typecheck/TcHsType.hs

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

    \section[TcMonoType]{Typechecking user-specified @MonoTypes@}
    -}

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

    module TcHsType (
            -- Type signatures
            kcHsSigType, tcClassSigType,
            tcHsSigType, tcHsSigWcType,
            funsSigCtxt, addSigCtxt,

            tcHsClsInstType,
            tcHsDeriv, tcHsVectInst,
            tcHsTypeApp,
            UserTypeCtxt(..),
            tcImplicitTKBndrs, tcImplicitTKBndrsType, tcExplicitTKBndrs,

                    -- Type checking type and class decls
            kcLookupKind, kcTyClTyVars, tcTyClTyVars,
            tcHsConArgType, tcDataKindSig,

            -- Kind-checking types
            -- No kind generalisation, no checkValidType
            tcWildCardBinders,
            kcHsTyVarBndrs,
            tcHsLiftedType,   tcHsOpenType,
            tcHsLiftedTypeNC, tcHsOpenTypeNC,
            tcLHsType, tcCheckLHsType,
            tcHsContext, tcLHsPredType, tcInferApps, tcInferArgs,
            solveEqualities, -- useful re-export

            kindGeneralize,

            -- Sort-checking kinds
            tcLHsKind,

            -- Pattern type signatures
            tcHsPatSigType, tcPatSig, funAppCtxt
       ) where

    #include "HsVersions.h"

    import HsSyn
    import TcRnMonad
    import TcEvidence
    import TcEnv
    import TcMType
    import TcValidity
    import TcUnify
    import TcIface
    import TcSimplify ( solveEqualities )
    import TcType
    import Inst   ( tcInstBinders, tcInstBindersX )
    import Type
    import Kind
    import RdrName( lookupLocalRdrOcc )
    import Var
    import VarSet
    import TyCon
    import ConLike
    import DataCon
    import TysPrim ( tYPE )
    import Class
    import Name
    import NameEnv
    import NameSet
    import VarEnv
    import TysWiredIn
    import BasicTypes
    import SrcLoc
    import Constants ( mAX_CTUPLE_SIZE )
    import ErrUtils( MsgDoc )
    import Unique
    import Util
    import UniqSupply
    import Outputable
    import FastString
    import PrelNames hiding ( wildCardName )
    import Pair
    import qualified GHC.LanguageExtensions as LangExt

    import Maybes
    import Data.List ( partition )
    import Control.Monad

    {-
            ----------------------------
                    General notes
            ----------------------------

    Unlike with expressions, type-checking types both does some checking and
    desugars at the same time. This is necessary because we often want to perform
    equality checks on the types right away, and it would be incredibly painful
    to do this on un-desugared types. Luckily, desugared types are close enough
    to HsTypes to make the error messages sane.

    During type-checking, we perform as little validity checking as possible.
    This is because some type-checking is done in a mutually-recursive knot, and
    if we look too closely at the tycons, we'll loop. This is why we always must
    use mkNakedTyConApp and mkNakedAppTys, etc., which never look at a tycon.
    The mkNamed... functions don't uphold Type invariants, but zonkTcTypeToType
    will repair this for us. Note that zonkTcType *is* safe within a knot, and
    can be done repeatedly with no ill effect: it just squeezes out metavariables.

    Generally, after type-checking, you will want to do validity checking, say
    with TcValidity.checkValidType.

    Validity checking
    ~~~~~~~~~~~~~~~~~
    Some of the validity check could in principle be done by the kind checker,
    but not all:

    - During desugaring, we normalise by expanding type synonyms.  Only
      after this step can we check things like type-synonym saturation
      e.g.  type T k = k Int
            type S a = a
      Then (T S) is ok, because T is saturated; (T S) expands to (S Int);
      and then S is saturated.  This is a GHC extension.

    - Similarly, also a GHC extension, we look through synonyms before complaining
      about the form of a class or instance declaration

    - Ambiguity checks involve functional dependencies, and it's easier to wait
      until knots have been resolved before poking into them

    Also, in a mutually recursive group of types, we can't look at the TyCon until we've
    finished building the loop.  So to keep things simple, we postpone most validity
    checking until step (3).

    Knot tying
    ~~~~~~~~~~
    During step (1) we might fault in a TyCon defined in another module, and it might
    (via a loop) refer back to a TyCon defined in this module. So when we tie a big
    knot around type declarations with ARecThing, so that the fault-in code can get
    the TyCon being defined.

    %************************************************************************
    %*                                                                      *
                  Check types AND do validity checking
    *                                                                      *
    ************************************************************************
    -}

    funsSigCtxt :: [Located Name] -> UserTypeCtxt
    -- Returns FunSigCtxt, with no redundant-context-reporting,
    -- form a list of located names
    funsSigCtxt (L _ name1 : _) = FunSigCtxt name1 False
    funsSigCtxt []              = panic "funSigCtxt"

    addSigCtxt :: UserTypeCtxt -> LHsType Name -> TcM a -> TcM a
    addSigCtxt ctxt sig_ty thing_inside
      = setSrcSpan (getLoc sig_ty) $
        addErrCtxt (pprSigCtxt ctxt empty (ppr sig_ty)) $
        thing_inside

    tcHsSigWcType :: UserTypeCtxt -> LHsSigWcType Name -> TcM Type
    -- This one is used when we have a LHsSigWcType, but in
    -- a place where wildards aren't allowed. The renamer has
    -- alrady checked this, so we can simply ignore it.
    tcHsSigWcType ctxt sig_ty = tcHsSigType ctxt (dropWildCards sig_ty)

    kcHsSigType :: [Located Name] -> LHsSigType Name -> TcM ()
    kcHsSigType names (HsIB { hsib_body = hs_ty
                            , hsib_vars = sig_vars })
      = addSigCtxt (funsSigCtxt names) hs_ty $
        discardResult $
        tcImplicitTKBndrsType sig_vars $
        tc_lhs_type typeLevelMode hs_ty liftedTypeKind

    tcClassSigType :: [Located Name] -> LHsSigType Name -> TcM Type
    -- Does not do validity checking; this must be done outside
    -- the recursive class declaration "knot"
    tcClassSigType names sig_ty
      = addSigCtxt (funsSigCtxt names) (hsSigType sig_ty) $
        do { ty <- tc_hs_sig_type sig_ty liftedTypeKind
           ; kindGeneralizeType ty }

    tcHsSigType :: UserTypeCtxt -> LHsSigType Name -> TcM Type
    -- Does validity checking
    tcHsSigType ctxt sig_ty
      = addSigCtxt ctxt (hsSigType sig_ty) $
        do { kind <- case expectedKindInCtxt ctxt of
                        AnythingKind -> newMetaKindVar
                        TheKind k    -> return k
                        OpenKind     -> do { rr <- newFlexiTyVarTy runtimeRepTy
                                           ; return $ tYPE rr }
                  -- The kind is checked by checkValidType, and isn't necessarily
                  -- of kind * in a Template Haskell quote eg [t| Maybe |]

           ; ty <- tc_hs_sig_type sig_ty kind
              -- Generalise here: see Note [Kind generalisation]
           ; ty <- maybeKindGeneralizeType ty -- also zonks
           ; checkValidType ctxt ty
           ; return ty }

    tc_hs_sig_type :: LHsSigType Name -> Kind -> TcM Type
    -- Does not do validity checking or zonking
    tc_hs_sig_type (HsIB { hsib_body = hs_ty
                         , hsib_vars = sig_vars }) kind
      = do { (tkvs, ty) <- solveEqualities $
                           tcImplicitTKBndrsType sig_vars $
                           tc_lhs_type typeLevelMode hs_ty kind
           ; return (mkSpecForAllTys tkvs ty) }

    -----------------
    tcHsDeriv :: LHsSigType Name -> TcM ([TyVar], Class, [Type], Kind)
    -- Like tcHsSigType, but for the ...deriving( C t1 ty2 ) clause
    -- Returns the C, [ty1, ty2, and the kind of C's *next* argument
    -- E.g.    class C (a::*) (b::k->k)
    --         data T a b = ... deriving( C Int )
    --    returns ([k], C, [k, Int],  k->k)
    -- Also checks that (C ty1 ty2 arg) :: Constraint
    -- if arg has a suitable kind
    tcHsDeriv hs_ty
      = do { arg_kind <- newMetaKindVar
                        -- always safe to kind-generalize, because there
                        -- can be no covars in an outer scope
           ; ty <- checkNoErrs $
                     -- avoid redundant error report with "illegal deriving", below
                   tc_hs_sig_type hs_ty (mkFunTy arg_kind constraintKind)
           ; ty <- kindGeneralizeType ty  -- also zonks
           ; arg_kind <- zonkTcType arg_kind
           ; let (tvs, pred) = splitForAllTys ty
           ; case getClassPredTys_maybe pred of
               Just (cls, tys) -> return (tvs, cls, tys, arg_kind)
               Nothing -> failWithTc (text "Illegal deriving item" <+> quotes (ppr hs_ty)) }

    tcHsClsInstType :: UserTypeCtxt    -- InstDeclCtxt or SpecInstCtxt
                    -> LHsSigType Name
                    -> TcM ([TyVar], ThetaType, Class, [Type])
    -- Like tcHsSigType, but for a class instance declaration
    tcHsClsInstType user_ctxt hs_inst_ty
      = setSrcSpan (getLoc (hsSigType hs_inst_ty)) $
        do { inst_ty <- tc_hs_sig_type hs_inst_ty constraintKind
           ; inst_ty <- kindGeneralizeType inst_ty
           ; checkValidInstance user_ctxt hs_inst_ty inst_ty }

    -- Used for 'VECTORISE [SCALAR] instance' declarations
    tcHsVectInst :: LHsSigType Name -> TcM (Class, [Type])
    tcHsVectInst ty
      | Just (L _ cls_name, tys) <- hsTyGetAppHead_maybe (hsSigType ty)
        -- Ignoring the binders looks pretty dodgy to me
      = do { (cls, cls_kind) <- tcClass cls_name
           ; (applied_class, _res_kind)
               <- tcInferApps typeLevelMode cls_name (mkClassPred cls []) cls_kind tys
           ; case tcSplitTyConApp_maybe applied_class of
               Just (_tc, args) -> ASSERT( _tc == classTyCon cls )
                                   return (cls, args)
               _ -> failWithTc (text "Too many arguments passed to" <+> ppr cls_name) }
      | otherwise
      = failWithTc $ text "Malformed instance type"

    ----------------------------------------------
    -- | Type-check a visible type application
    tcHsTypeApp :: LHsWcType Name -> Kind -> TcM Type
    tcHsTypeApp wc_ty kind
      | HsWC { hswc_wcs = sig_wcs, hswc_ctx = extra, hswc_body = hs_ty } <- wc_ty
      = ASSERT( isNothing extra )  -- handled in RnTypes.rnExtraConstraintWildCard
        tcWildCardBinders sig_wcs $ \ _ ->
        do { ty <- tcCheckLHsType hs_ty kind
           ; ty <- zonkTcType ty
           ; checkValidType TypeAppCtxt ty
           ; return ty }
            -- NB: we don't call emitWildcardHoleConstraints here, because
            -- we want any holes in visible type applications to be used
            -- without fuss. No errors, warnings, extensions, etc.

    {-
            These functions are used during knot-tying in
            type and class declarations, when we have to
            separate kind-checking, desugaring, and validity checking

    ************************************************************************
    *                                                                      *
                The main kind checker: no validity checks here
    %*                                                                      *
    %************************************************************************

            First a couple of simple wrappers for kcHsType
    -}

    tcHsConArgType :: NewOrData ->  LHsType Name -> TcM Type
    -- Permit a bang, but discard it
    tcHsConArgType NewType  bty = tcHsLiftedType (getBangType bty)
      -- Newtypes can't have bangs, but we don't check that
      -- until checkValidDataCon, so do not want to crash here

    tcHsConArgType DataType bty = tcHsOpenType (getBangType bty)
      -- Can't allow an unlifted type for newtypes, because we're effectively
      -- going to remove the constructor while coercing it to a lifted type.
      -- And newtypes can't be bang'd

    ---------------------------
    tcHsOpenType, tcHsLiftedType,
      tcHsOpenTypeNC, tcHsLiftedTypeNC :: LHsType Name -> TcM TcType
    -- Used for type signatures
    -- Do not do validity checking
    tcHsOpenType ty   = addTypeCtxt ty $ tcHsOpenTypeNC ty
    tcHsLiftedType ty = addTypeCtxt ty $ tcHsLiftedTypeNC ty

    tcHsOpenTypeNC   ty = do { ek <- ekOpen
                             ; tc_lhs_type typeLevelMode ty ek }
    tcHsLiftedTypeNC ty = tc_lhs_type typeLevelMode ty liftedTypeKind

    -- Like tcHsType, but takes an expected kind
    tcCheckLHsType :: LHsType Name -> Kind -> TcM Type
    tcCheckLHsType hs_ty exp_kind
      = addTypeCtxt hs_ty $
        tc_lhs_type typeLevelMode hs_ty exp_kind

    tcLHsType :: LHsType Name -> TcM (TcType, TcKind)
    -- Called from outside: set the context
    tcLHsType ty = addTypeCtxt ty (tc_infer_lhs_type typeLevelMode ty)

    ---------------------------
    -- | Should we generalise the kind of this type?
    -- We *should* generalise if the type is mentions no scoped type variables
    -- or if NoMonoLocalBinds is set. Otherwise, nope.
    decideKindGeneralisationPlan :: Type -> TcM Bool
    decideKindGeneralisationPlan ty
      = do { mono_locals <- xoptM LangExt.MonoLocalBinds
           ; in_scope <- getInLocalScope
           ; let fvs        = tyCoVarsOfTypeList ty
                 should_gen = not mono_locals || all (not . in_scope . getName) fvs
           ; traceTc "decideKindGeneralisationPlan"
               (vcat [ text "type:" <+> ppr ty
                     , text "ftvs:" <+> ppr fvs
                     , text "should gen?" <+> ppr should_gen ])
           ; return should_gen }

    maybeKindGeneralizeType :: TcType -> TcM Type
    maybeKindGeneralizeType ty
      = do { should_gen <- decideKindGeneralisationPlan ty
           ; if should_gen
             then kindGeneralizeType ty
             else zonkTcType ty }

    {-
    ************************************************************************
    *                                                                      *
          Type-checking modes
    *                                                                      *
    ************************************************************************

    The kind-checker is parameterised by a TcTyMode, which contains some
    information about where we're checking a type.

    The renamer issues errors about what it can. All errors issued here must
    concern things that the renamer can't handle.

    -}

    -- | Info about the context in which we're checking a type. Currently,
    -- differentiates only between types and kinds, but this will likely
    -- grow, at least to include the distinction between patterns and
    -- not-patterns.
    newtype TcTyMode
      = TcTyMode { mode_level :: TypeOrKind  -- True <=> type, False <=> kind
                 }

    typeLevelMode :: TcTyMode
    typeLevelMode = TcTyMode { mode_level = TypeLevel }

    kindLevelMode :: TcTyMode
    kindLevelMode = TcTyMode { mode_level = KindLevel }

    -- switch to kind level
    kindLevel :: TcTyMode -> TcTyMode
    kindLevel mode = mode { mode_level = KindLevel }

    instance Outputable TcTyMode where
      ppr = ppr . mode_level

    {-
    Note [Bidirectional type checking]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    In expressions, whenever we see a polymorphic identifier, say `id`, we are
    free to instantiate it with metavariables, knowing that we can always
    re-generalize with type-lambdas when necessary. For example:

      rank2 :: (forall a. a -> a) -> ()
      x = rank2 id

    When checking the body of `x`, we can instantiate `id` with a metavariable.
    Then, when we're checking the application of `rank2`, we notice that we really
    need a polymorphic `id`, and then re-generalize over the unconstrained
    metavariable.

    In types, however, we're not so lucky, because *we cannot re-generalize*!
    There is no lambda. So, we must be careful only to instantiate at the last
    possible moment, when we're sure we're never going to want the lost polymorphism
    again. This is done in calls to tcInstBinders and tcInstBindersX.

    To implement this behavior, we use bidirectional type checking, where we
    explicitly think about whether we know the kind of the type we're checking
    or not. Note that there is a difference between not knowing a kind and
    knowing a metavariable kind: the metavariables are TauTvs, and cannot become
    forall-quantified kinds. Previously (before dependent types), there were
    no higher-rank kinds, and so we could instantiate early and be sure that
    no types would have polymorphic kinds, and so we could always assume that
    the kind of a type was a fresh metavariable. Not so anymore, thus the
    need for two algorithms.

    For HsType forms that can never be kind-polymorphic, we implement only the
    "down" direction, where we safely assume a metavariable kind. For HsType forms
    that *can* be kind-polymorphic, we implement just the "up" (functions with
    "infer" in their name) version, as we gain nothing by also implementing the
    "down" version.

    Note [Future-proofing the type checker]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    As discussed in Note [Bidirectional type checking], each HsType form is
    handled in *either* tc_infer_hs_type *or* tc_hs_type. These functions
    are mutually recursive, so that either one can work for any type former.
    But, we want to make sure that our pattern-matches are complete. So,
    we have a bunch of repetitive code just so that we get warnings if we're
    missing any patterns.
    -}

    -- | Check and desugar a type, returning the core type and its
    -- possibly-polymorphic kind. Much like 'tcInferRho' at the expression
    -- level.
    tc_infer_lhs_type :: TcTyMode -> LHsType Name -> TcM (TcType, TcKind)
    tc_infer_lhs_type mode (L span ty)
      = setSrcSpan span $
        do { traceTc "tc_infer_lhs_type:" (ppr ty)
           ; tc_infer_hs_type mode ty }

    -- | Infer the kind of a type and desugar. This is the "up" type-checker,
    -- as described in Note [Bidirectional type checking]
    tc_infer_hs_type :: TcTyMode -> HsType Name -> TcM (TcType, TcKind)
    tc_infer_hs_type mode (HsTyVar (L _ tv)) = tcTyVar mode tv
    tc_infer_hs_type mode (HsAppTy ty1 ty2)
      = do { let (fun_ty, arg_tys) = splitHsAppTys ty1 [ty2]
           ; (fun_ty', fun_kind) <- tc_infer_lhs_type mode fun_ty
           ; fun_kind' <- zonkTcType fun_kind
           ; tcInferApps mode fun_ty fun_ty' fun_kind' arg_tys }
    tc_infer_hs_type mode (HsParTy t)     = tc_infer_lhs_type mode t
    tc_infer_hs_type mode (HsOpTy lhs (L _ op) rhs)
      | not (op `hasKey` funTyConKey)
      = do { (op', op_kind) <- tcTyVar mode op
           ; op_kind' <- zonkTcType op_kind
           ; tcInferApps mode op op' op_kind' [lhs, rhs] }
    tc_infer_hs_type mode (HsKindSig ty sig)
      = do { sig' <- tc_lhs_kind (kindLevel mode) sig
           ; ty' <- tc_lhs_type mode ty sig'
           ; return (ty', sig') }
    tc_infer_hs_type mode (HsDocTy ty _) = tc_infer_lhs_type mode ty
    tc_infer_hs_type _    (HsCoreTy ty)  = return (ty, typeKind ty)
    tc_infer_hs_type mode other_ty
      = do { kv <- newMetaKindVar
           ; ty' <- tc_hs_type mode other_ty kv
           ; return (ty', kv) }

    tc_lhs_type :: TcTyMode -> LHsType Name -> TcKind -> TcM TcType
    tc_lhs_type mode (L span ty) exp_kind
      = setSrcSpan span $
        do { traceTc "tc_lhs_type:" (ppr ty $$ ppr exp_kind)
           ; tc_hs_type mode ty exp_kind }

    ------------------------------------------
    tc_fun_type :: TcTyMode -> LHsType Name -> LHsType Name -> TcKind -> TcM TcType
    tc_fun_type mode ty1 ty2 exp_kind = case mode_level mode of
      TypeLevel ->
        do { arg_rr <- newFlexiTyVarTy runtimeRepTy
           ; res_rr <- newFlexiTyVarTy runtimeRepTy
           ; ty1' <- tc_lhs_type mode ty1 (tYPE arg_rr)
           ; ty2' <- tc_lhs_type mode ty2 (tYPE res_rr)
           ; checkExpectedKind (mkFunTy ty1' ty2') liftedTypeKind exp_kind }
      KindLevel ->  -- no representation polymorphism in kinds. yet.
        do { ty1' <- tc_lhs_type mode ty1 liftedTypeKind
           ; ty2' <- tc_lhs_type mode ty2 liftedTypeKind
           ; checkExpectedKind (mkFunTy ty1' ty2') liftedTypeKind exp_kind }

    ------------------------------------------
    -- See also Note [Bidirectional type checking]
    tc_hs_type :: TcTyMode -> HsType Name -> TcKind -> TcM TcType
    tc_hs_type mode (HsParTy ty)   exp_kind = tc_lhs_type mode ty exp_kind
    tc_hs_type mode (HsDocTy ty _) exp_kind = tc_lhs_type mode ty exp_kind
    tc_hs_type _ ty@(HsBangTy {}) _
        -- While top-level bangs at this point are eliminated (eg !(Maybe Int)),
        -- other kinds of bangs are not (eg ((!Maybe) Int)). These kinds of
        -- bangs are invalid, so fail. (#7210)
        = failWithTc (text "Unexpected strictness annotation:" <+> ppr ty)
    tc_hs_type _ ty@(HsRecTy _)      _
          -- Record types (which only show up temporarily in constructor
          -- signatures) should have been removed by now
        = failWithTc (text "Record syntax is illegal here:" <+> ppr ty)

    -- This should never happen; type splices are expanded by the renamer
    tc_hs_type _ ty@(HsSpliceTy {}) _exp_kind
      = failWithTc (text "Unexpected type splice:" <+> ppr ty)

    ---------- Functions and applications
    tc_hs_type mode (HsFunTy ty1 ty2) exp_kind
      = tc_fun_type mode ty1 ty2 exp_kind

    tc_hs_type mode (HsOpTy ty1 (L _ op) ty2) exp_kind
      | op `hasKey` funTyConKey
      = tc_fun_type mode ty1 ty2 exp_kind

    --------- Foralls
    tc_hs_type mode (HsForAllTy { hst_bndrs = hs_tvs, hst_body = ty }) exp_kind
      = fmap fst $
        tcExplicitTKBndrs hs_tvs $ \ tvs' ->
        -- Do not kind-generalise here!  See Note [Kind generalisation]
        -- Why exp_kind?  See Note [Body kind of HsForAllTy]
        do { ty' <- tc_lhs_type mode ty exp_kind
           ; let bound_vars = allBoundVariables ty'
                 bndrs      = mkNamedBinders Specified tvs'
           ; return (mkForAllTys bndrs ty', bound_vars) }

    tc_hs_type mode (HsQualTy { hst_ctxt = ctxt, hst_body = ty }) exp_kind
      | null (unLoc ctxt)
      = tc_lhs_type mode ty exp_kind

      | otherwise
      = do { ctxt' <- tc_hs_context mode ctxt

             -- See Note [Body kind of a HsQualTy]
           ; ty' <- if isConstraintKind exp_kind
                    then tc_lhs_type mode ty constraintKind
                    else do { ek <- ekOpen -- The body kind (result of the
                                           -- function) can be * or #, hence ekOpen
                            ; ty <- tc_lhs_type mode ty ek
                            ; checkExpectedKind ty liftedTypeKind exp_kind }

           ; return (mkPhiTy ctxt' ty') }

    --------- Lists, arrays, and tuples
    tc_hs_type mode (HsListTy elt_ty) exp_kind
      = do { tau_ty <- tc_lhs_type mode elt_ty liftedTypeKind
           ; checkWiredInTyCon listTyCon
           ; checkExpectedKind (mkListTy tau_ty) liftedTypeKind exp_kind }

    tc_hs_type mode (HsPArrTy elt_ty) exp_kind
      = do { MASSERT( isTypeLevel (mode_level mode) )
           ; tau_ty <- tc_lhs_type mode elt_ty liftedTypeKind
           ; checkWiredInTyCon parrTyCon
           ; checkExpectedKind (mkPArrTy tau_ty) liftedTypeKind exp_kind }

    -- See Note [Distinguishing tuple kinds] in HsTypes
    -- See Note [Inferring tuple kinds]
    tc_hs_type mode (HsTupleTy HsBoxedOrConstraintTuple hs_tys) exp_kind
         -- (NB: not zonking before looking at exp_k, to avoid left-right bias)
      | Just tup_sort <- tupKindSort_maybe exp_kind
      = traceTc "tc_hs_type tuple" (ppr hs_tys) >>
        tc_tuple mode tup_sort hs_tys exp_kind
      | otherwise
      = do { traceTc "tc_hs_type tuple 2" (ppr hs_tys)
           ; (tys, kinds) <- mapAndUnzipM (tc_infer_lhs_type mode) hs_tys
           ; kinds <- mapM zonkTcType kinds
               -- Infer each arg type separately, because errors can be
               -- confusing if we give them a shared kind.  Eg Trac #7410
               -- (Either Int, Int), we do not want to get an error saying
               -- "the second argument of a tuple should have kind *->*"

           ; let (arg_kind, tup_sort)
                   = case [ (k,s) | k <- kinds
                                  , Just s <- [tupKindSort_maybe k] ] of
                        ((k,s) : _) -> (k,s)
                        [] -> (liftedTypeKind, BoxedTuple)
             -- In the [] case, it's not clear what the kind is, so guess *

           ; tys' <- sequence [ setSrcSpan loc $
                                checkExpectedKind ty kind arg_kind
                              | ((L loc _),ty,kind) <- zip3 hs_tys tys kinds ]

           ; finish_tuple tup_sort tys' (map (const arg_kind) tys') exp_kind }

    tc_hs_type mode (HsTupleTy hs_tup_sort tys) exp_kind
      = tc_tuple mode tup_sort tys exp_kind
      where
        tup_sort = case hs_tup_sort of  -- Fourth case dealt with above
                      HsUnboxedTuple    -> UnboxedTuple
                      HsBoxedTuple      -> BoxedTuple
                      HsConstraintTuple -> ConstraintTuple
                      _                 -> panic "tc_hs_type HsTupleTy"

    --------- Promoted lists and tuples
    tc_hs_type mode (HsExplicitListTy _k tys) exp_kind
      = do { tks <- mapM (tc_infer_lhs_type mode) tys
           ; (taus', kind) <- unifyKinds tks
           ; let ty = (foldr (mk_cons kind) (mk_nil kind) taus')
           ; checkExpectedKind ty (mkListTy kind) exp_kind }
      where
        mk_cons k a b = mkTyConApp (promoteDataCon consDataCon) [k, a, b]
        mk_nil  k     = mkTyConApp (promoteDataCon nilDataCon) [k]

    tc_hs_type mode (HsExplicitTupleTy _ tys) exp_kind
      -- using newMetaKindVar means that we force instantiations of any polykinded
      -- types. At first, I just used tc_infer_lhs_type, but that led to #11255.
      = do { ks   <- replicateM arity newMetaKindVar
           ; taus <- zipWithM (tc_lhs_type mode) tys ks
           ; let kind_con   = tupleTyCon           Boxed arity
                 ty_con     = promotedTupleDataCon Boxed arity
                 tup_k      = mkTyConApp kind_con ks
           ; checkExpectedKind (mkTyConApp ty_con (ks ++ taus)) tup_k exp_kind }
      where
        arity = length tys

    --------- Constraint types
    tc_hs_type mode (HsIParamTy n ty) exp_kind
      = do { MASSERT( isTypeLevel (mode_level mode) )
           ; ty' <- tc_lhs_type mode ty liftedTypeKind
           ; let n' = mkStrLitTy $ hsIPNameFS n
           ; ipClass <- tcLookupClass ipClassName
           ; checkExpectedKind (mkClassPred ipClass [n',ty'])
               constraintKind exp_kind }

    tc_hs_type mode (HsEqTy ty1 ty2) exp_kind
      = do { (ty1', kind1) <- tc_infer_lhs_type mode ty1
           ; (ty2', kind2) <- tc_infer_lhs_type mode ty2
           ; ty2'' <- checkExpectedKind ty2' kind2 kind1
           ; eq_tc <- tcLookupTyCon eqTyConName
           ; let ty' = mkNakedTyConApp eq_tc [kind1, ty1', ty2'']
           ; checkExpectedKind ty' constraintKind exp_kind }

    --------- Literals
    tc_hs_type _ (HsTyLit (HsNumTy _ n)) exp_kind
      = do { checkWiredInTyCon typeNatKindCon
           ; checkExpectedKind (mkNumLitTy n) typeNatKind exp_kind }

    tc_hs_type _ (HsTyLit (HsStrTy _ s)) exp_kind
      = do { checkWiredInTyCon typeSymbolKindCon
           ; checkExpectedKind (mkStrLitTy s) typeSymbolKind exp_kind }

    --------- Potentially kind-polymorphic types: call the "up" checker
    -- See Note [Future-proofing the type checker]
    tc_hs_type mode ty@(HsTyVar {})   ek = tc_infer_hs_type_ek mode ty ek
    tc_hs_type mode ty@(HsAppTy {})   ek = tc_infer_hs_type_ek mode ty ek
    tc_hs_type mode ty@(HsOpTy {})    ek = tc_infer_hs_type_ek mode ty ek
    tc_hs_type mode ty@(HsKindSig {}) ek = tc_infer_hs_type_ek mode ty ek
    tc_hs_type mode ty@(HsCoreTy {})  ek = tc_infer_hs_type_ek mode ty ek

    tc_hs_type _ (HsWildCardTy wc) exp_kind
      = do { let name = wildCardName wc
           ; tv <- tcLookupTyVar name
           ; checkExpectedKind (mkTyVarTy tv) (tyVarKind tv) exp_kind }

    -- disposed of by renamer
    tc_hs_type _ ty@(HsAppsTy {}) _
      = pprPanic "tc_hs_tyep HsAppsTy" (ppr ty)

    ---------------------------
    -- | Call 'tc_infer_hs_type' and check its result against an expected kind.
    tc_infer_hs_type_ek :: TcTyMode -> HsType Name -> TcKind -> TcM TcType
    tc_infer_hs_type_ek mode ty ek
      = do { (ty', k) <- tc_infer_hs_type mode ty
           ; checkExpectedKind ty' k ek }

    ---------------------------
    tupKindSort_maybe :: TcKind -> Maybe TupleSort
    tupKindSort_maybe k
      | Just (k', _) <- splitCastTy_maybe k = tupKindSort_maybe k'
      | Just k'      <- coreView k          = tupKindSort_maybe k'
      | isConstraintKind k = Just ConstraintTuple
      | isLiftedTypeKind k = Just BoxedTuple
      | otherwise          = Nothing

    tc_tuple :: TcTyMode -> TupleSort -> [LHsType Name] -> TcKind -> TcM TcType
    tc_tuple mode tup_sort tys exp_kind
      = do { arg_kinds <- case tup_sort of
               BoxedTuple      -> return (nOfThem arity liftedTypeKind)
               UnboxedTuple    -> do { rrs <- newFlexiTyVarTys arity runtimeRepTy
                                     ; return $ map tYPE rrs }
               ConstraintTuple -> return (nOfThem arity constraintKind)
           ; tau_tys <- zipWithM (tc_lhs_type mode) tys arg_kinds
           ; finish_tuple tup_sort tau_tys arg_kinds exp_kind }
      where
        arity   = length tys

    finish_tuple :: TupleSort
                 -> [TcType]    -- ^ argument types
                 -> [TcKind]    -- ^ of these kinds
                 -> TcKind      -- ^ expected kind of the whole tuple
                 -> TcM TcType
    finish_tuple tup_sort tau_tys tau_kinds exp_kind
      = do { traceTc "finish_tuple" (ppr res_kind $$ ppr tau_kinds $$ ppr exp_kind)
           ; let arg_tys  = case tup_sort of
                       -- See also Note [Unboxed tuple RuntimeRep vars] in TyCon
                     UnboxedTuple    -> map (getRuntimeRepFromKind "finish_tuple") tau_kinds
                                        ++ tau_tys
                     BoxedTuple      -> tau_tys
                     ConstraintTuple -> tau_tys
           ; tycon <- case tup_sort of
               ConstraintTuple
                 | arity > mAX_CTUPLE_SIZE
                             -> failWith (bigConstraintTuple arity)
                 | otherwise -> tcLookupTyCon (cTupleTyConName arity)
               BoxedTuple    -> do { let tc = tupleTyCon Boxed arity
                                   ; checkWiredInTyCon tc
                                   ; return tc }
               UnboxedTuple  -> return (tupleTyCon Unboxed arity)
           ; checkExpectedKind (mkTyConApp tycon arg_tys) res_kind exp_kind }
      where
        arity = length tau_tys
        res_kind = case tup_sort of
                     UnboxedTuple    -> tYPE unboxedTupleRepDataConTy
                     BoxedTuple      -> liftedTypeKind
                     ConstraintTuple -> constraintKind

    bigConstraintTuple :: Arity -> MsgDoc
    bigConstraintTuple arity
      = hang (text "Constraint tuple arity too large:" <+> int arity
              <+> parens (text "max arity =" <+> int mAX_CTUPLE_SIZE))
           2 (text "Instead, use a nested tuple")

    ---------------------------
    -- | Apply a type of a given kind to a list of arguments. This instantiates
    -- invisible parameters as necessary. However, it does *not* necessarily
    -- apply all the arguments, if the kind runs out of binders.
    -- This takes an optional @VarEnv Kind@ which maps kind variables to kinds.
    -- These kinds should be used to instantiate invisible kind variables;
    -- they come from an enclosing class for an associated type/data family.
    -- This version will instantiate all invisible arguments left over after
    -- the visible ones.
    tcInferArgs :: Outputable fun
                => fun                      -- ^ the function
                -> [TyBinder]               -- ^ function kind's binders
                -> Maybe (VarEnv Kind)      -- ^ possibly, kind info (see above)
                -> [LHsType Name]           -- ^ args
                -> TcM (TCvSubst, [TyBinder], [TcType], [LHsType Name], Int)
                   -- ^ (instantiating subst, un-insted leftover binders,
                   --   typechecked args, untypechecked args, n)
    tcInferArgs fun binders mb_kind_info args
      = do { (subst, leftover_binders, args', leftovers, n)
               <- tc_infer_args typeLevelMode fun binders mb_kind_info args 1
            -- now, we need to instantiate any remaining invisible arguments
           ; let (invis_bndrs, other_binders) = span isInvisibleBinder leftover_binders
           ; (subst', invis_args)
               <- tcInstBindersX subst mb_kind_info invis_bndrs
           ; return ( subst'
                    , other_binders
                    , args' `chkAppend` invis_args
                    , leftovers, n ) }

    -- | See comments for 'tcInferArgs'. But this version does not instantiate
    -- any remaining invisible arguments.
    tc_infer_args :: Outputable fun
                  => TcTyMode
                  -> fun                      -- ^ the function
                  -> [TyBinder]               -- ^ function kind's binders (zonked)
                  -> Maybe (VarEnv Kind)      -- ^ possibly, kind info (see above)
                  -> [LHsType Name]           -- ^ args
                  -> Int                      -- ^ number to start arg counter at
                  -> TcM (TCvSubst, [TyBinder], [TcType], [LHsType Name], Int)
    tc_infer_args mode orig_ty binders mb_kind_info orig_args n0
      = do { traceTc "tcInferApps" (ppr binders $$ ppr orig_args)
           ; go emptyTCvSubst binders orig_args n0 [] }
      where
        go subst binders []   n acc
          = return ( subst, binders, reverse acc, [], n )
        -- when we call this when checking type family patterns, we really
        -- do want to instantiate all invisible arguments. During other
        -- typechecking, we don't.

        go subst binders all_args n acc
          | (inv_binders, other_binders) <- span isInvisibleBinder binders
          , not (null inv_binders)
          = do { (subst', args') <- tcInstBindersX subst mb_kind_info inv_binders
               ; go subst' other_binders all_args n (reverse args' ++ acc) }

        go subst (binder:binders) (arg:args) n acc
          = ASSERT( isVisibleBinder binder )
            do { arg' <- addErrCtxt (funAppCtxt orig_ty arg n) $
                         tc_lhs_type mode arg (substTyUnchecked subst $ binderType binder)
               ; let subst' = case binderVar_maybe binder of
                       Just tv -> extendTvSubst subst tv arg'
                       Nothing -> subst
               ; go subst' binders args (n+1) (arg' : acc) }

        go subst [] all_args n acc
          = return (subst, [], reverse acc, all_args, n)

    -- | Applies a type to a list of arguments. Always consumes all the
    -- arguments.
    tcInferApps :: Outputable fun
                 => TcTyMode
                 -> fun                  -- ^ Function (for printing only)
                 -> TcType               -- ^ Function (could be knot-tied)
                 -> TcKind               -- ^ Function kind (zonked)
                 -> [LHsType Name]       -- ^ Args
                 -> TcM (TcType, TcKind) -- ^ (f args, result kind)
    tcInferApps mode orig_ty ty ki args = go ty ki args 1
      where
        go fun fun_kind []   _ = return (fun, fun_kind)
        go fun fun_kind args n
          | let (binders, res_kind) = splitPiTys fun_kind
          , not (null binders)
          = do { (subst, leftover_binders, args', leftover_args, n')
                    <- tc_infer_args mode orig_ty binders Nothing args n
               ; let fun_kind' = substTyUnchecked subst $
                                 mkForAllTys leftover_binders res_kind
               ; go (mkNakedAppTys fun args') fun_kind' leftover_args n' }

        go fun fun_kind all_args@(arg:args) n
          = do { (co, arg_k, res_k) <- matchExpectedFunKind (length all_args)
                                                            fun fun_kind
               ; arg' <- addErrCtxt (funAppCtxt orig_ty arg n) $
                         tc_lhs_type mode arg arg_k
               ; go (mkNakedAppTy (fun `mkNakedCastTy` co) arg')
                    res_k args (n+1) }

    --------------------------
    checkExpectedKind :: TcType               -- the type whose kind we're checking
                      -> TcKind               -- the known kind of that type, k
                      -> TcKind               -- the expected kind, exp_kind
                      -> TcM TcType    -- a possibly-inst'ed, casted type :: exp_kind
    -- Instantiate a kind (if necessary) and then call unifyType
    --      (checkExpectedKind ty act_kind exp_kind)
    -- checks that the actual kind act_kind is compatible
    --      with the expected kind exp_kind
    checkExpectedKind ty act_kind exp_kind
     = do { (ty', act_kind') <- instantiate ty act_kind exp_kind
          ; let origin = TypeEqOrigin { uo_actual   = act_kind'
                                      , uo_expected = mkCheckExpType exp_kind
                                      , uo_thing    = Just $ mkTypeErrorThing ty'
                                      }
          ; co_k <- uType origin KindLevel act_kind' exp_kind
          ; traceTc "checkExpectedKind" (vcat [ ppr act_kind
                                              , ppr exp_kind
                                              , ppr co_k ])
          ; let result_ty = ty' `mkNakedCastTy` co_k
          ; return result_ty }
      where
        -- we need to make sure that both kinds have the same number of implicit
        -- foralls out front. If the actual kind has more, instantiate accordingly.
        -- Otherwise, just pass the type & kind through -- the errors are caught
        -- in unifyType.
        instantiate :: TcType    -- the type
                    -> TcKind    -- of this kind
                    -> TcKind   -- but expected to be of this one
                    -> TcM ( TcType   -- the inst'ed type
                           , TcKind ) -- its new kind
        instantiate ty act_ki exp_ki
          = let (exp_bndrs, _) = splitPiTysInvisible exp_ki in
            instantiateTyN (length exp_bndrs) ty act_ki

    -- | Instantiate a type to have at most @n@ invisible arguments.
    instantiateTyN :: Int    -- ^ @n@
                   -> TcType -- ^ the type
                   -> TcKind -- ^ its kind
                   -> TcM (TcType, TcKind)   -- ^ The inst'ed type with kind
    instantiateTyN n ty ki
      = let (bndrs, inner_ki)            = splitPiTysInvisible ki
            num_to_inst                  = length bndrs - n
               -- NB: splitAt is forgiving with invalid numbers
            (inst_bndrs, leftover_bndrs) = splitAt num_to_inst bndrs
        in
        if num_to_inst <= 0 then return (ty, ki) else
        do { (subst, inst_args) <- tcInstBinders inst_bndrs
           ; let rebuilt_ki = mkForAllTys leftover_bndrs inner_ki
                 ki'        = substTy subst rebuilt_ki
           ; return (mkNakedAppTys ty inst_args, ki') }

    ---------------------------
    tcHsContext :: LHsContext Name -> TcM [PredType]
    tcHsContext = tc_hs_context typeLevelMode

    tcLHsPredType :: LHsType Name -> TcM PredType
    tcLHsPredType = tc_lhs_pred typeLevelMode

    tc_hs_context :: TcTyMode -> LHsContext Name -> TcM [PredType]
    tc_hs_context mode ctxt = mapM (tc_lhs_pred mode) (unLoc ctxt)

    tc_lhs_pred :: TcTyMode -> LHsType Name -> TcM PredType
    tc_lhs_pred mode pred = tc_lhs_type mode pred constraintKind

    ---------------------------
    tcTyVar :: TcTyMode -> Name -> TcM (TcType, TcKind)
    -- See Note [Type checking recursive type and class declarations]
    -- in TcTyClsDecls
    tcTyVar mode name         -- Could be a tyvar, a tycon, or a datacon
      = do { traceTc "lk1" (ppr name)
           ; thing <- tcLookup name
           ; case thing of
               ATyVar _ tv -> return (mkTyVarTy tv, tyVarKind tv)

               ATcTyCon tc_tc -> do { check_tc tc_tc
                                    ; tc <- get_loopy_tc name tc_tc
                                    ; handle_tyfams tc tc_tc }
                                 -- mkNakedTyConApp: see Note [Type-checking inside the knot]
                     -- NB: we really should check if we're at the kind level
                     -- and if the tycon is promotable if -XNoTypeInType is set.
                     -- But this is a terribly large amount of work! Not worth it.

               AGlobal (ATyCon tc)
                 -> do { check_tc tc
                       ; handle_tyfams tc tc }

               AGlobal (AConLike (RealDataCon dc))
                 -> do { data_kinds <- xoptM LangExt.DataKinds
                       ; unless (data_kinds || specialPromotedDc dc) $
                           promotionErr name NoDataKindsDC
                       ; type_in_type <- xoptM LangExt.TypeInType
                       ; unless ( type_in_type ||
                                  ( isTypeLevel (mode_level mode) &&
                                    isLegacyPromotableDataCon dc ) ||
                                  ( isKindLevel (mode_level mode) &&
                                    specialPromotedDc dc ) ) $
                           promotionErr name NoTypeInTypeDC
                       ; let tc = promoteDataCon dc
                       ; return (mkNakedTyConApp tc [], tyConKind tc) }

               APromotionErr err -> promotionErr name err

               _  -> wrongThingErr "type" thing name }
      where
        check_tc :: TyCon -> TcM ()
        check_tc tc = do { type_in_type <- xoptM LangExt.TypeInType
                         ; data_kinds   <- xoptM LangExt.DataKinds
                         ; unless (isTypeLevel (mode_level mode) ||
                                   data_kinds ||
                                   isKindTyCon tc) $
                           promotionErr name NoDataKindsTC
                         ; unless (isTypeLevel (mode_level mode) ||
                                   type_in_type ||
                                   isLegacyPromotableTyCon tc) $
                           promotionErr name NoTypeInTypeTC }

        -- if we are type-checking a type family tycon, we must instantiate
        -- any invisible arguments right away. Otherwise, we get #11246
        handle_tyfams :: TyCon   -- the tycon to instantiate (might be loopy)
                      -> TyCon   -- a non-loopy version of the tycon
                      -> TcM (TcType, TcKind)
        handle_tyfams tc tc_tc
          | mightBeUnsaturatedTyCon tc_tc
          = do { traceTc "tcTyVar2a" (ppr tc_tc $$ ppr tc_kind)
               ; return (ty, tc_kind) }

          | otherwise
          = do { (tc_ty, kind) <- instantiateTyN 0 ty tc_kind
               -- tc and tc_ty must not be traced here, because that would
               -- force the evaluation of a potentially knot-tied variable (tc),
               -- and the typechecker would hang, as per #11708
               ; traceTc "tcTyVar2b" (vcat [ ppr tc_tc <+> dcolon <+> ppr tc_kind
                                           , ppr kind ])
               ; return (tc_ty, kind) }
          where
            ty      = mkNakedTyConApp tc []
            tc_kind = tyConKind tc_tc

        get_loopy_tc :: Name -> TyCon -> TcM TyCon
        -- Return the knot-tied global TyCon if there is one
        -- Otherwise the local TcTyCon; we must be doing kind checking
        -- but we still want to return a TyCon of some sort to use in
        -- error messages
        get_loopy_tc name tc_tc
          = do { env <- getGblEnv
               ; case lookupNameEnv (tcg_type_env env) name of
                    Just (ATyCon tc) -> return tc
                    _                -> do { traceTc "lk1 (loopy)" (ppr name)
                                           ; return tc_tc } }

    tcClass :: Name -> TcM (Class, TcKind)
    tcClass cls     -- Must be a class
      = do { thing <- tcLookup cls
           ; case thing of
               ATcTyCon tc -> return (aThingErr "tcClass" cls, tyConKind tc)
               AGlobal (ATyCon tc)
                 | Just cls <- tyConClass_maybe tc
                 -> return (cls, tyConKind tc)
               _ -> wrongThingErr "class" thing cls }

    aThingErr :: String -> Name -> b
    -- The type checker for types is sometimes called simply to
    -- do *kind* checking; and in that case it ignores the type
    -- returned. Which is a good thing since it may not be available yet!
    aThingErr str x = pprPanic "AThing evaluated unexpectedly" (text str <+> ppr x)

    {-
    Note [Type-checking inside the knot]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Suppose we are checking the argument types of a data constructor.  We
    must zonk the types before making the DataCon, because once built we
    can't change it.  So we must traverse the type.

    BUT the parent TyCon is knot-tied, so we can't look at it yet.

    So we must be careful not to use "smart constructors" for types that
    look at the TyCon or Class involved.

      * Hence the use of mkNakedXXX functions. These do *not* enforce
        the invariants (for example that we use (ForAllTy (Anon s) t) rather
        than (TyConApp (->) [s,t])).

      * The zonking functions establish invariants (even zonkTcType, a change from
        previous behaviour). So we must never inspect the result of a
        zonk that might mention a knot-tied TyCon. This is generally OK
        because we zonk *kinds* while kind-checking types. And the TyCons
        in kinds shouldn't be knot-tied, because they come from a previous
        mutually recursive group.

      * TcHsSyn.zonkTcTypeToType also can safely check/establish
        invariants.

    This is horribly delicate.  I hate it.  A good example of how
    delicate it is can be seen in Trac #7903.

    Note [Body kind of a HsForAllTy]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The body of a forall is usually a type, but in principle
    there's no reason to prohibit *unlifted* types.
    In fact, GHC can itself construct a function with an
    unboxed tuple inside a for-all (via CPR analyis; see
    typecheck/should_compile/tc170).

    Moreover in instance heads we get forall-types with
    kind Constraint.

    It's tempting to check that the body kind is either * or #. But this is
    wrong. For example:

      class C a b
      newtype N = Mk Foo deriving (C a)

    We're doing newtype-deriving for C. But notice how `a` isn't in scope in
    the predicate `C a`. So we quantify, yielding `forall a. C a` even though
    `C a` has kind `* -> Constraint`. The `forall a. C a` is a bit cheeky, but
    convenient. Bottom line: don't check for * or # here.

    Note [Body kind of a HsQualTy]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    If ctxt is non-empty, the HsQualTy really is a /function/, so the
    kind of the result really is '*', and in that case the kind of the
    body-type can be lifted or unlifted.

    However, consider
        instance Eq a => Eq [a] where ...
    or
        f :: (Eq a => Eq [a]) => blah
    Here both body-kind of the HsQualTy is Constraint rather than *.
    Rather crudely we tell the difference by looking at exp_kind. It's
    very convenient to typecheck instance types like any other HsSigType.

    Admittedly the '(Eq a => Eq [a]) => blah' case is erroneous, but it's
    better to reject in checkValidType.  If we say that the body kind
    should be '*' we risk getting TWO error messages, one saying that Eq
    [a] doens't have kind '*', and one saying that we need a Constraint to
    the left of the outer (=>).

    How do we figure out the right body kind?  Well, it's a bit of a
    kludge: I just look at the expected kind.  If it's Constraint, we
    must be in this instance situation context. It's a kludge because it
    wouldn't work if any unification was involved to compute that result
    kind -- but it isn't.  (The true way might be to use the 'mode'
    parameter, but that seemed like a sledgehammer to crack a nut.)

    Note [Inferring tuple kinds]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Give a tuple type (a,b,c), which the parser labels as HsBoxedOrConstraintTuple,
    we try to figure out whether it's a tuple of kind * or Constraint.
      Step 1: look at the expected kind
      Step 2: infer argument kinds

    If after Step 2 it's not clear from the arguments that it's
    Constraint, then it must be *.  Once having decided that we re-check
    the Check the arguments again to give good error messages
    in eg. `(Maybe, Maybe)`

    Note that we will still fail to infer the correct kind in this case:

      type T a = ((a,a), D a)
      type family D :: Constraint -> Constraint

    While kind checking T, we do not yet know the kind of D, so we will default the
    kind of T to * -> *. It works if we annotate `a` with kind `Constraint`.

    Note [Desugaring types]
    ~~~~~~~~~~~~~~~~~~~~~~~
    The type desugarer is phase 2 of dealing with HsTypes.  Specifically:

      * It transforms from HsType to Type

      * It zonks any kinds.  The returned type should have no mutable kind
        or type variables (hence returning Type not TcType):
          - any unconstrained kind variables are defaulted to (Any *) just
            as in TcHsSyn.
          - there are no mutable type variables because we are
            kind-checking a type
        Reason: the returned type may be put in a TyCon or DataCon where
        it will never subsequently be zonked.

    You might worry about nested scopes:
            ..a:kappa in scope..
                let f :: forall b. T '[a,b] -> Int
    In this case, f's type could have a mutable kind variable kappa in it;
    and we might then default it to (Any *) when dealing with f's type
    signature.  But we don't expect this to happen because we can't get a
    lexically scoped type variable with a mutable kind variable in it.  A
    delicate point, this.  If it becomes an issue we might need to
    distinguish top-level from nested uses.

    Moreover
      * it cannot fail,
      * it does no unifications
      * it does no validity checking, except for structural matters, such as
            (a) spurious ! annotations.
            (b) a class used as a type

    Note [Kind of a type splice]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider these terms, each with TH type splice inside:
         [| e1 :: Maybe $(..blah..) |]
         [| e2 :: $(..blah..) |]
    When kind-checking the type signature, we'll kind-check the splice
    $(..blah..); we want to give it a kind that can fit in any context,
    as if $(..blah..) :: forall k. k.

    In the e1 example, the context of the splice fixes kappa to *.  But
    in the e2 example, we'll desugar the type, zonking the kind unification
    variables as we go.  When we encounter the unconstrained kappa, we
    want to default it to '*', not to (Any *).

    Help functions for type applications
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    -}

    addTypeCtxt :: LHsType Name -> TcM a -> TcM a
            -- Wrap a context around only if we want to show that contexts.
            -- Omit invisble ones and ones user's won't grok
    addTypeCtxt (L _ ty) thing
      = addErrCtxt doc thing
      where
        doc = text "In the type" <+> quotes (ppr ty)

    {-
    ************************************************************************
    *                                                                      *
                    Type-variable binders
    %*                                                                      *
    %************************************************************************

    Note [Scope-check inferred kinds]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider

      data SameKind :: k -> k -> *
      foo :: forall a (b :: Proxy a) (c :: Proxy d). SameKind b c

    d has no binding site. So it gets bound implicitly, at the top. The
    problem is that d's kind mentions `a`. So it's all ill-scoped.

    The way we check for this is to gather all variables *bound* in a
    type variable's scope. The type variable's kind should not mention
    any of these variables. That is, d's kind can't mention a, b, or c.
    We can't just check to make sure that d's kind is in scope, because
    we might be about to kindGeneralize.

    A little messy, but it works.

    Note [Dependent LHsQTyVars]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We track (in the renamer) which explicitly bound variables in a
    LHsQTyVars are manifestly dependent; only precisely these variables
    may be used within the LHsQTyVars. We must do this so that kcHsTyVarBndrs
    can produce the right TcTyBinders, and tell Anon vs. Named. Earlier,
    I thought it would work simply to do a free-variable check during
    kcHsTyVarBndrs, but this is bogus, because there may be unsolved
    equalities about. And we don't want to eagerly solve the equalities,
    because we may get further information after kcHsTyVarBndrs is called.
    (Recall that kcHsTyVarBndrs is usually called from getInitialKind.
    The only other case is in kcConDecl.) This is what implements the rule
    that all variables intended to be dependent must be manifestly so.

    Sidenote: It's quite possible that later, we'll consider (t -> s)
    as a degenerate case of some (pi (x :: t) -> s) and then this will
    all get more permissive.

    -}

    tcWildCardBinders :: [Name]
                      -> ([(Name, TcTyVar)] -> TcM a)
                      -> TcM a
    -- Use the Unique form the specified Name; don't clone it.  There is
    -- no need to clone, and not doing so avoids the need to return a list
    -- of pairs to bring into scope.
    tcWildCardBinders wcs thing_inside
      = do { wcs <- mapM new_wildcard wcs
           ; tcExtendTyVarEnv2 wcs $
             thing_inside wcs }
      where
       new_wildcard :: Name -> TcM (Name, TcTyVar)
       new_wildcard name = do { kind <- newMetaKindVar
                              ; tv   <- newFlexiTyVar kind
                              ; return (name, tv) }

    -- | Kind-check a 'LHsQTyVars'. If the decl under consideration has a complete,
    -- user-supplied kind signature (CUSK), generalise the result.
    -- Used in 'getInitialKind' (for tycon kinds and other kinds)
    -- and in kind-checking (but not for tycon kinds, which are checked with
    -- tcTyClDecls). See also Note [Complete user-supplied kind signatures] in
    -- HsDecls.
    --
    -- This function does not do telescope checking.
    kcHsTyVarBndrs :: Bool    -- ^ True <=> the decl being checked has a CUSK
                   -> Bool    -- ^ True <=> the decl is an open type/data family
                   -> Bool    -- ^ True <=> all the hsq_implicit are *kind* vars
                              -- (will give these kind * if -XNoTypeInType)
                   -> LHsQTyVars Name
                   -> TcM (Kind, r)  -- ^ the result kind, possibly with other info
                   -> TcM ([TcTyBinder], TcKind, r)
                         -- ^ The bound variables in the kind, the result kind,
                         -- with the other info.
                         -- Always returns Named binders; sort out Named vs. Anon
                         -- yourself.
    kcHsTyVarBndrs cusk open_fam all_kind_vars
      (HsQTvs { hsq_implicit = kv_ns, hsq_explicit = hs_tvs
              , hsq_dependent = dep_names }) thing_inside
      | cusk
      = do { kv_kinds <- mk_kv_kinds
           ; let scoped_kvs = zipWith new_skolem_tv kv_ns kv_kinds
           ; tcExtendTyVarEnv2 (kv_ns `zip` scoped_kvs) $
        do { (tvs, binders, res_kind, stuff) <- solveEqualities $
                                                bind_telescope hs_tvs thing_inside

               -- Now, because we're in a CUSK, quantify over the mentioned
               -- kind vars, in dependency order.
           ; binders  <- mapM zonkTcTyBinder binders
           ; res_kind <- zonkTcType res_kind
           ; let qkvs = tyCoVarsOfTypeWellScoped (mkForAllTys binders res_kind)
                       -- the visibility of tvs doesn't matter here; we just
                       -- want the free variables not to include the tvs

              -- if there are any meta-tvs left, the user has lied about having
              -- a CUSK. Error.
           ; let (meta_tvs, good_tvs) = partition isMetaTyVar qkvs
           ; when (not (null meta_tvs)) $
             report_non_cusk_tvs (qkvs ++ tvs)

           ; return ( mkNamedBinders Specified good_tvs ++ binders
                    , res_kind, stuff ) }}

      | otherwise
      = do { kv_kinds <- mk_kv_kinds
           ; scoped_kvs <- zipWithM newSigTyVar kv_ns kv_kinds
                         -- the names must line up in splitTelescopeTvs
           ; (_, binders, res_kind, stuff)
               <- tcExtendTyVarEnv2 (kv_ns `zip` scoped_kvs) $
                  bind_telescope hs_tvs thing_inside
           ; return (binders, res_kind, stuff) }
      where
          -- if -XNoTypeInType and we know all the implicits are kind vars,
          -- just give the kind *. This prevents test
          -- dependent/should_fail/KindLevelsB from compiling, as it should
        mk_kv_kinds :: TcM [Kind]
        mk_kv_kinds = do { typeintype <- xoptM LangExt.TypeInType
                         ; if not typeintype && all_kind_vars
                           then return (map (const liftedTypeKind) kv_ns)
                           else mapM (const newMetaKindVar) kv_ns }

          -- there may be dependency between the explicit "ty" vars. So, we have
          -- to handle them one at a time.
        bind_telescope :: [LHsTyVarBndr Name]
                       -> TcM (Kind, r)
                       -> TcM ([TcTyVar], [TyBinder], TcKind, r)
        bind_telescope [] thing
          = do { (res_kind, stuff) <- thing
               ; return ([], [], res_kind, stuff) }
        bind_telescope (L _ hs_tv : hs_tvs) thing
          = do { tv_pair@(tv, _) <- kc_hs_tv hs_tv
                   -- NB: Bring all tvs into scope, even non-dependent ones,
                   -- as they're needed in type synonyms, data constructors, etc.
               ; (tvs, binders, res_kind, stuff) <- bind_unless_scoped tv_pair $
                                                    bind_telescope hs_tvs $
                                                    thing
                      -- See Note [Dependent LHsQTyVars]
               ; let new_binder | hsTyVarName hs_tv `elemNameSet` dep_names
                                = mkNamedBinder Visible tv
                                | otherwise
                                = mkAnonBinder (tyVarKind tv)
               ; return ( tv : tvs
                        , new_binder : binders
                        , res_kind, stuff ) }

        -- | Bind the tyvar in the env't unless the bool is True
        bind_unless_scoped :: (TcTyVar, Bool) -> TcM a -> TcM a
        bind_unless_scoped (_, True)   thing_inside = thing_inside
        bind_unless_scoped (tv, False) thing_inside
          = tcExtendTyVarEnv [tv] thing_inside

        kc_hs_tv :: HsTyVarBndr Name -> TcM (TcTyVar, Bool)
        kc_hs_tv (UserTyVar (L _ name))
          = do { tv_pair@(tv, scoped) <- tcHsTyVarName Nothing name

                  -- Open type/data families default their variables to kind *.
               ; when (open_fam && not scoped) $ -- (don't default class tyvars)
                 discardResult $ unifyKind (Just (mkTyVarTy tv)) liftedTypeKind
                                                                 (tyVarKind tv)

               ; return tv_pair }

        kc_hs_tv (KindedTyVar (L _ name) lhs_kind)
          = do { kind <- tcLHsKind lhs_kind
               ; tcHsTyVarName (Just kind) name }

        report_non_cusk_tvs all_tvs
          = do { all_tvs <- mapM zonkTyCoVarKind all_tvs
               ; let (_, tidy_tvs)         = tidyOpenTyCoVars emptyTidyEnv all_tvs
                     (meta_tvs, other_tvs) = partition isMetaTyVar tidy_tvs

               ; addErr $
                 vcat [ text "You have written a *complete user-suppled kind signature*,"
                      , hang (text "but the following variable" <> plural meta_tvs <+>
                              isOrAre meta_tvs <+> text "undetermined:")
                           2 (vcat (map pp_tv meta_tvs))
                      , text "Perhaps add a kind signature."
                      , hang (text "Inferred kinds of user-written variables:")
                           2 (vcat (map pp_tv other_tvs)) ] }
          where
            pp_tv tv = ppr tv <+> dcolon <+> ppr (tyVarKind tv)

    tcImplicitTKBndrs :: [Name]
                      -> TcM (a, TyVarSet)   -- vars are bound somewhere in the scope
                                             -- see Note [Scope-check inferred kinds]
                      -> TcM ([TcTyVar], a)
    tcImplicitTKBndrs = tcImplicitTKBndrsX (tcHsTyVarName Nothing)

    -- | Convenient specialization
    tcImplicitTKBndrsType :: [Name]
                          -> TcM Type
                          -> TcM ([TcTyVar], Type)
    tcImplicitTKBndrsType var_ns thing_inside
      = tcImplicitTKBndrs var_ns $
        do { res_ty <- thing_inside
           ; return (res_ty, allBoundVariables res_ty) }

    -- this more general variant is needed in tcHsPatSigType.
    -- See Note [Pattern signature binders]
    tcImplicitTKBndrsX :: (Name -> TcM (TcTyVar, Bool))  -- new_tv function
                       -> [Name]
                       -> TcM (a, TyVarSet)
                       -> TcM ([TcTyVar], a)
    -- Returned TcTyVars have the supplied Names
    --   i.e. no cloning of fresh names
    tcImplicitTKBndrsX new_tv var_ns thing_inside
      = do { tkvs_pairs <- mapM new_tv var_ns
           ; let must_scope_tkvs = [ tkv | (tkv, False) <- tkvs_pairs ]
                 tkvs            = map fst tkvs_pairs
           ; (result, bound_tvs) <- tcExtendTyVarEnv must_scope_tkvs $
                                    thing_inside

             -- it's possible that we guessed the ordering of variables
             -- wrongly. Adjust.
           ; tkvs <- mapM zonkTyCoVarKind tkvs
           ; let extra = text "NB: Implicitly-bound variables always come" <+>
                         text "before other ones."
           ; checkValidInferredKinds tkvs bound_tvs extra

           ; let final_tvs = toposortTyVars tkvs
           ; traceTc "tcImplicitTKBndrs" (ppr var_ns $$ ppr final_tvs)

           ; return (final_tvs, result) }

    tcExplicitTKBndrs :: [LHsTyVarBndr Name]
                      -> ([TyVar] -> TcM (a, TyVarSet))
                            -- ^ Thing inside returns the set of variables bound
                            -- in the scope. See Note [Scope-check inferred kinds]
                      -> TcM (a, TyVarSet)  -- ^ returns augmented bound vars
    -- No cloning: returned TyVars have the same Name as the incoming LHsTyVarBndrs
    tcExplicitTKBndrs orig_hs_tvs thing_inside
      = go orig_hs_tvs $ \ tvs ->
        do { (result, bound_tvs) <- thing_inside tvs

             -- Issue an error if the ordering is bogus.
             -- See Note [Bad telescopes] in TcValidity.
           ; tvs <- checkZonkValidTelescope (interppSP orig_hs_tvs) tvs empty
           ; checkValidInferredKinds tvs bound_tvs empty

           ; traceTc "tcExplicitTKBndrs" $
               vcat [ text "Hs vars:" <+> ppr orig_hs_tvs
                    , text "tvs:" <+> sep (map pprTvBndr tvs) ]

           ; return (result, bound_tvs `unionVarSet` mkVarSet tvs)
           }
      where
        go [] thing = thing []
        go (L _ hs_tv : hs_tvs) thing
          = do { tv <- tcHsTyVarBndr hs_tv
               ; tcExtendTyVarEnv [tv] $
                 go hs_tvs $ \ tvs ->
                 thing (tv : tvs) }

    tcHsTyVarBndr :: HsTyVarBndr Name -> TcM TcTyVar
    -- Return a SkolemTv TcTyVar, initialised with a kind variable.
    -- Typically the Kind inside the HsTyVarBndr will be a tyvar
    -- with a mutable kind in it.
    -- NB: These variables must not be in scope. This function
    -- is not appropriate for use with associated types, for example.
    --
    -- Returned TcTyVar has the same name; no cloning
    --
    -- See also Note [Associated type tyvar names] in Class
    tcHsTyVarBndr (UserTyVar (L _ name))
      = do { kind <- newMetaKindVar
           ; return (mkTcTyVar name kind (SkolemTv False)) }
    tcHsTyVarBndr (KindedTyVar (L _ name) kind)
      = do { kind <- tcLHsKind kind
           ; return (mkTcTyVar name kind (SkolemTv False)) }

    -- | Produce a tyvar of the given name (with the kind provided, or
    -- otherwise a meta-var kind). If
    -- the name is already in scope, return the scoped variable, checking
    -- to make sure the known kind matches any kind provided. The
    -- second return value says whether the variable is in scope (True)
    -- or not (False). (Use this for associated types, for example.)
    tcHsTyVarName :: Maybe Kind -> Name -> TcM (TcTyVar, Bool)
    tcHsTyVarName m_kind name
      = do { mb_tv <- tcLookupLcl_maybe name
           ; case mb_tv of
               Just (ATyVar _ tv)
                 -> do { whenIsJust m_kind $ \ kind ->
                         discardResult $
                         unifyKind (Just (mkTyVarTy tv)) kind (tyVarKind tv)
                       ; return (tv, True) }
               _ -> do { kind <- maybe newMetaKindVar return m_kind
                       ; return (mkTcTyVar name kind vanillaSkolemTv, False) }}

    -- makes a new skolem tv
    new_skolem_tv :: Name -> Kind -> TcTyVar
    new_skolem_tv n k = mkTcTyVar n k vanillaSkolemTv

    ------------------
    kindGeneralizeType :: Type -> TcM Type
    -- Result is zonked
    kindGeneralizeType ty
      = do { kvs <- kindGeneralize ty
           ; zonkTcType (mkInvForAllTys kvs ty) }

    kindGeneralize :: TcType -> TcM [KindVar]
    kindGeneralize ty
      = do { gbl_tvs <- tcGetGlobalTyCoVars -- Already zonked
           ; kvs <- zonkTcTypeAndFV ty
           ; quantifyZonkedTyVars gbl_tvs (Pair kvs emptyVarSet) }

    {-
    Note [Kind generalisation]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~
    We do kind generalisation only at the outer level of a type signature.
    For example, consider
      T :: forall k. k -> *
      f :: (forall a. T a -> Int) -> Int
    When kind-checking f's type signature we generalise the kind at
    the outermost level, thus:
      f1 :: forall k. (forall (a:k). T k a -> Int) -> Int  -- YES!
    and *not* at the inner forall:
      f2 :: (forall k. forall (a:k). T k a -> Int) -> Int  -- NO!
    Reason: same as for HM inference on value level declarations,
    we want to infer the most general type.  The f2 type signature
    would be *less applicable* than f1, because it requires a more
    polymorphic argument.

    NB: There are no explicit kind variables written in f's signature.
    When there are, the renamer adds these kind variables to the list of
    variables bound by the forall, so you can indeed have a type that's
    higher-rank in its kind. But only by explicit request.

    Note [Kinds of quantified type variables]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    tcTyVarBndrsGen quantifies over a specified list of type variables,
    *and* over the kind variables mentioned in the kinds of those tyvars.

    Note that we must zonk those kinds (obviously) but less obviously, we
    must return type variables whose kinds are zonked too. Example
        (a :: k7)  where  k7 := k9 -> k9
    We must return
        [k9, a:k9->k9]
    and NOT
        [k9, a:k7]
    Reason: we're going to turn this into a for-all type,
       forall k9. forall (a:k7). blah
    which the type checker will then instantiate, and instantiate does not
    look through unification variables!

    Hence using zonked_kinds when forming tvs'.

    Note [Typechecking telescopes]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The function tcTyClTyVars has to bind the scoped type and kind
    variables in a telescope. For example:

    class Foo k (t :: Proxy k -> k2) where ...

    By the time [kt]cTyClTyVars is called, we know *something* about the kind of Foo,
    at least that it has the form

      Foo :: forall (k2 :: mk2). forall (k :: mk1) -> (Proxy mk1 k -> k2) -> Constraint

    if it has a CUSK (Foo does not, in point of fact) or

      Foo :: forall (k :: mk1) -> (Proxy mk1 k -> k2) -> Constraint

    if it does not, where mk1 and mk2 are meta-kind variables (mk1, mk2 :: *).

    When calling tcTyClTyVars, this kind is further generalized w.r.t. any
    free variables appearing in mk1 or mk2. So, mk_tvs must handle
    that possibility. Perhaps we discover that mk1 := Maybe k3 and mk2 := *,
    so we have

    Foo :: forall (k3 :: *). forall (k2 :: *). forall (k :: Maybe k3) ->
           (Proxy (Maybe k3) k -> k2) -> Constraint

    We now have several sorts of variables to think about:
    1) The variable k3 is not mentioned in the source at all. It is neither
       explicitly bound nor ever used. It is *not* a scoped kind variable,
       and should not be bound when type-checking the scope of the telescope.

    2) The variable k2 is mentioned in the source, but it is not explicitly
       bound. It *is* a scoped kind variable, and will appear in the
       hsq_implicit field of a LHsTyVarBndrs.

       2a) In the non-CUSK case, these variables won't have been generalized
           yet and don't appear in the looked-up kind. So we just return these
           in a NameSet.

    3) The variable k is mentioned in the source with an explicit binding.
       It *is* a scoped type variable, and will appear in the
       hsq_explicit field of a LHsTyVarBndrs.

    4) The variable t is bound in the source, but it is never mentioned later
       in the kind. It *is* a scoped variable (it can appear in the telescope
       scope, even though it is non-dependent), and will appear in the
       hsq_explicit field of a LHsTyVarBndrs.

    splitTelescopeTvs walks through the output of a splitPiTys on the
    telescope head's kind (Foo, in our example), creating a list of tyvars
    to be bound within the telescope scope. It must simultaneously walk
    through the hsq_implicit and hsq_explicit fields of a LHsTyVarBndrs.
    Comments in the code refer back to the cases in this Note.

    Cases (1) and (2) can be mixed together, but these cases must appear before
    cases (3) and (4) (the implicitly bound vars always precede the explicitly
    bound ones). So, we handle the lists in two stages (mk_tvs and
    mk_tvs2).

    As a further wrinkle, it's possible that the variables in case (2) have
    been reordered. This is because hsq_implicit is ordered by the renamer,
    but there may be dependency among the variables. Of course, the order in
    the kind must take dependency into account. So we use a NameSet to keep
    these straightened out.

    Note [Free-floating kind vars]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider

      data T = MkT (forall (a :: k). Proxy a)
      -- from test ghci/scripts/T7873

    This is not an existential datatype, but a higher-rank one. Note that
    the forall to the right of MkT. Also consider

      data S a = MkS (Proxy (a :: k))

    According to the rules around implicitly-bound kind variables, those
    k's scope over the whole declarations. The renamer grabs it and adds it
    to the hsq_implicits field of the HsQTyVars of the tycon. So it must
    be in scope during type-checking, but we want to reject T while accepting
    S.

    Why reject T? Because the kind variable isn't fixed by anything. For
    a variable like k to be implicit, it needs to be mentioned in the kind
    of a tycon tyvar. But it isn't.

    Why accept S? Because kind inference tells us that a has kind k, so it's
    all OK.

    Here's the approach: in the first pass ("kind-checking") we just bring
    k into scope. In the second pass, we certainly hope that k has been
    integrated into the type's (generalized) kind, and so it should be found
    by splitTelescopeTvs. If it's not, then we must have a definition like
    T, and we reject. (But see Note [Tiresome kind checking] about some extra
    processing necessary in the second pass.)

    Note [Tiresome kind matching]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Because of the use of SigTvs in kind inference (see #11203, for example)
    sometimes kind variables come into tcTyClTyVars (the second, desugaring
    pass in TcTyClDecls) with the wrong names. The best way to fix this up
    is just to unify the kinds, again. So we return HsKind/Kind pairs from
    splitTelescopeTvs that can get unified in tcTyClTyVars, but only if there
    are kind vars the didn't link up in splitTelescopeTvs.

    -}

    --------------------
    -- getInitialKind has made a suitably-shaped kind for the type or class
    -- Unpack it, and attribute those kinds to the type variables
    -- Extend the env with bindings for the tyvars, taken from
    -- the kind of the tycon/class.  Give it to the thing inside, and
    -- check the result kind matches
    kcLookupKind :: Name -> TcM ([TyBinder], Kind)
    kcLookupKind nm
      = do { tc_ty_thing <- tcLookup nm
           ; case tc_ty_thing of
               ATcTyCon tc         -> return (tyConBinders tc, tyConResKind tc)
               AGlobal (ATyCon tc) -> return (tyConBinders tc, tyConResKind tc)
               _                   -> pprPanic "kcLookupKind" (ppr tc_ty_thing) }

    -- See Note [Typechecking telescopes]
    splitTelescopeTvs :: [TyBinder]   -- telescope binders
                      -> LHsQTyVars Name
                      -> ( [TyVar]    -- scoped type variables
                         , NameSet    -- ungeneralized implicit variables (case 2a)
                         , [TyVar]    -- implicit type variables (cases 1 & 2)
                         , [TyVar]    -- explicit type variables (cases 3 & 4)
                         , [(LHsKind Name, Kind)] ) -- see Note [Tiresome kind matching]
    splitTelescopeTvs bndrs tvbs@(HsQTvs { hsq_implicit = hs_kvs
                                         , hsq_explicit = hs_tvs })
      = let (scoped_tvs, non_cusk_imp_names, imp_tvs, exp_tvs, kind_matches)
              = mk_tvs [] [] bndrs (mkNameSet hs_kvs) hs_tvs
        in
        (scoped_tvs, non_cusk_imp_names, imp_tvs, exp_tvs, kind_matches)
      where
        mk_tvs :: [TyVar]    -- scoped tv accum (reversed)
               -> [TyVar]    -- implicit tv accum (reversed)
               -> [TyBinder]
               -> NameSet             -- implicit variables
               -> [LHsTyVarBndr Name] -- explicit variables
               -> ( [TyVar]           -- the tyvars to be lexically bound
                  , NameSet           -- Case 2a names
                  , [TyVar]           -- implicit tyvars
                  , [TyVar]           -- explicit tyvars
                  , [(LHsKind Name, Kind)] ) -- tiresome kind matches
        mk_tvs scoped_tv_acc imp_tv_acc (bndr : bndrs) all_hs_kvs all_hs_tvs
          | Just tv <- binderVar_maybe bndr
          , isInvisibleBinder bndr
          , let tv_name = getName tv
          , tv_name `elemNameSet` all_hs_kvs
          = mk_tvs (tv : scoped_tv_acc) (tv : imp_tv_acc)
                   bndrs (all_hs_kvs `delFromNameSet` tv_name) all_hs_tvs      -- Case (2)

          | Just tv <- binderVar_maybe bndr
          , isInvisibleBinder bndr
          = mk_tvs scoped_tv_acc (tv : imp_tv_acc)
                   bndrs all_hs_kvs all_hs_tvs  -- Case (1)

         -- there may actually still be some hs_kvs, if we're kind checking
         -- a non-CUSK. The kinds *aren't* generalized, so we won't see them
         -- here.
        mk_tvs scoped_tv_acc imp_tv_acc all_bndrs all_hs_kvs all_hs_tvs
          = let (scoped, exp_tvs, kind_matches)
                  = mk_tvs2 scoped_tv_acc [] [] all_bndrs all_hs_tvs in
            (scoped, all_hs_kvs, reverse imp_tv_acc, exp_tvs, kind_matches)
               -- no more Case (1) or (2)

        -- This can't handle Case (1) or Case (2) from [Typechecking telescopes]
        mk_tvs2 :: [TyVar]
                -> [TyVar]   -- new parameter: explicit tv accum (reversed)
                -> [(LHsKind Name, Kind)]  -- tiresome kind matches accum
                -> [TyBinder]
                -> [LHsTyVarBndr Name]
                -> ( [TyVar]
                   , [TyVar]   -- explicit tvs only
                   , [(LHsKind Name, Kind)] )  -- tiresome kind matches
        mk_tvs2 scoped_tv_acc exp_tv_acc kind_match_acc (bndr : bndrs) (hs_tv : hs_tvs)
          | Just tv <- binderVar_maybe bndr
          = ASSERT2( isVisibleBinder bndr, err_doc )
            ASSERT( getName tv == hsLTyVarName hs_tv )
            mk_tvs2 (tv : scoped_tv_acc) (tv : exp_tv_acc) kind_match_acc' bndrs hs_tvs
                -- Case (3)

          | otherwise
          = ASSERT( isVisibleBinder bndr )
            let tv = mkTyVar (hsLTyVarName hs_tv) (binderType bndr) in
            mk_tvs2 (tv : scoped_tv_acc) (tv : exp_tv_acc) kind_match_acc' bndrs hs_tvs
                   -- Case (4)
          where
            err_doc = vcat [ ppr (bndr : bndrs)
                           , ppr (hs_tv : hs_tvs)
                           , ppr tvbs ]

            kind_match_acc' = case hs_tv of
              L _ (UserTyVar {})          -> kind_match_acc
              L _ (KindedTyVar _ hs_kind) -> (hs_kind, kind) : kind_match_acc
                where kind = binderType bndr

        mk_tvs2 scoped_tv_acc exp_tv_acc kind_match_acc [] [] -- All done!
          = ( reverse scoped_tv_acc
            , reverse exp_tv_acc
            , kind_match_acc )   -- no need to reverse; it's not ordered

        mk_tvs2 _ _ _ all_bndrs all_hs_tvs
          = pprPanic "splitTelescopeTvs 2" (vcat [ ppr all_bndrs
                                                 , ppr all_hs_tvs ])

    -----------------------
    -- | "Kind check" the tyvars to a tycon. This is used during the "kind-checking"
    -- pass in TcTyClsDecls. (Never in getInitialKind, never in the
    -- "type-checking"/desugaring pass.) It works only for LHsQTyVars associated
    -- with a tycon, whose kind is known (partially) via getInitialKinds.
    -- Never emits constraints, though the thing_inside might.
    kcTyClTyVars :: Name   -- ^ of the tycon
                 -> LHsQTyVars Name
                 -> TcM a -> TcM a
    kcTyClTyVars tycon hs_tvs thing_inside
      = do { (binders, res_kind) <- kcLookupKind tycon
           ; let (scoped_tvs, non_cusk_kv_name_set, all_kvs, all_tvs, _)
                   = splitTelescopeTvs binders hs_tvs
           ; traceTc "kcTyClTyVars splitTelescopeTvs:"
               (vcat [ text "Tycon:" <+> ppr tycon
                     , text "Binders:" <+> ppr binders
                     , text "res_kind:" <+> ppr res_kind
                     , text "hs_tvs:" <+> ppr hs_tvs
                     , text "scoped tvs:" <+> pprWithCommas pprTvBndr scoped_tvs
                     , text "implicit tvs:" <+> pprWithCommas pprTvBndr all_kvs
                     , text "explicit tvs:" <+> pprWithCommas pprTvBndr all_tvs
                     , text "non-CUSK kvs:" <+> ppr non_cusk_kv_name_set ] )

                -- need to look up the non-cusk kvs in order to get their
                -- kinds right, in case the kinds were informed by
                -- the getInitialKinds pass
           ; let non_cusk_kv_names = nameSetElems non_cusk_kv_name_set
                 free_kvs          = tyCoVarsOfTypes $
                                     map tyVarKind (all_kvs ++ all_tvs)
                 mk_kv kv_name     = case lookupVarSetByName free_kvs kv_name of
                                       Just kv -> return kv
                                       Nothing ->
                                         -- this kv isn't mentioned in the
                                         -- LHsQTyVars at all. But maybe
                                         -- it's mentioned in the body
                                         -- In any case, just gin up a
                                         -- meta-kind for it
                                         do { kv_kind <- newMetaKindVar
                                            ; return (new_skolem_tv kv_name kv_kind) }
           ; non_cusk_kvs <- mapM mk_kv non_cusk_kv_names

                 -- The non_cusk_kvs are still scoped; they are mentioned by
                 -- name by the user
           ; tcExtendTyVarEnv (non_cusk_kvs ++ scoped_tvs) $
             thing_inside }

    tcTyClTyVars :: Name -> LHsQTyVars Name      -- LHS of the type or class decl
                 -> ([TyVar] -> [TyVar] -> [TyBinder] -> Kind -> TcM a) -> TcM a
    -- ^ Used for the type variables of a type or class decl
    -- on the second full pass (type-checking/desugaring) in TcTyClDecls.
    -- This is *not* used in the initial-kind run, nor in the "kind-checking" pass.
    -- Accordingly, everything passed to the continuation is fully zonked.
    --
    -- (tcTyClTyVars T [a,b] thing_inside)
    --   where T : forall k1 k2 (a:k1 -> *) (b:k1). k2 -> *
    --   calls thing_inside with arguments
    --      [k1,k2] [a,b] [k1:*, k2:*, a:k1 -> *, b:k1] (k2 -> *)
    --   having also extended the type environment with bindings
    --   for k1,k2,a,b
    --
    -- Never emits constraints.
    --
    -- The LHsTyVarBndrs is always user-written, and the full, generalised
    -- kind of the tycon is available in the local env.
    tcTyClTyVars tycon hs_tvs thing_inside
      = do { (binders, res_kind) <- kcLookupKind tycon
           ; let ( scoped_tvs, float_kv_name_set, all_kvs
                   , all_tvs, kind_matches )
                     = splitTelescopeTvs binders hs_tvs
           ; traceTc "tcTyClTyVars splitTelescopeTvs:"
               (vcat [ text "Tycon:" <+> ppr tycon
                     , text "Binders:" <+> ppr binders
                     , text "res_kind:" <+> ppr res_kind
                     , text "hs_tvs.hsq_implicit:" <+> ppr (hsq_implicit hs_tvs)
                     , text "hs_tvs.hsq_explicit:" <+> ppr (hsq_explicit hs_tvs)
                     , text "scoped tvs:" <+> pprWithCommas pprTvBndr scoped_tvs
                     , text "implicit tvs:" <+> pprWithCommas pprTvBndr all_kvs
                     , text "explicit tvs:" <+> pprWithCommas pprTvBndr all_tvs
                     , text "floating kvs:" <+> ppr float_kv_name_set
                     , text "Tiresome kind matches:" <+> ppr kind_matches ] )

           ; float_kvs <- deal_with_float_kvs float_kv_name_set kind_matches
                                              scoped_tvs all_tvs

           ; tcExtendTyVarEnv (float_kvs ++ scoped_tvs) $
               -- the float_kvs are already in the all_kvs
             thing_inside all_kvs all_tvs binders res_kind }
      where
             -- See Note [Free-floating kind vars]
        deal_with_float_kvs float_kv_name_set kind_matches scoped_tvs all_tvs
          | isEmptyNameSet float_kv_name_set
          = return []

          | otherwise
          = do { -- the floating kvs might just be renamed
                 -- see Note [Tiresome kind matching]
               ; let float_kv_names = nameSetElems float_kv_name_set
               ; float_kv_kinds <- mapM (const newMetaKindVar) float_kv_names
               ; float_kvs <- zipWithM newSigTyVar float_kv_names float_kv_kinds
               ; discardResult $
                 tcExtendTyVarEnv (float_kvs ++ scoped_tvs) $
                 solveEqualities $
                 forM kind_matches $ \ (hs_kind, kind) ->
                 do { tc_kind <- tcLHsKind hs_kind
                    ; unifyKind noThing tc_kind kind }

               ; zonked_kvs <- mapM ((return . tcGetTyVar "tcTyClTyVars") <=< zonkTcTyVar)
                                    float_kvs
               ; let (still_sigs, matched_up) = partition isSigTyVar zonked_kvs
                   -- the still_sigs didn't match with anything. They must be
                   -- "free-floating", as in Note [Free-floating kind vars]
               ; checkNoErrs $ mapM_ (report_floating_kv all_tvs) still_sigs

                   -- the matched up kvs are proper scoped kvs.
               ; return matched_up }

        report_floating_kv all_tvs kv
          = addErr $
            vcat [ text "Kind variable" <+> quotes (ppr kv) <+>
                   text "is implicitly bound in datatype"
                 , quotes (ppr tycon) <> comma <+>
                   text "but does not appear as the kind of any"
                 , text "of its type variables. Perhaps you meant"
                 , text "to bind it (with TypeInType) explicitly somewhere?"
                 , if null all_tvs then empty else
                   hang (text "Type variables with inferred kinds:")
                      2 (pprTvBndrs all_tvs) ]

    -----------------------------------
    tcDataKindSig :: Kind -> TcM ([TyVar], [TyBinder], Kind)
    -- GADT decls can have a (perhaps partial) kind signature
    --      e.g.  data T :: * -> * -> * where ...
    -- This function makes up suitable (kinded) type variables for
    -- the argument kinds, and checks that the result kind is indeed *.
    -- We use it also to make up argument type variables for for data instances.
    -- Never emits constraints.
    -- Returns the new TyVars, the extracted TyBinders, and the new, reduced
    -- result kind (which should always be Type or a synonym thereof)
    tcDataKindSig kind
      = do  { checkTc (isLiftedTypeKind res_kind) (badKindSig kind)
            ; span <- getSrcSpanM
            ; us   <- newUniqueSupply
            ; rdr_env <- getLocalRdrEnv
            ; let uniqs = uniqsFromSupply us
                  occs  = [ occ | str <- allNameStrings
                                , let occ = mkOccName tvName str
                                , isNothing (lookupLocalRdrOcc rdr_env occ) ]
                     -- Note [Avoid name clashes for associated data types]

                -- NB: Use the tv from a binder if there is one. Otherwise,
                -- we end up inventing a new Unique for it, and any other tv
                -- that mentions the first ends up with the wrong kind.
            ; return ( [ tv
                       | ((bndr, occ), uniq) <- bndrs `zip` occs `zip` uniqs
                       , let tv | Just bndr_tv <- binderVar_maybe bndr
                                = bndr_tv
                                | otherwise
                                = mk_tv span uniq occ (binderType bndr) ]
                     , bndrs, res_kind ) }
      where
        (bndrs, res_kind) = splitPiTys kind
        mk_tv loc uniq occ kind
          = mkTyVar (mkInternalName uniq occ loc) kind

    badKindSig :: Kind -> SDoc
    badKindSig kind
     = hang (text "Kind signature on data type declaration has non-* return kind")
            2 (ppr kind)

    {-
    Note [Avoid name clashes for associated data types]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider    class C a b where
                   data D b :: * -> *
    When typechecking the decl for D, we'll invent an extra type variable
    for D, to fill out its kind.  Ideally we don't want this type variable
    to be 'a', because when pretty printing we'll get
                class C a b where
                   data D b a0
    (NB: the tidying happens in the conversion to IfaceSyn, which happens
    as part of pretty-printing a TyThing.)

    That's why we look in the LocalRdrEnv to see what's in scope. This is
    important only to get nice-looking output when doing ":info C" in GHCi.
    It isn't essential for correctness.

    ************************************************************************
    *                                                                      *
                    Scoped type variables
    *                                                                      *
    ************************************************************************

    tcAddScopedTyVars is used for scoped type variables added by pattern
    type signatures
            e.g.  \ ((x::a), (y::a)) -> x+y
    They never have explicit kinds (because this is source-code only)
    They are mutable (because they can get bound to a more specific type).

    Usually we kind-infer and expand type splices, and then
    tupecheck/desugar the type.  That doesn't work well for scoped type
    variables, because they scope left-right in patterns.  (e.g. in the
    example above, the 'a' in (y::a) is bound by the 'a' in (x::a).

    The current not-very-good plan is to
      * find all the types in the patterns
      * find their free tyvars
      * do kind inference
      * bring the kinded type vars into scope
      * BUT throw away the kind-checked type
            (we'll kind-check it again when we type-check the pattern)

    This is bad because throwing away the kind checked type throws away
    its splices.  But too bad for now.  [July 03]

    Historical note:
        We no longer specify that these type variables must be universally
        quantified (lots of email on the subject).  If you want to put that
        back in, you need to
            a) Do a checkSigTyVars after thing_inside
            b) More insidiously, don't pass in expected_ty, else
               we unify with it too early and checkSigTyVars barfs
               Instead you have to pass in a fresh ty var, and unify
               it with expected_ty afterwards
    -}

    tcHsPatSigType :: UserTypeCtxt
                   -> LHsSigWcType Name           -- The type signature
                   -> TcM ( Type                  -- The signature
                          , [TcTyVar]     -- The new bit of type environment, binding
                                          -- the scoped type variables
                          , [(Name, TcTyVar)] )   -- The wildcards
    -- Used for type-checking type signatures in
    -- (a) patterns           e.g  f (x::Int) = e
    -- (b) RULE forall bndrs  e.g. forall (x::Int). f x = x
    --
    -- This may emit constraints

    tcHsPatSigType ctxt sig_ty
      | HsIB { hsib_vars = sig_vars, hsib_body = wc_ty } <- sig_ty
      , HsWC { hswc_wcs = sig_wcs, hswc_ctx = extra, hswc_body = hs_ty } <- wc_ty
      = ASSERT( isNothing extra )  -- No extra-constraint wildcard in pattern sigs
        addSigCtxt ctxt hs_ty $
        tcWildCardBinders sig_wcs $ \ wcs ->
        do  { emitWildCardHoleConstraints wcs
            ; (vars, sig_ty) <- tcImplicitTKBndrsX new_tkv sig_vars $
                                do { ty <- tcHsLiftedType hs_ty
                                   ; return (ty, allBoundVariables ty) }
            ; sig_ty <- zonkTcType sig_ty
                  -- don't use zonkTcTypeToType; it mistreats wildcards
            ; checkValidType ctxt sig_ty
            ; traceTc "tcHsPatSigType" (ppr sig_vars)
            ; return (sig_ty, vars, wcs) }
      where
        new_tkv name   -- See Note [Pattern signature binders]
          = (, False) <$>  -- "False" means that these tyvars aren't yet in scope
            do { kind <- newMetaKindVar
               ; case ctxt of
                   RuleSigCtxt {} -> return $ new_skolem_tv name kind
                   _              -> newSigTyVar name kind }
                                       -- See Note [Unifying SigTvs]

    tcPatSig :: Bool                    -- True <=> pattern binding
             -> LHsSigWcType Name
             -> ExpSigmaType
             -> TcM (TcType,            -- The type to use for "inside" the signature
                     [TcTyVar],         -- The new bit of type environment, binding
                                        -- the scoped type variables
                     [(Name, TcTyVar)], -- The wildcards
                     HsWrapper)         -- Coercion due to unification with actual ty
                                        -- Of shape:  res_ty ~ sig_ty
    tcPatSig in_pat_bind sig res_ty
      = do  { (sig_ty, sig_tvs, sig_wcs) <- tcHsPatSigType PatSigCtxt sig
            -- sig_tvs are the type variables free in 'sig',
            -- and not already in scope. These are the ones
            -- that should be brought into scope

            ; if null sig_tvs then do {
                    -- Just do the subsumption check and return
                      wrap <- addErrCtxtM (mk_msg sig_ty) $
                              tcSubTypeET_NC PatSigCtxt res_ty sig_ty
                    ; return (sig_ty, [], sig_wcs, wrap)
            } else do
                    -- Type signature binds at least one scoped type variable

                    -- A pattern binding cannot bind scoped type variables
                    -- It is more convenient to make the test here
                    -- than in the renamer
            { when in_pat_bind (addErr (patBindSigErr sig_tvs))

                    -- Check that all newly-in-scope tyvars are in fact
                    -- constrained by the pattern.  This catches tiresome
                    -- cases like
                    --      type T a = Int
                    --      f :: Int -> Int
                    --      f (x :: T a) = ...
                    -- Here 'a' doesn't get a binding.  Sigh
            ; let bad_tvs = [ tv | tv <- sig_tvs
                                 , not (tv `elemVarSet` exactTyCoVarsOfType sig_ty) ]
            ; checkTc (null bad_tvs) (badPatSigTvs sig_ty bad_tvs)

            -- Now do a subsumption check of the pattern signature against res_ty
            ; wrap <- addErrCtxtM (mk_msg sig_ty) $
                      tcSubTypeET_NC PatSigCtxt res_ty sig_ty

            -- Phew!
            ; return (sig_ty, sig_tvs, sig_wcs, wrap)
            } }
      where
        mk_msg sig_ty tidy_env
           = do { (tidy_env, sig_ty) <- zonkTidyTcType tidy_env sig_ty
                ; res_ty <- readExpType res_ty   -- should be filled in by now
                ; (tidy_env, res_ty) <- zonkTidyTcType tidy_env res_ty
                ; let msg = vcat [ hang (text "When checking that the pattern signature:")
                                      4 (ppr sig_ty)
                                 , nest 2 (hang (text "fits the type of its context:")
                                              2 (ppr res_ty)) ]
                ; return (tidy_env, msg) }

    patBindSigErr :: [TcTyVar] -> SDoc
    patBindSigErr sig_tvs
      = hang (text "You cannot bind scoped type variable" <> plural sig_tvs
              <+> pprQuotedList sig_tvs)
           2 (text "in a pattern binding signature")

    {-
    Note [Pattern signature binders]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider
       data T = forall a. T a (a->Int)
       f (T x (f :: a->Int) = blah)

    Here
     * The pattern (T p1 p2) creates a *skolem* type variable 'a_sk',
       It must be a skolem so that that it retains its identity, and
       TcErrors.getSkolemInfo can thereby find the binding site for the skolem.

     * The type signature pattern (f :: a->Int) binds "a" -> a_sig in the envt

     * Then unification makes a_sig := a_sk

    That's why we must make a_sig a MetaTv (albeit a SigTv),
    not a SkolemTv, so that it can unify to a_sk.

    For RULE binders, though, things are a bit different (yuk).
      RULE "foo" forall (x::a) (y::[a]).  f x y = ...
    Here this really is the binding site of the type variable so we'd like
    to use a skolem, so that we get a complaint if we unify two of them
    together.

    Note [Unifying SigTvs]
    ~~~~~~~~~~~~~~~~~~~~~~
    ALAS we have no decent way of avoiding two SigTvs getting unified.
    Consider
      f (x::(a,b)) (y::c)) = [fst x, y]
    Here we'd really like to complain that 'a' and 'c' are unified. But
    for the reasons above we can't make a,b,c into skolems, so they
    are just SigTvs that can unify.  And indeed, this would be ok,
      f x (y::c) = case x of
                     (x1 :: a1, True) -> [x,y]
                     (x1 :: a2, False) -> [x,y,y]
    Here the type of x's first component is called 'a1' in one branch and
    'a2' in the other.  We could try insisting on the same OccName, but
    they definitely won't have the sane lexical Name.

    I think we could solve this by recording in a SigTv a list of all the
    in-scope variables that it should not unify with, but it's fiddly.

    ************************************************************************
    *                                                                      *
            Checking kinds
    *                                                                      *
    ************************************************************************

    -}

    -- | Produce an 'TcKind' suitable for a checking a type that can be * or #.
    ekOpen :: TcM TcKind
    ekOpen = do { rr <- newFlexiTyVarTy runtimeRepTy
                ; return (tYPE rr) }

    unifyKinds :: [(TcType, TcKind)] -> TcM ([TcType], TcKind)
    unifyKinds act_kinds
      = do { kind <- newMetaKindVar
           ; let check (ty, act_kind) = checkExpectedKind ty act_kind kind
           ; tys' <- mapM check act_kinds
           ; return (tys', kind) }

    {-
    ************************************************************************
    *                                                                      *
            Sort checking kinds
    *                                                                      *
    ************************************************************************

    tcLHsKind converts a user-written kind to an internal, sort-checked kind.
    It does sort checking and desugaring at the same time, in one single pass.
    -}

    tcLHsKind :: LHsKind Name -> TcM Kind
    tcLHsKind = tc_lhs_kind kindLevelMode

    tc_lhs_kind :: TcTyMode -> LHsKind Name -> TcM Kind
    tc_lhs_kind mode k
      = addErrCtxt (text "In the kind" <+> quotes (ppr k)) $
        tc_lhs_type (kindLevel mode) k liftedTypeKind

    promotionErr :: Name -> PromotionErr -> TcM a
    promotionErr name err
      = failWithTc (hang (pprPECategory err <+> quotes (ppr name) <+> text "cannot be used here")
                       2 (parens reason))
      where
        reason = case err of
                   FamDataConPE   -> text "it comes from a data family instance"
                   NoDataKindsTC  -> text "Perhaps you intended to use DataKinds"
                   NoDataKindsDC  -> text "Perhaps you intended to use DataKinds"
                   NoTypeInTypeTC -> text "Perhaps you intended to use TypeInType"
                   NoTypeInTypeDC -> text "Perhaps you intended to use TypeInType"
                   PatSynPE       -> text "Pattern synonyms cannot be promoted"
                   _ -> text "it is defined and used in the same recursive group"

    {-
    ************************************************************************
    *                                                                      *
                    Scoped type variables
    *                                                                      *
    ************************************************************************
    -}

    badPatSigTvs :: TcType -> [TyVar] -> SDoc
    badPatSigTvs sig_ty bad_tvs
      = vcat [ fsep [text "The type variable" <> plural bad_tvs,
                     quotes (pprWithCommas ppr bad_tvs),
                     text "should be bound by the pattern signature" <+> quotes (ppr sig_ty),
                     text "but are actually discarded by a type synonym" ]
             , text "To fix this, expand the type synonym"
             , text "[Note: I hope to lift this restriction in due course]" ]

    {-
    ************************************************************************
    *                                                                      *
              Error messages and such
    *                                                                      *
    ************************************************************************
    -}

    -- | Make an appropriate message for an error in a function argument.
    -- Used for both expressions and types.
    funAppCtxt :: (Outputable fun, Outputable arg) => fun -> arg -> Int -> SDoc
    funAppCtxt fun arg arg_no
      = hang (hsep [ text "In the", speakNth arg_no, ptext (sLit "argument of"),
                        quotes (ppr fun) <> text ", namely"])
           2 (quotes (ppr arg))