigin' to use when instantiating
    -- the "actual" type
    tcSubTypeDS_O :: Outputable a
                  => CtOrigin -> UserTypeCtxt
                  -> Maybe a -> TcSigmaType -> ExpRhoType
                  -> TcM HsWrapper
    tcSubTypeDS_O orig ctxt maybe_thing ty_actual ty_expected
      = addSubTypeCtxt ty_actual ty_expected $
        do { traceTc "tcSubTypeDS_O" (vcat [ pprCtOrigin orig
                                           , pprUserTypeCtxt ctxt
                                           , ppr ty_actual
                                           , ppr ty_expected ])
           ; tcSubTypeDS_NC_O orig ctxt maybe_thing ty_actual ty_expected }

    addSubTypeCtxt :: TcType -> ExpType -> TcM a -> TcM a
    addSubTypeCtxt ty_actual ty_expected thing_inside
     | isRhoTy ty_actual        -- If there is no polymorphism involved, the
     , isRhoExpTy ty_expected   -- TypeEqOrigin stuff (added by the _NC functions)
     = thing_inside             -- gives enough context by itself
     | otherwise
     = addErrCtxtM mk_msg thing_inside
      where
        mk_msg tidy_env
          = do { (tidy_env, ty_actual)   <- zonkTidyTcType tidy_env ty_actual
                       -- might not be filled if we're debugging. ugh.
               ; mb_ty_expected          <- readExpType_maybe ty_expected
               ; (tidy_env, ty_expected) <- case mb_ty_expected of
                                              Just ty -> second mkCheckExpType <$>
                                                         zonkTidyTcType tidy_env ty
                                              Nothing -> return (tidy_env, ty_expected)
               ; ty_expected             <- readExpType ty_expected
               ; (tidy_env, ty_expected) <- zonkTidyTcType tidy_env ty_expected
               ; let msg = vcat [ hang (text "When checking that:")
                                     4 (ppr ty_actual)
                                , nest 2 (hang (text "is more polymorphic than:")
                                             2 (ppr ty_expected)) ]
               ; return (tidy_env, msg) }

    ---------------
    -- The "_NC" variants do not add a typechecker-error context;
    -- the caller is assumed to do that

    tcSubType_NC :: UserTypeCtxt -> TcSigmaType -> ExpSigmaType -> TcM HsWrapper
    tcSubType_NC ctxt ty_actual ty_expected
      = do { traceTc "tcSubType_NC" (vcat [pprUserTypeCtxt ctxt, ppr ty_actual, ppr ty_expected])
           ; tc_sub_type origin origin ctxt ty_actual ty_expected }
      where
        origin = TypeEqOrigin { uo_actual   = ty_actual
                              , uo_expected = ty_expected
                              , uo_thing    = Nothing }

    tcSubTypeDS_NC :: Outputable a
                   => UserTypeCtxt
                   -> Maybe a  -- ^ If present, this has type ty_actual
                   -> TcSigmaType -> ExpRhoType -> TcM HsWrapper
    tcSubTypeDS_NC ctxt maybe_thing ty_actual ty_expected
      = do { traceTc "tcSubTypeDS_NC" (vcat [pprUserTypeCtxt ctxt, ppr ty_actual, ppr ty_expected])
           ; tcSubTypeDS_NC_O origin ctxt maybe_thing ty_actual ty_expected }
      where
        origin = TypeEqOrigin { uo_actual   = ty_actual
                              , uo_expected = ty_expected
                              , uo_thing    = mkErrorThing <$> maybe_thing }

    tcSubTypeDS_NC_O :: Outputable a
                     => CtOrigin   -- origin used for instantiation only
                     -> UserTypeCtxt
                     -> Maybe a
                     -> TcSigmaType -> ExpRhoType -> TcM HsWrapper
    -- Just like tcSubType, but with the additional precondition that
    -- ty_expected is deeply skolemised
    tcSubTypeDS_NC_O inst_orig ctxt m_thing ty_actual et
      = uExpTypeX eq_orig ty_actual et
          (return . mkWpCastN)
          (tc_sub_type_ds eq_orig inst_orig ctxt ty_actual)
      where
        eq_orig = TypeEqOrigin { uo_actual = ty_actual, uo_expected = et
                               , uo_thing = mkErrorThing <$> m_thing }

    ---------------
    tc_sub_type :: CtOrigin   -- origin used when calling uType
                -> CtOrigin   -- origin used when instantiating
                -> UserTypeCtxt -> TcSigmaType -> ExpSigmaType -> TcM HsWrapper
    tc_sub_type eq_orig inst_orig ctxt ty_actual et
      = uExpTypeX eq_orig ty_actual et
          (return . mkWpCastN)
          (tc_sub_tc_type eq_orig inst_orig ctxt ty_actual)

    tc_sub_tc_type :: CtOrigin   -- used when calling uType
                   -> CtOrigin   -- used when instantiating
                   -> UserTypeCtxt -> TcSigmaType -> TcSigmaType -> TcM HsWrapper
    tc_sub_tc_type eq_orig inst_orig ctxt ty_actual ty_expected
      | Just tv_actual <- tcGetTyVar_maybe ty_actual -- See Note [Higher rank types]
      = do { lookup_res <- lookupTcTyVar tv_actual
           ; case lookup_res of
               Filled ty_actual' -> tc_sub_tc_type eq_orig inst_orig
                                                   ctxt ty_actual' ty_expected

                 -- It's tempting to see if tv_actual can unify with a polytype
                 -- and, if so, call uType; otherwise, skolemise first. But this
                 -- is wrong, because skolemising will bump the TcLevel and the
                 -- unification will fail anyway.
                 -- It's also tempting to call uUnfilledVar directly, but calling
                 -- uType seems safer in the presence of possible refactoring
                 -- later.
               Unfilled _        -> mkWpCastN <$>
                                    uType eq_orig TypeLevel ty_actual ty_expected }

      | otherwise  -- See Note [Deep skolemisation]
      = do { (sk_wrap, inner_wrap) <- tcSkolemise ctxt ty_expected $
                                      \ _ sk_rho ->
                                      tc_sub_type_ds eq_orig inst_orig ctxt
                                                     ty_actual sk_rho
           ; return (sk_wrap <.> inner_wrap) }

    ---------------
    tc_sub_type_ds :: CtOrigin    -- used when calling uType
                   -> CtOrigin    -- used when instantiating
                   -> UserTypeCtxt -> TcSigmaType -> TcRhoType -> TcM HsWrapper
    -- Just like tcSubType, but with the additional precondition that
    -- ty_expected is deeply skolemised
    tc_sub_type_ds eq_orig inst_orig ctxt ty_actual ty_expected
      = go ty_actual ty_expected
      where
        go ty_a ty_e | Just ty_a' <- coreView ty_a = go ty_a' ty_e
                     | Just ty_e' <- coreView ty_e = go ty_a  ty_e'

        go (TyVarTy tv_a) ty_e
          = do { lookup_res <- lookupTcTyVar tv_a
               ; case lookup_res of
                   Filled ty_a' ->
                     do { traceTc "tcSubTypeDS_NC_O following filled act meta-tyvar:"
                            (ppr tv_a <+> text "-->" <+> ppr ty_a')
                        ; tc_sub_type_ds eq_orig inst_orig ctxt ty_a' ty_e }
                   Unfilled _   -> unify }

        -- Historical note (Sept 16): there was a case here for
        --    go ty_a (TyVarTy alpha)
        -- which, in the impredicative case unified  alpha := ty_a
        -- where th_a is a polytype.  Not only is this probably bogus (we
        -- simply do not have decent story for imprdicative types), but it
        -- caused Trac #12616 because (also bizarrely) 'deriving' code had
        -- -XImpredicativeTypes on.  I deleted the entire case.

        go (ForAllTy (Anon act_arg) act_res) (ForAllTy (Anon exp_arg) exp_res)
          | not (isPredTy act_arg)
          , not (isPredTy exp_arg)
          = -- See Note [Co/contra-variance of subsumption checking]
            do { res_wrap <- tc_sub_type_ds eq_orig inst_orig ctxt act_res exp_res
               ; arg_wrap
                   <- tc_sub_tc_type eq_orig (GivenOrigin
                                              (SigSkol GenSigCtxt
                                               (mkCheckExpType exp_arg)))
                                     ctxt exp_arg act_arg
               ; return (mkWpFun arg_wrap res_wrap exp_arg exp_res) }
                   -- arg_wrap :: exp_arg ~ act_arg
                   -- res_wrap :: act-res ~ exp_res

        go ty_a ty_e
          | let (tvs, theta, _) = tcSplitSigmaTy ty_a
          , not (null tvs && null theta)
          = do { (in_wrap, in_rho) <- topInstantiate inst_orig ty_a
               ; body_wrap <- tc_sub_type_ds
                                (eq_orig { uo_actual = in_rho
                                         , uo_expected =
                                             mkCheckExpType ty_expected })
                                inst_orig ctxt in_rho ty_e
               ; return (body_wrap <.> in_wrap) }

          | otherwise   -- Revert to unification
          = inst_and_unify
             -- It's still possible that ty_actual has nested foralls. Instantiate
             -- these, as there's no way unification will succeed with them in.
             -- See typecheck/should_compile/T11305 for an example of when this
             -- is important. The problem is that we're checking something like
             --  a -> forall b. b -> b     <=   alpha beta gamma
             -- where we end up with alpha := (->)

        inst_and_unify = do { (wrap, rho_a) <- deeplyInstantiate inst_orig ty_actual

                               -- if we haven't recurred through an arrow, then
                               -- the eq_orig will list ty_actual. In this case,
                               -- we want to update the origin to reflect the
                               -- instantiation. If we *have* recurred through
                               -- an arrow, it's better not to update.
                            ; let eq_orig' = case eq_orig of
                                    TypeEqOrigin { uo_actual   = orig_ty_actual }
                                      |  orig_ty_actual `tcEqType` ty_actual
                                      ,  not (isIdHsWrapper wrap)
                                      -> eq_orig { uo_actual = rho_a }
                                    _ -> eq_orig

                            ; cow <- uType eq_orig' TypeLevel rho_a ty_expected
                            ; return (mkWpCastN cow <.> wrap) }

         -- use versions without synonyms expanded
        unify = mkWpCastN <$> uType eq_orig TypeLevel ty_actual ty_expected

    -----------------
    -- needs both un-type-checked (for origins) and type-checked (for wrapping)
    -- expressions
    tcWrapResult :: HsExpr Name -> HsExpr TcId -> TcSigmaType -> ExpRhoType
                 -> TcM (HsExpr TcId)
    tcWrapResult rn_expr = tcWrapResultO (exprCtOrigin rn_expr)

    -- | Sometimes we don't have a @HsExpr Name@ to hand, and this is more
    -- convenient.
    tcWrapResultO :: CtOrigin -> HsExpr TcId -> TcSigmaType -> ExpRhoType
                   -> TcM (HsExpr TcId)
    tcWrapResultO orig expr actual_ty res_ty
      = do { traceTc "tcWrapResult" (vcat [ text "Actual:  " <+> ppr actual_ty
                                          , text "Expected:" <+> ppr res_ty ])
           ; cow <- tcSubTypeDS_NC_O orig GenSigCtxt
                                     (Just expr) actual_ty res_ty
           ; return (mkHsWrap cow expr) }

    -----------------------------------
    wrapFunResCoercion
            :: [TcType]        -- Type of args
            -> HsWrapper       -- HsExpr a -> HsExpr b
            -> TcM HsWrapper   -- HsExpr (arg_tys -> a) -> HsExpr (arg_tys -> b)
    wrapFunResCoercion arg_tys co_fn_res
      | isIdHsWrapper co_fn_res
      = return idHsWrapper
      | null arg_tys
      = return co_fn_res
      | otherwise
      = do  { arg_ids <- newSysLocalIds (fsLit "sub") arg_tys
            ; return (mkWpLams arg_ids <.> co_fn_res <.> mkWpEvVarApps arg_ids) }

    -----------------------------------
    -- | Infer a type using a fresh ExpType
    -- See also Note [ExpType] in TcMType
    tcInfer :: (ExpRhoType -> TcM a) -> TcM (a, TcType)
    tcInfer tc_check
      = do { res_ty <- newOpenInferExpType
           ; result <- tc_check res_ty
           ; res_ty <- readExpType res_ty
           ; return (result, res_ty) }

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Generalisation}
    *                                                                      *
    ************************************************************************
    -}

    -- | Take an "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@.
    tcSkolemise :: UserTypeCtxt -> TcSigmaType
                -> ([TcTyVar] -> TcType -> TcM result)
             -- ^ These are only ever used for scoped type variables.
                -> TcM (HsWrapper, result)
            -- ^ The expression has type: spec_ty -> expected_ty

    tcSkolemise ctxt expected_ty thing_inside
       -- We expect expected_ty to be a forall-type
       -- If not, the call is a no-op
      = do  { traceTc "tcSkolemise" Outputable.empty
            ; (wrap, tvs', given, rho') <- deeplySkolemise expected_ty

            ; lvl <- getTcLevel
            ; when debugIsOn $
                  traceTc "tcSkolemise" $ vcat [
                    ppr lvl,
                    text "expected_ty" <+> ppr expected_ty,
                    text "inst tyvars" <+> ppr tvs',
                    text "given"       <+> ppr given,
                    text "inst type"   <+> ppr rho' ]

            -- Generally we must check that the "forall_tvs" havn't been constrained
            -- The interesting bit here is that we must include the free variables
            -- of the expected_ty.  Here's an example:
            --       runST (newVar True)
            -- Here, if we don't make a check, we'll get a type (ST s (MutVar s Bool))
            -- for (newVar True), with s fresh.  Then we unify with the runST's arg type
            -- forall s'. ST s' a. That unifies s' with s, and a with MutVar s Bool.
            -- So now s' isn't unconstrained because it's linked to a.
            --
            -- However [Oct 10] now that the untouchables are a range of
            -- TcTyVars, all this is handled automatically with no need for
            -- extra faffing around

            -- Use the *instantiated* type in the SkolemInfo
            -- so that the names of displayed type variables line up
            ; let skol_info = SigSkol ctxt (mkCheckExpType $
                                            mkFunTys (map varType given) rho')

            ; (ev_binds, result) <- checkConstraints skol_info tvs' given $
                                    thing_inside tvs' rho'

            ; return (wrap <.> mkWpLet ev_binds, 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 _ et@(Infer {}) thing_inside
      = (idHsWrapper, ) <$> thing_inside et
    tcSkolemiseET ctxt (Check ty) thing_inside
      = tcSkolemise ctxt ty $ \_ -> thing_inside . mkCheckExpType

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

    checkConstraints skol_info skol_tvs given thing_inside
      = do { (implics, ev_binds, result)
                <- buildImplication skol_info skol_tvs given thing_inside
           ; emitImplications implics
           ; return (ev_binds, result) }

    buildImplication :: SkolemInfo
                     -> [TcTyVar]           -- Skolems
                     -> [EvVar]             -- Given
                     -> TcM result
                     -> TcM (Bag Implication, TcEvBinds, result)
    buildImplication skol_info skol_tvs given thing_inside
      = do { tc_lvl <- getTcLevel
           ; deferred_type_errors <- goptM Opt_DeferTypeErrors <||>
                                     goptM Opt_DeferTypedHoles
           ; if null skol_tvs && null given && (not deferred_type_errors ||
                                                not (isTopTcLevel tc_lvl))
             then do { res <- thing_inside
                     ; return (emptyBag, emptyTcEvBinds, res) }
          -- Fast path.  We check every function argument with
          -- tcPolyExpr, which uses tcSkolemise and hence checkConstraints.
          -- But with the solver producing unlifted equalities, we need
          -- to have an EvBindsVar for them when they might be deferred to
          -- runtime. Otherwise, they end up as top-level unlifted bindings,
          -- which are verboten. See also Note [Deferred errors for coercion holes]
          -- in TcErrors.
             else
        do { (tclvl, wanted, result) <- pushLevelAndCaptureConstraints thing_inside
           ; (implics, ev_binds) <- buildImplicationFor tclvl skol_info skol_tvs given wanted
           ; return (implics, ev_binds, result) }}

    buildImplicationFor :: TcLevel -> SkolemInfo -> [TcTyVar]
                       -> [EvVar] -> WantedConstraints
                       -> TcM (Bag Implication, TcEvBinds)
    buildImplicationFor tclvl skol_info skol_tvs given wanted
      | isEmptyWC wanted && null 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
      = return (emptyBag, emptyTcEvBinds)

      | otherwise
      = ASSERT2( all isTcTyVar skol_tvs, ppr skol_tvs )
        ASSERT2( all isSkolemTyVar skol_tvs, ppr skol_tvs )
        do { ev_binds_var <- newTcEvBinds
           ; env <- getLclEnv
           ; let implic = Implic { ic_tclvl = tclvl
                                 , ic_skols = skol_tvs
                                 , ic_no_eqs = False
                                 , ic_given = given
                                 , ic_wanted = wanted
                                 , ic_status  = IC_Unsolved
                                 , ic_binds = Just ev_binds_var
                                 , ic_env = env
                                 , ic_info = skol_info }

           ; return (unitBag implic, TcEvBinds ev_binds_var) }

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

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

    -- | Unify two types, discarding a resultant coercion. Any constraints
    -- generated will still need to be solved, however.
    unifyType_ :: Outputable a => Maybe a  -- ^ If present, has type 'ty1'
               -> TcTauType -> TcTauType -> TcM ()
    unifyType_ thing ty1 ty2 = void $ unifyType thing ty1 ty2

    unifyType :: Outputable a => Maybe a   -- ^ If present, has type 'ty1'
              -> TcTauType -> TcTauType -> TcM TcCoercionN
    -- Actual and expected types
    -- Returns a coercion : ty1 ~ ty2
    unifyType thing ty1 ty2 = uType origin TypeLevel ty1 ty2
      where
        origin = TypeEqOrigin { uo_actual = ty1, uo_expected = mkCheckExpType ty2
                              , uo_thing  = mkErrorThing <$> thing }

    -- | Variant of 'unifyType' that takes an 'ExpType' as its second type
    unifyExpType :: Outputable a => Maybe a
                 -> TcTauType -> ExpType -> TcM TcCoercionN
    unifyExpType mb_thing ty1 ty2
      = uExpType ty_orig ty1 ty2
      where
        ty_orig   = TypeEqOrigin { uo_actual   = ty1
                                 , uo_expected = ty2
                                 , uo_thing    = mkErrorThing <$> mb_thing }

    -- | Use this instead of 'Nothing' when calling 'unifyType' without
    -- a good "thing" (where the "thing" has the "actual" type passed in)
    -- This has an 'Outputable' instance, avoiding amgiguity problems.
    noThing :: Maybe (HsExpr Name)
    noThing = Nothing

    unifyKind :: Outputable a => Maybe a -> TcKind -> TcKind -> TcM CoercionN
    unifyKind thing ty1 ty2 = uType origin KindLevel ty1 ty2
      where origin = TypeEqOrigin { uo_actual = ty1, uo_expected = mkCheckExpType ty2
                                  , uo_thing  = mkErrorThing <$> thing }

    ---------------
    unifyPred :: PredType -> PredType -> TcM TcCoercionN
    -- Actual and expected types
    unifyPred = unifyType noThing

    ---------------
    unifyTheta :: TcThetaType -> TcThetaType -> TcM [TcCoercionN]
    -- Actual and expected types
    unifyTheta theta1 theta2
      = do  { checkTc (equalLength theta1 theta2)
                      (vcat [text "Contexts differ in length",
                             nest 2 $ parens $ text "Use RelaxedPolyRec to allow this"])
            ; zipWithM unifyPred theta1 theta2 }

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

    uType is the heart of the unifier.
    -}

    uExpType :: CtOrigin -> TcType -> ExpType -> TcM CoercionN
    uExpType orig ty1 et
      = uExpTypeX orig ty1 et return $
        uType orig TypeLevel ty1

    -- | Tries to unify with an ExpType. If the ExpType is filled in, calls the first
    -- continuation with the produced coercion. Otherwise, calls the second
    -- continuation. This can happen either with a Check or with an untouchable
    -- ExpType that reverts to a tau-type. See Note [TcLevel of ExpType]
    uExpTypeX :: CtOrigin -> TcType -> ExpType
              -> (TcCoercionN -> TcM a)   -- Infer case, co :: TcType ~N ExpType
              -> (TcType -> TcM a)        -- Check / untouchable case
              -> TcM a
    uExpTypeX orig ty1 et@(Infer _ tc_lvl ki _) coercion_cont type_cont
      = do { cur_lvl <- getTcLevel
           ; if cur_lvl `sameDepthAs` tc_lvl
             then do { ki_co <- uType kind_orig KindLevel (typeKind ty1) ki
                     ; writeExpType et (ty1 `mkCastTy` ki_co)
                     ; coercion_cont $ mkTcNomReflCo ty1 `mkTcCoherenceRightCo` ki_co }
             else do { traceTc "Preventing writing to untouchable ExpType" empty
                     ; tau <- expTypeToType et -- See Note [TcLevel of ExpType]
                     ; type_cont tau }}
      where
        kind_orig = KindEqOrigin ty1 Nothing orig (Just TypeLevel)
    uExpTypeX _ _ (Check ty2) _ type_cont
      = type_cont ty2

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

    --------------
    -- It is always safe to defer unification to the main constraint solver
    -- See Note [Deferred unification]
    uType_defer origin t_or_k ty1 ty2
      = do { hole <- newCoercionHole
           ; loc <- getCtLocM origin (Just t_or_k)
           ; emitSimple $ mkNonCanonical $
                 CtWanted { ctev_dest = HoleDest hole
                          , ctev_pred = mkPrimEqPred ty1 ty2
                          , ctev_loc = loc }

           -- Error trace only
           -- NB. do *not* call mkErrInfo unless tracing is on, because
           -- it is hugely expensive (#5631)
           ; whenDOptM Opt_D_dump_tc_trace $ do
                { ctxt <- getErrCtxt
                ; doc <- mkErrInfo emptyTidyEnv ctxt
                ; traceTc "utype_defer" (vcat [ppr hole, ppr ty1,
                                               ppr ty2, pprCtOrigin origin, doc])
                }
           ; return (mkHoleCo hole Nominal ty1 ty2) }

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

            -- Variables; go for uVar
            -- 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 tv1) ty2
          = do { lookup_res <- lookupTcTyVar tv1
               ; case lookup_res of
                   Filled ty1   -> do { traceTc "found filled tyvar" (ppr tv1 <+> text ":->" <+> ppr ty1)
                                      ; go ty1 ty2 }
                   Unfilled _ -> uUnfilledVar origin t_or_k NotSwapped tv1 ty2 }
        go ty1 (TyVarTy tv2)
          = do { lookup_res <- lookupTcTyVar tv2
               ; case lookup_res of
                   Filled ty2   -> do { traceTc "found filled tyvar" (ppr tv2 <+> text ":->" <+> ppr ty2)
                                      ; go ty1 ty2 }
                   Unfilled _ -> uUnfilledVar origin t_or_k IsSwapped tv2 ty1 }

          -- See Note [Expanding synonyms during unification]
        go ty1@(TyConApp tc1 []) (TyConApp tc2 [])
          | tc1 == tc2
          = return $ mkReflCo Nominal 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 Trac #4535.
        go ty1 ty2
          | Just ty1' <- coreView ty1 = go ty1' ty2
          | Just ty2' <- coreView ty2 = go ty1  ty2'

        go (CastTy t1 co1) t2
          = do { co_tys <- go t1 t2
               ; return (mkCoherenceLeftCo co_tys co1) }

        go t1 (CastTy t2 co2)
          = do { co_tys <- go t1 t2
               ; return (mkCoherenceRightCo co_tys co2) }

            -- Functions (or predicate functions) just check the two parts
        go (ForAllTy (Anon fun1) arg1) (ForAllTy (Anon fun2) arg2)
          = do { co_l <- uType origin t_or_k fun1 fun2
               ; co_r <- uType origin t_or_k arg1 arg2
               ; return $ mkFunCo Nominal co_l co_r }

            -- Always defer if a type synonym family (type function)
            -- is involved.  (Data families behave rigidly.)
        go ty1@(TyConApp tc1 _) ty2
          | isTypeFamilyTyCon tc1 = defer ty1 ty2
        go ty1 ty2@(TyConApp tc2 _)
          | isTypeFamilyTyCon tc2 = defer ty1 ty2

        go (TyConApp tc1 tys1) (TyConApp tc2 tys2)
          -- See Note [Mismatched type lists and application decomposition]
          | tc1 == tc2, length tys1 == length tys2
          = ASSERT2( isGenerativeTyCon tc1 Nominal, ppr tc1 )
            do { cos <- zipWithM (uType origin t_or_k) tys1 tys2
               ; return $ mkTyConAppCo Nominal tc1 cos }

        go (LitTy m) ty@(LitTy n)
          | m == n
          = return $ mkNomReflCo 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 s1 t1) (AppTy s2 t2)
          = go_app s1 t1 s2 t2

        go (AppTy s1 t1) (TyConApp tc2 ts2)
          | Just (ts2', t2') <- snocView ts2
          = ASSERT( mightBeUnsaturatedTyCon tc2 )
            go_app s1 t1 (TyConApp tc2 ts2') t2'

        go (TyConApp tc1 ts1) (AppTy s2 t2)
          | Just (ts1', t1') <- snocView ts1
          = ASSERT( mightBeUnsaturatedTyCon tc1 )
            go_app (TyConApp tc1 ts1') t1' s2 t2

        go (CoercionTy co1) (CoercionTy co2)
          = do { let ty1 = coercionType co1
                     ty2 = coercionType co2
               ; kco <- uType (KindEqOrigin orig_ty1 (Just orig_ty2) origin
                                            (Just t_or_k))
                              KindLevel
                              ty1 ty2
               ; return $ mkProofIrrelCo Nominal kco co1 co2 }

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

        ------------------
        defer ty1 ty2   -- See Note [Check for equality before deferring]
          | ty1 `tcEqType` ty2 = return (mkNomReflCo ty1)
          | otherwise          = uType_defer origin t_or_k ty1 ty2

        ------------------
        go_app s1 t1 s2 t2
          = do { co_s <- uType origin t_or_k s1 s2
               ; co_t <- uType origin t_or_k t1 t2
               ; return $ mkAppCo co_s 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 Trac #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

     * 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.

    ************************************************************************
    *                                                                      *
                     uVar and friends
    *                                                                      *
    ************************************************************************

    @uVar@ 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
                 -> 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 origin t_or_k swapped tv1 ty2
      = do { ty2 <- zonkTcType 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
           ; uUnfilledVar1 origin t_or_k swapped tv1 ty2 }

    ----------
    uUnfilledVar1 :: CtOrigin
                  -> TypeOrKind
                  -> SwapFlag
                  -> TcTyVar        -- Tyvar 1
                  -> TcTauType      -- Type 2, zonked
                  -> TcM Coercion
    uUnfilledVar1 origin t_or_k swapped tv1 ty2
      | Just tv2 <- tcGetTyVar_maybe ty2
      = go tv2

      | otherwise
      = uUnfilledVar2 origin t_or_k swapped tv1 ty2

      where
        -- 'go' handles the case where both are
        -- tyvars so we might want to swap
        go tv2 | tv1 == tv2  -- Same type variable => no-op
               = return (mkNomReflCo (mkTyVarTy tv1))

               | swapOverTyVars tv1 tv2   -- Distinct type variables
               = uUnfilledVar2 origin t_or_k (flipSwap swapped)
                               tv2 (mkTyVarTy tv1)

               | otherwise
               = uUnfilledVar2 origin t_or_k swapped tv1 ty2

    ----------
    uUnfilledVar2 :: CtOrigin
                  -> TypeOrKind
                  -> SwapFlag
                  -> TcTyVar        -- Tyvar 1
                  -> TcTauType      -- Type 2, zonked
                  -> TcM Coercion
    uUnfilledVar2 origin t_or_k swapped tv1 ty2
      = do { dflags  <- getDynFlags
           ; cur_lvl <- getTcLevel
           ; go dflags cur_lvl }
      where
        go dflags cur_lvl
          | canSolveByUnification cur_lvl tv1 ty2
          , Just ty2' <- metaTyVarUpdateOK dflags tv1 ty2
          = do { co_k <- uType kind_origin KindLevel (typeKind ty2') (tyVarKind tv1)
               ; co   <- updateMeta tv1 ty2' co_k
               ; return (maybe_sym swapped co) }

          | otherwise
          = unSwap swapped (uType_defer origin t_or_k) ty1 ty2
                   -- Occurs check or an untouchable: just defer
                   -- NB: occurs check isn't necessarily fatal:
                   --     eg tv1 occured in type family parameter

        ty1 = mkTyVarTy tv1
        kind_origin = KindEqOrigin ty1 (Just ty2) origin (Just t_or_k)

    -- | apply sym iff swapped
    maybe_sym :: SwapFlag -> Coercion -> Coercion
    maybe_sym IsSwapped  = mkSymCo
    maybe_sym NotSwapped = id

    ----------------
    metaTyVarUpdateOK :: DynFlags
                      -> TcTyVar             -- tv :: k1
                      -> TcType              -- ty :: k2
                      -> Maybe TcType        -- possibly-expanded ty
    -- (metaTyFVarUpdateOK tv ty)
    -- We are about to update the meta-tyvar tv with ty.
    -- Check (a) that tv doesn't occur in ty (occurs check)
    --       (b) that ty does not have any foralls
    --           (in the impredicative case), or type functions
    --
    -- We have two possible outcomes:
    -- (1) Return the type to update the type variable with,
    --        [we know the update is ok]
    -- (2) Return Nothing,
    --        [the update might be dodgy]
    --
    -- Note that "Nothing" does not mean "definite error".  For example
    --   type family F a
    --   type instance F Int = Int
    -- consider
    --   a ~ F a
    -- This is perfectly reasonable, if we later get a ~ Int.  For now, though,
    -- we return Nothing, leaving it to the later constraint simplifier to
    -- sort matters out.

    metaTyVarUpdateOK dflags tv ty
      = case defer_me impredicative ty of
          OC_OK _   -> Just ty
          OC_Forall -> Nothing  -- forall or type function
          OC_Occurs -> occCheckExpand tv ty
      where
        details       = tcTyVarDetails tv
        impredicative = canUnifyWithPolyType dflags details

        defer_me :: Bool    -- True <=> foralls are ok
                 -> TcType
                 -> OccCheckResult ()
        -- Checks for (a) occurrence of tv
        --            (b) type family applications
        --            (c) foralls if the Bool is false
        -- See Note [Prevent unification with type families]
        -- See Note [Refactoring hazard: checkTauTvUpdate]
        -- See Note [Checking for foralls] in TcType

        defer_me _ (TyVarTy tv')
           | tv == tv' = OC_Occurs
           | otherwise = defer_me True (tyVarKind tv')
        defer_me b (TyConApp tc tys)
           | isTypeFamilyTyCon tc = OC_Forall   -- We use OC_Forall to signal
                                                -- forall /or/ type function
           | not (isTauTyCon tc)  = OC_Forall
           | otherwise            = mapM (defer_me b) tys >> OC_OK ()

        defer_me b (ForAllTy (Named tv' _) t)
           | not b     = OC_Forall
           | tv == tv' = OC_OK ()
           | otherwise = do { defer_me True (tyVarKind tv'); defer_me b t }

        defer_me _ (LitTy {})        = OC_OK ()
        defer_me b (ForAllTy (Anon fun) arg) = defer_me b fun >> defer_me b arg
        defer_me b (AppTy fun arg)   = defer_me b fun >> defer_me b arg
        defer_me b (CastTy ty co)    = defer_me b ty  >> defer_me_co co
        defer_me _ (CoercionTy co)   = defer_me_co co

          -- We don't really care if there are type families in a coercion,
          -- but we still can't have an occurs-check failure
        defer_me_co co | tv `elemVarSet` tyCoVarsOfCo co = OC_Occurs
                       | otherwise                       = OC_OK ()

    swapOverTyVars :: TcTyVar -> TcTyVar -> Bool
    -- See Note [Canonical orientation for tyvar/tyvar equality constraints]
    swapOverTyVars tv1 tv2
      | Just lvl1 <- metaTyVarTcLevel_maybe tv1
          -- If tv1 is touchable, swap only if tv2 is also
          -- touchable and it's strictly better to update the latter
          -- But see Note [Avoid unnecessary swaps]
      = case metaTyVarTcLevel_maybe tv2 of
          Nothing   -> False
          Just lvl2 | lvl2 `strictlyDeeperThan` lvl1 -> True
                    | lvl1 `strictlyDeeperThan` lvl2 -> False
                    | otherwise                      -> nicer_to_update tv2

      -- So tv1 is not a meta tyvar
      -- If only one is a meta tyvar, put it on the left
      -- This is not because it'll be solved; but because
      -- the floating step looks for meta tyvars on the left
      | isMetaTyVar tv2 = True

      -- So neither is a meta tyvar (including FlatMetaTv)

      -- If only one is a flatten skolem, put it on the left
      -- See Note [Eliminate flat-skols]
      | not (isFlattenTyVar tv1), isFlattenTyVar tv2 = True

      | otherwise = False

      where
        nicer_to_update tv2
          =  (isSigTyVar tv1                 && not (isSigTyVar tv2))
          || (isSystemName (Var.varName tv2) && not (isSystemName (Var.varName tv1)))

    -- @trySpontaneousSolve wi@ solves equalities where one side is a
    -- touchable unification variable.
    -- Returns True <=> spontaneous solve happened
    canSolveByUnification :: TcLevel -> TcTyVar -> TcType -> Bool
    canSolveByUnification tclvl tv xi
      | isTouchableMetaTyVar tclvl tv
      = case metaTyVarInfo tv of
          SigTv -> is_tyvar xi
          _     -> True

      | otherwise    -- Untouchable
      = False
      where
        is_tyvar xi
          = case tcGetTyVar_maybe xi of
              Nothing -> False
              Just tv -> case tcTyVarDetails tv of
                           MetaTv { mtv_info = info }
                                       -> case info of
                                            SigTv -> True
                                            _     -> False
                           SkolemTv {} -> True
                           FlatSkol {} -> False
                           RuntimeUnk  -> True

    {-
    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. (We're not helped by the fact that
    the flattener re-flattens all the arguments every time around.) 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.

    Note [Refactoring hazard: checkTauTvUpdate]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    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 TcInteract,
    function @occ_check_ok@.

    Note [Non-TcTyVars in TcUnify]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    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 TcTyClsDecls"). 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.
    -}

    data LookupTyVarResult  -- The result of a lookupTcTyVar call
      = Unfilled TcTyVarDetails     -- SkolemTv or virgin MetaTv
      | Filled   TcType

    lookupTcTyVar :: TcTyVar -> TcM LookupTyVarResult
    lookupTcTyVar tyvar
      | MetaTv { mtv_ref = ref } <- details
      = do { meta_details <- readMutVar ref
           ; case meta_details of
               Indirect ty -> return (Filled ty)
               Flexi -> do { is_touchable <- isTouchableTcM tyvar
                                 -- Note [Unifying untouchables]
                           ; if is_touchable then
                                return (Unfilled details)
                             else
                                return (Unfilled vanillaSkolemTv) } }
      | otherwise
      = return (Unfilled details)
      where
        details = tcTyVarDetails tyvar

    -- | Fill in a meta-tyvar
    updateMeta :: TcTyVar            -- ^ tv to fill in, tv :: k1
               -> TcType             -- ^ ty2 :: k2
               -> Coercion           -- ^ kind_co :: k2 ~N k1
               -> TcM Coercion       -- ^ :: tv ~N ty2 (= ty2 |> kind_co ~N ty2)
    updateMeta tv1 ty2 kind_co
      = do { let ty2'     = ty2 `mkCastTy` kind_co
                 ty2_refl = mkNomReflCo ty2
                 co       = mkCoherenceLeftCo ty2_refl kind_co
           ; writeMetaTyVar tv1 ty2'
           ; return co }

    {-
    Note [Unifying untouchables]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We treat an untouchable type variable as if it was a skolem.  That
    ensures it won't unify with anything.  It's a slight had, because
    we return a made-up TcTyVarDetails, but I think it works smoothly.
    -}

    -- | Breaks apart a function kind into its pieces.
    matchExpectedFunKind :: Arity           -- ^ # of args remaining, only for errors
                         -> TcType          -- ^ type, only for errors
                         -> TcKind          -- ^ function kind
                         -> TcM (Coercion, TcKind, TcKind)
                                      -- ^ co :: old_kind ~ arg -> res
    matchExpectedFunKind num_args_remaining ty = go
      where
        go k | Just k' <- coreView k = go k'

        go k@(TyVarTy kvar)
          | isTcTyVar kvar, isMetaTyVar kvar
          = do { maybe_kind <- readMetaTyVar kvar
               ; case maybe_kind of
                    Indirect fun_kind -> go fun_kind
                    Flexi ->             defer k }

        go k@(ForAllTy (Anon arg) res)
          = return (mkNomReflCo k, arg, res)

        go other = defer other

        defer k
          = do { arg_kind <- newMetaKindVar
               ; res_kind <- newMetaKindVar
               ; let new_fun = mkFunTy arg_kind res_kind
                     thing   = mkTypeErrorThingArgs ty num_args_remaining
                     origin  = TypeEqOrigin { uo_actual   = k
                                            , uo_expected = mkCheckExpType new_fun
                                            , uo_thing    = Just thing
                                            }
               ; co <- uType origin KindLevel k new_fun
               ; return (co, arg_kind, res_kind) }
          where