content: {-
  (c) The University of Glasgow 2006
  (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
  
  
  Type subsumption and unification
  -}
  {-# LANGUAGE CPP #-}
  
  module Eta.TypeCheck.TcUnify (
    -- Full-blown subsumption
    tcWrapResult, tcGen,
    tcSubType, tcSubType_NC, tcSubTypeDS, tcSubTypeDS_NC,
    checkConstraints, buildImplication, buildImplicationFor,
  
    -- Various unifications
    unifyType, unifyTypeList, unifyTheta,
    unifyKindX,
  
    --------------------------------
    -- Holes
    tcInfer,
    matchExpectedListTy,
    matchExpectedPArrTy,
    matchExpectedTyConApp,
    matchExpectedAppTy,
    matchExpectedFunTys,
    matchExpectedFunKind,
    wrapFunResCoercion
  
    ) where
  
  import Eta.HsSyn.HsSyn
  import Eta.Types.TypeRep
  import Eta.TypeCheck.TcMType
  import Eta.TypeCheck.TcRnMonad
  import Eta.TypeCheck.TcType
  import Eta.Types.Type
  import Eta.TypeCheck.TcEvidence
  import Eta.TypeCheck.Inst
  import Eta.Types.Kind
  import Eta.Types.TyCon
  import Eta.Prelude.TysWiredIn
  import Eta.BasicTypes.Var
  import qualified Eta.BasicTypes.Var as Var
  import Eta.BasicTypes.VarEnv
  import Eta.BasicTypes.VarSet
  import Eta.Main.ErrUtils
  import Eta.Main.DynFlags
  import Eta.BasicTypes.BasicTypes
  import Eta.Utils.Maybes ( isJust )
  import Eta.Utils.Bag
  import Eta.Utils.Util
  import Eta.Utils.Outputable
  import qualified Eta.Utils.Outputable as Outputable
  import Eta.Utils.FastString
  import Eta.BasicTypes.Name
  import Control.Monad
  
  #include "HsVersions.h"
  
  {-
  ************************************************************************
  *                                                                      *
               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 an (Expected rho) 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 is not (currently) where deep skolemisation occurs;
  matchExpectedFunTys does not skolmise nested foralls in the
  expected type, because it expects that to have been done already
  -}
  
  matchExpectedFunTys :: SDoc     -- See Note [Herald for matchExpectedFunTys]
                      -> Arity
                      -> TcRhoType
                      -> TcM (TcCoercion, [TcSigmaType], TcRhoType)
  
  -- If    matchExpectFunTys n ty = (co, [t1,..,tn], ty_r)
  -- then  co : ty ~ (t1 -> ... -> tn -> ty_r)
  --
  -- 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
  
  matchExpectedFunTys herald arity orig_ty
    = go arity orig_ty
    where
      -- If     go n ty = (co, [t1,..,tn], ty_r)
      -- then   co : ty ~ t1 -> .. -> tn -> ty_r
  
      go n_req ty
        | n_req == 0 = return (mkTcNomReflCo ty, [], ty)
  
      go n_req ty
        | Just ty' <- tcView ty = go n_req ty'
  
      go n_req (FunTy arg_ty res_ty)
        | not (isPredTy arg_ty)
        = do { (co, tys, ty_r) <- go (n_req-1) res_ty
             ; return (mkTcFunCo Nominal (mkTcNomReflCo arg_ty) co, arg_ty:tys, ty_r) }
  
      go n_req ty@(TyVarTy tv)
        | ASSERT( isTcTyVar tv) isMetaTyVar tv
        = do { cts <- readMetaTyVar tv
             ; case cts of
                 Indirect ty' -> go n_req ty'
                 Flexi        -> defer n_req ty (isReturnTyVar tv) }
  
         -- 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_req ty = addErrCtxtM mk_ctxt $
                    defer n_req ty False
  
      ------------
      -- If we decide that a ReturnTv (see Note [ReturnTv] in TcType) should
      -- really be a function type, then we need to allow the argument and
      -- result types also to be ReturnTvs.
      defer n_req fun_ty is_return
        = do { arg_tys <- mapM new_ty_var_ty (nOfThem n_req openTypeKind)
                          -- See Note [Foralls to left of arrow]
             ; res_ty  <- new_ty_var_ty openTypeKind
             ; co   <- unifyType fun_ty (mkFunTys arg_tys res_ty)
             ; return (co, arg_tys, res_ty) }
        where
          new_ty_var_ty | is_return = newReturnTyVarTy
                        | otherwise = newFlexiTyVarTy
  
      ------------
      mk_ctxt :: TidyEnv -> TcM (TidyEnv, MsgDoc)
      mk_ctxt env = do { (env', ty) <- zonkTidyTcType env orig_ty
                       ; let (args, _) = tcSplitFunTys ty
                             n_actual = length args
                             (env'', orig_ty') = tidyOpenType env' orig_ty
                       ; return (env'', mk_msg orig_ty' ty n_actual) }
  
      mk_msg orig_ty ty n_args
        = herald <+> speakNOf arity (ptext (sLit "argument")) <> comma $$
          if n_args == arity
            then ptext (sLit "its type is") <+> quotes (pprType orig_ty) <>
                 comma $$
                 ptext (sLit "it is specialized to") <+> quotes (pprType ty)
            else sep [ptext (sLit "but its type") <+> quotes (pprType ty),
                      if n_args == 0 then ptext (sLit "has none")
                      else ptext (sLit "has only") <+> speakN n_args]
  
  {-
  Note [Foralls to left of arrow]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  Consider
    f (x :: forall a. a -> a) = x
  We give 'f' the type (alpha -> beta), and then want to unify
  the alpha with (forall a. a->a).  We want to the arg and result
  of (->) to have openTypeKind, and this also permits foralls, so
  we are ok.
  -}
  
  ----------------------
  matchExpectedListTy :: TcRhoType -> TcM (TcCoercion, TcRhoType)
  -- Special case for lists
  matchExpectedListTy exp_ty
   = do { (co, [elt_ty]) <- matchExpectedTyConApp listTyCon exp_ty
        ; return (co, elt_ty) }
  
  ----------------------
  matchExpectedPArrTy :: TcRhoType -> TcM (TcCoercion, 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 (TcCoercion,     -- T k1 k2 k3 a b c ~ 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' <- tcView 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 = ASSERT2( isSubOpenTypeKind res_kind, ppr tc )
              do { kappa_tys <- mapM (const newMetaKindVar) kvs
                 ; let arg_kinds' = map (substKiWith kvs kappa_tys) arg_kinds
                 ; tau_tys <- mapM newFlexiTyVarTy arg_kinds'
                 ; co <- unifyType (mkTyConApp tc (kappa_tys ++ tau_tys)) orig_ty
                 ; return (co, kappa_tys ++ tau_tys) }
  
      (kvs, body)           = splitForAllTys (tyConKind tc)
      (arg_kinds, res_kind) = splitKindFunTys body
  
  ----------------------
  matchExpectedAppTy :: TcRhoType                         -- orig_ty
                     -> TcM (TcCoercion,                   -- m a ~ 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' <- tcView 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 (mkAppTy ty1 ty2) orig_ty
                 ; return (co, (ty1, ty2)) }
  
      orig_kind = typeKind orig_ty
      kind1 = mkArrowKind liftedTypeKind (defaultKind orig_kind)
      kind2 = liftedTypeKind    -- m :: * -> k
                                -- arg type :: *
          -- The defaultKind is a bit smelly.  If you remove it,
          -- try compiling        f x = do { x }
          -- and you'll get a kind mis-match.  It smells, but
          -- not enough to lose sleep over.
  
  {-
  ************************************************************************
  *                                                                      *
                  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.
  
  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 predicatble 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 tcGen (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
  tc_sub_type_ds, 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.
  -}
  
  tcSubType :: UserTypeCtxt -> TcSigmaType -> TcSigmaType -> TcM HsWrapper
  -- Checks that actual <= expected
  -- Returns HsWrapper :: actual ~ expected
  tcSubType ctxt ty_actual ty_expected
    = addSubTypeCtxt ty_actual ty_expected $
      tcSubType_NC ctxt ty_actual ty_expected
  
  tcSubTypeDS :: UserTypeCtxt -> TcSigmaType -> TcRhoType -> TcM HsWrapper
  -- Just like tcSubType, but with the additional precondition that
  -- ty_expected is deeply skolemised (hence "DS")
  tcSubTypeDS ctxt ty_actual ty_expected
    = addSubTypeCtxt ty_actual ty_expected $
      tcSubTypeDS_NC ctxt ty_actual ty_expected
  
  
  addSubTypeCtxt :: TcType -> TcType -> TcM a -> TcM a
  addSubTypeCtxt ty_actual ty_expected thing_inside
   | isRhoTy ty_actual     -- If there is no polymorphism involved, the
   , isRhoTy 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
             ; (tidy_env, ty_expected) <- zonkTidyTcType tidy_env ty_expected
             ; let msg = vcat [ hang (ptext (sLit "When checking that:"))
                                   4 (ppr ty_actual)
                              , nest 2 (hang (ptext (sLit "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 -> TcSigmaType -> 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 ctxt ty_actual ty_expected }
    where
      origin = TypeEqOrigin { uo_actual = ty_actual, uo_expected = ty_expected }
  
  tcSubTypeDS_NC :: UserTypeCtxt -> TcSigmaType -> TcRhoType -> TcM HsWrapper
  tcSubTypeDS_NC ctxt ty_actual ty_expected
    = do { traceTc "tcSubTypeDS_NC" (vcat [pprUserTypeCtxt ctxt, ppr ty_actual, ppr ty_expected])
         ; tc_sub_type_ds origin ctxt ty_actual ty_expected }
    where
      origin = TypeEqOrigin { uo_actual = ty_actual, uo_expected = ty_expected }
  
  ---------------
  tc_sub_type :: CtOrigin -> UserTypeCtxt -> TcSigmaType -> TcSigmaType -> TcM HsWrapper
  tc_sub_type origin ctxt ty_actual ty_expected
    | isTyVarTy ty_actual  -- See Note [Higher rank types]
    = do { cow <- uType origin ty_actual ty_expected
         ; return (coToHsWrapper cow) }
  
    | otherwise  -- See Note [Deep skolemisation]
    = do { (sk_wrap, inner_wrap) <- tcGen ctxt ty_expected $ \ _ sk_rho ->
                                    tc_sub_type_ds origin ctxt ty_actual sk_rho
         ; return (sk_wrap <.> inner_wrap) }
  
  ---------------
  tc_sub_type_ds :: CtOrigin -> UserTypeCtxt -> TcSigmaType -> TcRhoType -> TcM HsWrapper
  -- Just like tcSubType, but with the additional precondition that
  -- ty_expected is deeply skolemised
  tc_sub_type_ds origin ctxt ty_actual ty_expected
    | Just (act_arg, act_res) <- tcSplitFunTy_maybe ty_actual
    , Just (exp_arg, exp_res) <- tcSplitFunTy_maybe ty_expected
    = -- See Note [Co/contra-variance of subsumption checking]
      do { res_wrap <- tc_sub_type_ds origin ctxt act_res exp_res
         ; arg_wrap <- tc_sub_type    origin 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
  
    | (tvs, theta, in_rho) <- tcSplitSigmaTy ty_actual
    , not (null tvs && null theta)
    = do { (subst, tvs') <- tcInstTyVars tvs
         ; let tys'    = mkTyVarTys tvs'
               theta'  = substTheta subst theta
               in_rho' = substTy subst in_rho
         ; in_wrap   <- instCall origin tys' theta'
         ; body_wrap <- tcSubTypeDS_NC ctxt in_rho' ty_expected
         ; return (body_wrap <.> in_wrap) }
  
    | otherwise   -- Revert to unification
    = do { cow <- uType origin ty_actual ty_expected
         ; return (coToHsWrapper cow) }
  
  -----------------
  tcWrapResult :: HsExpr TcId -> TcRhoType -> TcRhoType -> TcM (HsExpr TcId)
  tcWrapResult expr actual_ty res_ty
    = do { cow <- tcSubTypeDS GenSigCtxt actual_ty res_ty
                  -- Both types are deeply skolemised
         ; 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 type "checking" function by passing in a ReturnTv,
  -- which can unify with *anything*. See also Note [ReturnTv] in TcType
  tcInfer :: (TcType -> TcM a) -> TcM (a, TcType)
  tcInfer tc_check
    = do { ret_tv  <- newReturnTyVar openTypeKind
         ; res <- tc_check (mkTyVarTy ret_tv)
         ; details <- readMetaTyVar ret_tv
         ; res_ty <- case details of
              Indirect ty -> return ty
              Flexi ->    -- Checking was uninformative
                       do { traceTc "Defaulting un-filled ReturnTv to a TauTv" (ppr ret_tv)
                          ; tau_ty <- newFlexiTyVarTy openTypeKind
                          ; writeMetaTyVar ret_tv tau_ty
                          ; return tau_ty }
         ; return (res, res_ty) }
  
  {-
  ************************************************************************
  *                                                                      *
  \subsection{Generalisation}
  *                                                                      *
  ************************************************************************
  -}
  
  tcGen :: UserTypeCtxt -> TcType
        -> ([TcTyVar] -> TcRhoType -> TcM result)
        -> TcM (HsWrapper, result)
          -- The expression has type: spec_ty -> expected_ty
  
  tcGen ctxt expected_ty thing_inside
     -- We expect expected_ty to be a forall-type
     -- If not, the call is a no-op
    = do  { traceTc "tcGen" Outputable.empty
          ; (wrap, tvs', given, rho') <- deeplySkolemise expected_ty
  
          ; when debugIsOn $
                traceTc "tcGen" $ vcat [
                             text "expected_ty" <+> ppr expected_ty,
                             text "inst ty" <+> ppr tvs' <+> 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 (mkPiTypes 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
  
  checkConstraints :: SkolemInfo
                   -> [TcTyVar]           -- Skolems
                   -> [EvVar]             -- Given
                   -> TcM result
                   -> TcM (TcEvBinds, result)
  
  checkConstraints skol_info skol_tvs given thing_inside
    | null skol_tvs && null given
    = do { res <- thing_inside; return (emptyTcEvBinds, res) }
        -- Just for efficiency.  We check every function argument with
        -- tcPolyExpr, which uses tcGen and hence checkConstraints.
  
    | otherwise
    = 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
   =  ASSERT2( all isTcTyVar skol_tvs, ppr skol_tvs )
      ASSERT2( all isSkolemTyVar skol_tvs, ppr skol_tvs )
      do { (result, tclvl, wanted) <- pushLevelAndCaptureConstraints thing_inside
  
         ; if 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
           then
              return (emptyBag, emptyTcEvBinds, result)
           else 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_insol = insolubleWC wanted
                               , ic_binds = ev_binds_var
                               , ic_env = env
                               , ic_info = skol_info }
  
         ; return (unitBag implic, TcEvBinds ev_binds_var, 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 (isSkolemTyVar <||> isSigTyVar) skol_tvs, ppr skol_tvs )
       -- Why allow SigTvs? Because implicitly declared kind variables in
       -- non-CUSK type declarations are SigTvs, and we need to bring them
       -- into scope as a skolem in an implication. This is OK, though,
       -- because SigTvs will always remain tyvars, even after unification.
     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_insol  = insolubleWC wanted
                                      , ic_binds  = 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.
  -}
  
  unifyType :: TcTauType -> TcTauType -> TcM TcCoercion
  -- Actual and expected types
  -- Returns a coercion : ty1 ~ ty2
  unifyType ty1 ty2 = uType origin ty1 ty2
    where
      origin = TypeEqOrigin { uo_actual = ty1, uo_expected = ty2 }
  
  ---------------
  unifyPred :: PredType -> PredType -> TcM TcCoercion
  -- Actual and expected types
  unifyPred = unifyType
  
  ---------------
  unifyTheta :: TcThetaType -> TcThetaType -> TcM [TcCoercion]
  -- Actual and expected types
  unifyTheta theta1 theta2
    = do  { checkTc (equalLength theta1 theta2)
                    (vcat [ptext (sLit "Contexts differ in length"),
                           nest 2 $ parens $ ptext (sLit "Use RelaxedPolyRec to allow this")])
          ; zipWithM unifyPred theta1 theta2 }
  
  {-
  @unifyTypeList@ takes a single list of @TauType@s and unifies them
  all together.  It is used, for example, when typechecking explicit
  lists, when all the elts should be of the same type.
  -}
  
  unifyTypeList :: [TcTauType] -> TcM ()
  unifyTypeList []                 = return ()
  unifyTypeList [_]                = return ()
  unifyTypeList (ty1:tys@(ty2:_)) = do { _ <- unifyType ty1 ty2
                                       ; unifyTypeList tys }
  
  {-
  ************************************************************************
  *                                                                      *
                   uType and friends
  *                                                                      *
  ************************************************************************
  
  uType is the heart of the unifier.  Each arg occurs twice, because
  we want to report errors in terms of synomyms if possible.  The first of
  the pair is used in error messages only; it is always the same as the
  second, except that if the first is a synonym then the second may be a
  de-synonym'd version.  This way we get better error messages.
  -}
  
  ------------
  uType, uType_defer
    :: CtOrigin
    -> TcType    -- ty1 is the *actual* type
    -> TcType    -- ty2 is the *expected* type
    -> TcM TcCoercion
  
  --------------
  -- It is always safe to defer unification to the main constraint solver
  -- See Note [Deferred unification]
  uType_defer origin ty1 ty2
    = do { eqv <- newEq ty1 ty2
         ; loc <- getCtLoc origin
         ; emitSimple $ mkNonCanonical $
               CtWanted { ctev_evar = eqv
                        , ctev_pred = mkTcEqPred 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 eqv, ppr ty1,
                                             ppr ty2, pprCtOrigin origin, doc])
              }
         ; return (mkTcCoVarCo eqv) }
  
  --------------
  -- unify_np (short for "no push" on the origin stack) does the work
  uType origin 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 isTcReflCo 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 TcCoercion
          -- 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   -> go ty1 ty2
                 Unfilled ds1 -> uUnfilledVar origin NotSwapped tv1 ds1 ty2 }
      go ty1 (TyVarTy tv2)
        = do { lookup_res <- lookupTcTyVar tv2
             ; case lookup_res of
                 Filled ty2   -> go ty1 ty2
                 Unfilled ds2 -> uUnfilledVar origin IsSwapped tv2 ds2 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' <- tcView ty1 = go ty1' ty2
        | Just ty2' <- tcView ty2 = go ty1  ty2'
  
          -- Functions (or predicate functions) just check the two parts
      go (FunTy fun1 arg1) (FunTy fun2 arg2)
        = do { co_l <- uType origin fun1 fun2
             ; co_r <- uType origin arg1 arg2
             ; return $ mkTcFunCo 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 = uType_defer origin ty1 ty2
      go ty1 ty2@(TyConApp tc2 _)
        | isTypeFamilyTyCon tc2 = uType_defer origin ty1 ty2
  
      go (TyConApp tc1 tys1) (TyConApp tc2 tys2)
        -- See Note [Mismatched type lists and application decomposition]
        | tc1 == tc2, length tys1 == length tys2
        = do { cos <- zipWithM (uType origin) tys1 tys2
             ; return $ mkTcTyConAppCo Nominal tc1 cos }
  
      go (LitTy m) ty@(LitTy n)
        | m == n
        = return $ mkTcNomReflCo 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( isDecomposableTyCon tc2 )
          go_app s1 t1 (TyConApp tc2 ts2') t2'
  
      go (TyConApp tc1 ts1) (AppTy s2 t2)
        | Just (ts1', t1') <- snocView ts1
        = ASSERT( isDecomposableTyCon tc1 )
          go_app (TyConApp tc1 ts1') t1' s2 t2
  
          -- Anything else fails
          -- E.g. unifying for-all types, which is relative unusual
      go ty1 ty2 = uType_defer origin ty1 ty2 -- failWithMisMatch origin
  
      ------------------
      go_app s1 t1 s2 t2
        = do { co_s <- uType origin s1 s2  -- See Note [Unifying AppTy]
             ; co_t <- uType origin t1 t2
             ; return $ mkTcAppCo co_s co_t }
  
  {-
  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 [Unifying AppTy]
  ~~~~~~~~~~~~~~~~~~~~~
  Consider unifying  (m Int) ~ (IO Int) where m is a unification variable
  that is now bound to (say) (Bool ->).  Then we want to report
       "Can't unify (Bool -> Int) with (IO Int)
  and not
       "Can't unify ((->) Bool) with IO"
  That is why we use the "_np" variant of uType, which does not alter the error
  message.
  
  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.
  
  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 unfications 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 unfication 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
               -> SwapFlag
               -> TcTyVar -> TcTyVarDetails       -- Tyvar 1
               -> TcTauType                       -- Type 2
               -> TcM TcCoercion
  -- "Unfilled" means that the variable is definitely not a filled-in meta tyvar
  --            It might be a skolem, or untouchable, or meta
  
  uUnfilledVar origin swapped tv1 details1 (TyVarTy tv2)
    | tv1 == tv2  -- Same type variable => no-op
    = return (mkTcNomReflCo (mkTyVarTy tv1))
  
    | otherwise  -- Distinct type variables
    = do  { lookup2 <- lookupTcTyVar tv2
          ; case lookup2 of
              Filled ty2'       -> uUnfilledVar origin swapped tv1 details1 ty2'
              Unfilled details2 -> uUnfilledVars origin swapped tv1 details1 tv2 details2
          }
  
  uUnfilledVar origin swapped tv1 details1 non_var_ty2  -- ty2 is not a type variable
    = case details1 of
        MetaTv { mtv_ref = ref1 }
          -> do { dflags <- getDynFlags
                ; mb_ty2' <- checkTauTvUpdate dflags tv1 non_var_ty2
                ; case mb_ty2' of
                    Just ty2' -> updateMeta tv1 ref1 ty2'
                    Nothing   -> do { traceTc "Occ/kind defer"
                                          (ppr tv1 <+> dcolon <+> ppr (tyVarKind tv1)
                                           $$ ppr non_var_ty2 $$ ppr (typeKind non_var_ty2))
                                    ; defer }
                }
  
        _other -> do { traceTc "Skolem defer" (ppr tv1); defer }  -- Skolems of all sorts
    where
      defer = unSwap swapped (uType_defer origin) (mkTyVarTy tv1) non_var_ty2
                 -- Occurs check or an untouchable: just defer
                 -- NB: occurs check isn't necessarily fatal:
                 --     eg tv1 occured in type family parameter
  
  ----------------
  uUnfilledVars :: CtOrigin
                -> SwapFlag
                -> TcTyVar -> TcTyVarDetails      -- Tyvar 1
                -> TcTyVar -> TcTyVarDetails      -- Tyvar 2
                -> TcM TcCoercion
  -- Invarant: The type variables are distinct,
  --           Neither is filled in yet
  
  uUnfilledVars origin swapped tv1 details1 tv2 details2
    = do { traceTc "uUnfilledVars" (    text "trying to unify" <+> ppr k1
                                    <+> text "with"            <+> ppr k2)
         ; mb_sub_kind <- unifyKindX k1 k2
         ; case mb_sub_kind of {
             Nothing -> unSwap swapped (uType_defer origin) (mkTyVarTy tv1) ty2 ;
             Just sub_kind ->
  
           case (sub_kind, details1, details2) of
             -- k1 < k2, so update tv2
             (LT, _, MetaTv { mtv_ref = ref2 }) -> updateMeta tv2 ref2 ty1
  
             -- k2 < k1, so update tv1
             (GT, MetaTv { mtv_ref = ref1 }, _) -> updateMeta tv1 ref1 ty2
  
             -- k1 = k2, so we are free to update either way
             (EQ, MetaTv { mtv_info = i1, mtv_ref = ref1 },
                  MetaTv { mtv_info = i2, mtv_ref = ref2 })
                  | nicer_to_update_tv1 tv1 i1 i2 -> updateMeta tv1 ref1 ty2
                  | otherwise                     -> updateMeta tv2 ref2 ty1
             (EQ, MetaTv { mtv_ref = ref1 }, _) -> updateMeta tv1 ref1 ty2
             (EQ, _, MetaTv { mtv_ref = ref2 }) -> updateMeta tv2 ref2 ty1
  
             -- Can't do it in-place, so defer
             -- This happens for skolems of all sorts
             (_, _, _) -> unSwap swapped (uType_defer origin) ty1 ty2 } }
    where
      k1  = tyVarKind tv1
      k2  = tyVarKind tv2
      ty1 = mkTyVarTy tv1
      ty2 = mkTyVarTy tv2
  
  nicer_to_update_tv1 :: TcTyVar -> MetaInfo -> MetaInfo -> Bool
  nicer_to_update_tv1 _   _     SigTv = True
  nicer_to_update_tv1 _   SigTv _     = False
  nicer_to_update_tv1 tv1 _     _     = isSystemName (Var.varName tv1)
          -- Try not to update SigTvs; and try to update sys-y type
          -- variables in preference to ones gotten (say) by
          -- instantiating a polymorphic function with a user-written
          -- type sig
  
  ----------------
  checkTauTvUpdate :: DynFlags -> TcTyVar -> TcType -> TcM (Maybe TcType)
  --    (checkTauTvUpdate tv ty)
  -- We are about to update the TauTv/ReturnTv tv with ty.
  -- Check (a) that tv doesn't occur in ty (occurs check)
  --       (b) that kind(ty) is a sub-kind of kind(tv)
  --
  -- 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.
  
  checkTauTvUpdate dflags tv ty
    | SigTv <- info
    = ASSERT( not (isTyVarTy ty) )
      return Nothing
    | otherwise
    = do { ty1   <- zonkTcType ty
         ; sub_k <- unifyKindX (tyVarKind tv) (typeKind ty1)
         ; case sub_k of
             Nothing           -> return Nothing
             Just LT           -> return Nothing
             _  | is_return_tv -> if tv `elemVarSet` tyVarsOfType ty1
                                  then return Nothing
                                  else return (Just ty1)
             _  | defer_me ty1   -- Quick test
                -> -- Failed quick test so try harder
                   case occurCheckExpand dflags tv ty1 of
                     OC_OK ty2 | defer_me ty2 -> return Nothing
                               | otherwise    -> return (Just ty2)
                     _ -> return Nothing
                | otherwise   -> return (Just ty1) }
    where
      details = ASSERT2( isMetaTyVar tv, ppr tv ) tcTyVarDetails tv
      info         = mtv_info details
      is_return_tv = isReturnTyVar tv
      impredicative = canUnifyWithPolyType dflags details (tyVarKind tv)
  
      defer_me :: TcType -> Bool
      -- Checks for (a) occurrence of tv
      --            (b) type family applications
      -- See Note [Conservative unification check]
      defer_me (LitTy {})        = False
      defer_me (TyVarTy tv')     = tv == tv'
      defer_me (TyConApp tc tys) = isTypeFamilyTyCon tc || any defer_me tys
      defer_me (FunTy arg res)   = defer_me arg || defer_me res
      defer_me (AppTy fun arg)   = defer_me fun || defer_me arg
      defer_me (ForAllTy _ ty)   = not impredicative || defer_me ty
  
  {-
  Note [Conservative unification check]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  When unifying (tv ~ rhs), w try to avoid creating deferred constraints
  only for efficiency.  However, we do not unify (the defer_me check) if
    a) There's an occurs check (tv is in fvs(rhs))
    b) There's a type-function call in 'rhs'
  
  If we fail defer_me we use occurCheckExpand to try to make it pass,
  (see Note [Type synonyms and the occur check]) and then use defer_me
  again to check.  Example: Trac #4917)
         a ~ Const a b
  where type Const a b = a.  We can solve this immediately, even when
  'a' is a skolem, just by expanding the synonym.
  
  We always defer type-function calls, even if it's be perfectly safe to
  unify, eg (a ~ F [b]).  Reason: this ensures that the constraint
  solver gets to see, and hence simplify the type-function call, which
  in turn might simplify the type of an inferred function.  Test ghci046
  is a case in point.
  
  More mysteriously, test T7010 gave a horrible error
    T7010.hs:29:21:
      Couldn't match type `Serial (ValueTuple Float)' with `IO Float'
      Expected type: (ValueTuple Vector, ValueTuple Vector)
        Actual type: (ValueTuple Vector, ValueTuple Vector)
  because an insoluble type function constraint got mixed up with
  a soluble one when flattening.  I never fully understood this, but
  deferring type-function applications made it go away :-(.
  T5853 also got a less-good error message with more aggressive
  unification of type functions.
  
  Moreover the Note [Type family sharing] gives another reason, but
  again I'm not sure if it's really valid.
  
  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 [Type family sharing]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~
  We must avoid eagerly unifying type variables to types that contain function symbols,
  because this may lead to loss of sharing, and in turn, in very poor performance of the
  constraint simplifier. Assume that we have a wanted constraint:
  {
    m1 ~ [F m2],
    m2 ~ [F m3],
    m3 ~ [F m4],
    D m1,
    D m2,
    D m3
  }
  where D is some type class. If we eagerly unify m1 := [F m2], m2 := [F m3], m3 := [F m4],
  then, after zonking, our constraint simplifier will be faced with the following wanted
  constraint:
  {
    D [F [F [F m4]]],
    D [F [F m4]],
    D [F m4]
  }
  which has to be flattened by the constraint solver. In the absence of
  a flat-cache, this may generate a polynomially larger number of
  flatten skolems and the constraint sets we are working with will be
  polynomially larger.
  
  Instead, if we defer the unifications m1 := [F m2], etc. we will only
  be generating three flatten skolems, which is the maximum possible
  sharing arising from the original constraint.  That's why we used to
  use a local "ok" function, a variant of TcType.occurCheckExpand.
  
  HOWEVER, we *do* now have a flat-cache, which effectively recovers the
  sharing, so there's no great harm in losing it -- and it's generally
  more efficient to do the unification up-front.
  -}
  
  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 = ASSERT2( isTcTyVar tyvar, ppr tyvar )
                tcTyVarDetails tyvar
  
  updateMeta :: TcTyVar -> TcRef MetaDetails -> TcType -> TcM TcCoercion
  updateMeta tv1 ref1 ty2
    = do { writeMetaTyVarRef tv1 ref1 ty2
         ; return (mkTcNomReflCo ty2) }
  
  {-
  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.
  
  
  ************************************************************************
  *                                                                      *
                  Kind unification
  *                                                                      *
  ************************************************************************
  
  Unifying kinds is much, much simpler than unifying types.
  
  One small wrinkle is that as far as the user is concerned, types of kind
  Constraint should only be allowed to occur where we expect *exactly* that kind.
  We SHOULD NOT allow a type of kind fact to appear in a position expecting
  one of argTypeKind or openTypeKind.
  
  The situation is different in the core of the compiler, where we are perfectly
  happy to have types of kind Constraint on either end of an arrow.
  
  Note [Kind variables can be untouchable]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  We must use the careful function lookupTcTyVar to see if a kind
  variable is filled or unifiable.  It checks for touchablity, and kind
  variables can certainly be untouchable --- for example the variable
  might be bound outside an enclosing existental pattern match that
  binds an inner kind variable, which we don't want to escape outside.
  
  This, or something closely related, was the cause of Trac #8985.
  
  Note [Unifying kind variables]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  Rather hackily, kind variables can be TyVars not just TcTyVars.
  Main reason is in
     data instance T (D (x :: k)) = ...con-decls...
  Here we bring into scope a kind variable 'k', and use it in the
  con-decls.  BUT the con-decls will be finished and frozen, and
  are not amenable to subsequent substitution, so it makes sense
  to have the *final* kind-variable (a KindVar, not a TcKindVar) in
  scope.  So at least during kind unification we can encounter a
  KindVar.
  
  Hence the isTcTyVar tests before calling lookupTcTyVar.
  -}
  
  matchExpectedFunKind :: TcKind -> TcM (Maybe (TcKind, TcKind))
  -- Like unifyFunTy, but does not fail; instead just returns Nothing
  
  matchExpectedFunKind (FunTy arg_kind res_kind)
    = return (Just (arg_kind,res_kind))
  
  matchExpectedFunKind (TyVarTy kvar)
    | isTcTyVar kvar, isMetaTyVar kvar
    = do { maybe_kind <- readMetaTyVar kvar
         ; case maybe_kind of
              Indirect fun_kind -> matchExpectedFunKind fun_kind
              Flexi ->
                  do { arg_kind <- newMetaKindVar
                     ; res_kind <- newMetaKindVar
                     ; writeMetaTyVar kvar (mkArrowKind arg_kind res_kind)
                     ; return (Just (arg_kind,res_kind)) } }
  
  matchExpectedFunKind _ = return Nothing
  
  -----------------
  unifyKindX :: TcKind           -- k1 (actual)
             -> TcKind           -- k2 (expected)
             -> TcM (Maybe Ordering)
                                -- Returns the relation between the kinds
                                -- Just LT <=> k1 is a sub-kind of k2
                                -- Nothing <=> incomparable
  
  -- unifyKindX deals with the top-level sub-kinding story
  -- but recurses into the simpler unifyKindEq for any sub-terms
  -- The sub-kinding stuff only applies at top level
  
  unifyKindX (TyVarTy kv1) k2 = uKVar NotSwapped unifyKindX kv1 k2
  unifyKindX k1 (TyVarTy kv2) = uKVar IsSwapped  unifyKindX kv2 k1
  
  unifyKindX k1 k2       -- See Note [Expanding synonyms during unification]
    | Just k1' <- tcView k1 = unifyKindX k1' k2
    | Just k2' <- tcView k2 = unifyKindX k1  k2'
  
  unifyKindX (TyConApp kc1 []) (TyConApp kc2 [])
    | kc1 == kc2               = return (Just EQ)
    | kc1 `tcIsSubKindCon` kc2 = return (Just LT)
    | kc2 `tcIsSubKindCon` kc1 = return (Just GT)
    | otherwise                = return Nothing
  
  unifyKindX k1 k2 = unifyKindEq k1 k2
    -- In all other cases, let unifyKindEq do the work
  
  -------------------
  uKVar :: SwapFlag -> (TcKind -> TcKind -> TcM (Maybe Ordering))
        -> MetaKindVar -> TcKind -> TcM (Maybe Ordering)
  uKVar swapped unify_kind kv1 k2
    | isTcTyVar kv1
    = do { lookup_res <- lookupTcTyVar kv1  -- See Note [Kind variables can be untouchable]
         ; case lookup_res of
             Filled k1    -> unSwap swapped unify_kind k1 k2
             Unfilled ds1 -> uUnfilledKVar kv1 ds1 k2 }
  
    | otherwise   -- See Note [Unifying kind variables]
    = uUnfilledKVar kv1 vanillaSkolemTv k2
  
  -------------------
  uUnfilledKVar :: MetaKindVar -> TcTyVarDetails -> TcKind -> TcM (Maybe Ordering)
  uUnfilledKVar kv1 ds1 (TyVarTy kv2)
    | kv1 == kv2
    = return (Just EQ)
  
    | isTcTyVar kv2
    = do { lookup_res <- lookupTcTyVar kv2
         ; case lookup_res of
             Filled k2    -> uUnfilledKVar  kv1 ds1 k2
             Unfilled ds2 -> uUnfilledKVars kv1 ds1 kv2 ds2 }
  
    | otherwise  -- See Note [Unifying kind variables]
    = uUnfilledKVars kv1 ds1 kv2 vanillaSkolemTv
  
  uUnfilledKVar kv1 ds1 non_var_k2
    = case ds1 of
        MetaTv { mtv_info = SigTv }
          -> return Nothing
        MetaTv { mtv_ref = ref1 }
          -> do { k2a <- zonkTcKind non_var_k2
                ; let k2b = defaultKind k2a
                       -- MetaKindVars must be bound only to simple kinds
  
                ; dflags <- getDynFlags
                ; case occurCheckExpand dflags kv1 k2b of
                     OC_OK k2c -> do { writeMetaTyVarRef kv1 ref1 k2c; return (Just EQ) }
                     _         -> return Nothing }
        _ -> return Nothing
  
  -------------------
  uUnfilledKVars :: MetaKindVar -> TcTyVarDetails
                 -> MetaKindVar -> TcTyVarDetails
                 -> TcM (Maybe Ordering)
  -- kv1 /= kv2
  uUnfilledKVars kv1 ds1 kv2 ds2
    = case (ds1, ds2) of
        (MetaTv { mtv_info = i1, mtv_ref = r1 },
         MetaTv { mtv_info = i2, mtv_ref = r2 })
                | nicer_to_update_tv1 kv1 i1 i2 -> do_update kv1 r1 kv2
                | otherwise                     -> do_update kv2 r2 kv1
        (MetaTv { mtv_ref = r1 }, _) -> do_update kv1 r1 kv2
        (_, MetaTv { mtv_ref = r2 }) -> do_update kv2 r2 kv1
        _ -> return Nothing
    where
      do_update kv1 r1 kv2
        = do { writeMetaTyVarRef kv1 r1 (mkTyVarTy kv2); return (Just EQ) }
  
  ---------------------------
  unifyKindEq :: TcKind -> TcKind -> TcM (Maybe Ordering)
  -- Unify two kinds looking for equality not sub-kinding
  -- So it returns Nothing or (Just EQ) only
  unifyKindEq (TyVarTy kv1) k2 = uKVar NotSwapped unifyKindEq kv1 k2
  unifyKindEq k1 (TyVarTy kv2) = uKVar IsSwapped  unifyKindEq kv2 k1
  
  unifyKindEq (FunTy a1 r1) (FunTy a2 r2)
    = do { mb1 <- unifyKindEq a1 a2; mb2 <- unifyKindEq r1 r2
         ; return (if isJust mb1 && isJust mb2 then Just EQ else Nothing) }
  
  unifyKindEq (TyConApp kc1 k1s) (TyConApp kc2 k2s)
    | kc1 == kc2
    = ASSERT(length k1s == length k2s)
         -- Should succeed since the kind constructors are the same,
         -- and the kinds are sort-checked, thus fully applied
      do { mb_eqs <- zipWithM unifyKindEq k1s k2s
         ; return (if all isJust mb_eqs
                   then Just EQ
                   else Nothing) }
  
  unifyKindEq _ _ = return Nothing
  