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

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

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

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

        tcHsDeriv, tcHsVectInst,
        tcImplicitTKBndrs, tcImplicitTKBndrsType, tcExplicitTKBndrs,

                -- Type checking type and class decls
        kcLookupTcTyCon, kcTyClTyVars, tcTyClTyVars,

        -- Kind-checking types
        -- No kind generalisation, no checkValidType
        kcLHsQTyVars, kcLHsTyVarBndrs,
        tcHsLiftedType,   tcHsOpenType,
        tcHsLiftedTypeNC, tcHsOpenTypeNC,
        tcLHsType, tcLHsTypeUnsaturated, tcCheckLHsType,
        tcHsContext, tcLHsPredType, tcInferApps,
        solveEqualities, -- useful re-export

        typeLevelMode, kindLevelMode,

        kindGeneralize, checkExpectedKindX, instantiateTyUntilN,


        -- Sort-checking kinds
        tcLHsKindSig, badKindSig,

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

#include "HsVersions.h"

import GhcPrelude

import HsSyn
import TcRnMonad
import TcEvidence
import TcEnv
import TcMType
import TcValidity
import TcUnify
import TcIface
import TcSimplify ( solveEqualities )
import TcType
import TcHsSyn( zonkSigType )
import Inst   ( tcInstBinders, tcInstBinder )
import TyCoRep( TyBinder(..) )  -- Used in tcDataKindSig
import Type
import Kind
import RdrName( lookupLocalRdrOcc )
import Var
import VarSet
import TyCon
import ConLike
import DataCon
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 qualified GHC.LanguageExtensions as LangExt

import Maybes
import Data.List ( partition, mapAccumR )
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 GhcRn -> TcM a -> TcM a
addSigCtxt ctxt hs_ty thing_inside
  = setSrcSpan (getLoc hs_ty) $
    addErrCtxt (pprSigCtxt ctxt hs_ty) $

pprSigCtxt :: UserTypeCtxt -> LHsType GhcRn -> SDoc
-- (pprSigCtxt ctxt <extra> <type>)
-- prints    In the type signature for 'f':
--              f :: <type>
-- The <extra> is either empty or "the ambiguity check for"
pprSigCtxt ctxt hs_ty
  | Just n <- isSigMaybe ctxt
  = hang (text "In the type signature:")
       2 (pprPrefixOcc n <+> dcolon <+> ppr hs_ty)

  | otherwise
  = hang (text "In" <+> pprUserTypeCtxt ctxt <> colon)
       2 (ppr hs_ty)

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

kcHsSigType :: [Located Name] -> LHsSigType GhcRn -> 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 GhcRn -> 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) $
    tc_hs_sig_type_and_gen sig_ty liftedTypeKind

tcHsSigType :: UserTypeCtxt -> LHsSigType GhcRn -> 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     -> newOpenTypeKind
              -- The kind is checked by checkValidType, and isn't necessarily
              -- of kind * in a Template Haskell quote eg [t| Maybe |]

          -- Generalise here: see Note [Kind generalisation]
       ; do_kind_gen <- decideKindGeneralisationPlan sig_ty
       ; ty <- if do_kind_gen
               then tc_hs_sig_type_and_gen sig_ty kind
               else tc_hs_sig_type         sig_ty kind >>= zonkTcType

       ; checkValidType ctxt ty
       ; return ty }

tc_hs_sig_type_and_gen :: LHsSigType GhcRn -> Kind -> TcM Type
-- Kind-checks/desugars an 'LHsSigType',
--   solve equalities,
--   and then kind-generalizes.
-- This will never emit constraints, as it uses solveEqualities interally.
-- No validity checking, but it does zonk en route to generalization
tc_hs_sig_type_and_gen hs_ty kind
  = do { ty <- solveEqualities $
               tc_hs_sig_type hs_ty kind
         -- NB the call to solveEqualities, which unifies all those
         --    kind variables floating about, immediately prior to
         --    kind generalisation
       ; kindGeneralizeType ty }

tc_hs_sig_type :: LHsSigType GhcRn -> Kind -> TcM Type
-- Kind-check/desugar a 'LHsSigType', but does not solve
-- the equalities that arise from doing so; instead it may
-- emit kind-equality constraints into the monad
-- No zonking or validity checking
tc_hs_sig_type (HsIB { hsib_vars = sig_vars
                     , hsib_body = hs_ty }) kind
  = do { (tkvs, ty) <- tcImplicitTKBndrsType sig_vars $
                       tc_lhs_type typeLevelMode hs_ty kind
       ; return (mkSpecForAllTys tkvs ty) }

tcHsDeriv :: LHsSigType GhcRn -> TcM ([TyVar], Class, [Type], [Kind])
-- Like tcHsSigType, but for the ...deriving( C t1 ty2 ) clause
-- Returns the C, [ty1, ty2, and the kinds of C's remaining arguments
-- E.g.    class C (a::*) (b::k->k)
--         data T a b = ... deriving( C Int )
--    returns ([k], C, [k, Int], [k->k])
tcHsDeriv hs_ty
  = do { cls_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_and_gen hs_ty cls_kind
       ; cls_kind <- zonkTcType cls_kind
       ; let (tvs, pred) = splitForAllTys ty
       ; let (args, _) = splitFunTys cls_kind
       ; case getClassPredTys_maybe pred of
           Just (cls, tys) -> return (tvs, cls, tys, args)
           Nothing -> failWithTc (text "Illegal deriving item" <+> quotes (ppr hs_ty)) }

tcHsClsInstType :: UserTypeCtxt    -- InstDeclCtxt or SpecInstCtxt
                -> LHsSigType GhcRn
                -> 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_and_gen hs_inst_ty constraintKind
       ; checkValidInstance user_ctxt hs_inst_ty inst_ty }

-- Used for 'VECTORISE [SCALAR] instance' declarations
tcHsVectInst :: LHsSigType GhcRn -> TcM (Class, [Type])
tcHsVectInst ty
  | let hs_cls_ty = hsSigType ty
  , Just (L _ cls_name, tys) <- hsTyGetAppHead_maybe hs_cls_ty
    -- Ignoring the binders looks pretty dodgy to me
  = do { (cls, cls_kind) <- tcClass cls_name
       ; (applied_class, _res_kind)
           <- tcTyApps typeLevelMode hs_cls_ty (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 GhcRn -> Kind -> TcM Type
tcHsTypeApp wc_ty kind
  | HsWC { hswc_wcs = sig_wcs, hswc_body = hs_ty } <- wc_ty
  = do { ty <- tcWildCardBindersX newWildTyVar sig_wcs $ \ _ ->
               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.

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

        First a couple of simple wrappers for kcHsType

tcHsOpenType, tcHsLiftedType,
  tcHsOpenTypeNC, tcHsLiftedTypeNC :: LHsType GhcRn -> 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 <- newOpenTypeKind
                         ; tc_lhs_type typeLevelMode ty ek }
tcHsLiftedTypeNC ty = tc_lhs_type typeLevelMode ty liftedTypeKind

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

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

-- Like tcLHsType, but use it in a context where type synonyms and type families
-- do not need to be saturated, like in a GHCi :kind call
tcLHsTypeUnsaturated :: LHsType GhcRn -> TcM (TcType, TcKind)
tcLHsTypeUnsaturated ty = addTypeCtxt ty (tc_infer_lhs_type mode ty)
    mode = allowUnsaturated typeLevelMode

-- | Should we generalise the kind of this type signature?
-- We *should* generalise if the type is closed
-- or if NoMonoLocalBinds is set. Otherwise, nope.
-- See Note [Kind generalisation plan]
decideKindGeneralisationPlan :: LHsSigType GhcRn -> TcM Bool
decideKindGeneralisationPlan sig_ty@(HsIB { hsib_closed = closed })
  = do { mono_locals <- xoptM LangExt.MonoLocalBinds
       ; let should_gen = not mono_locals || closed
       ; traceTc "decideKindGeneralisationPlan"
           (ppr sig_ty $$ text "should gen?" <+> ppr should_gen)
       ; return should_gen }

{- Note [Kind generalisation plan]
When should we do kind-generalisation for user-written type signature?
Answer: we use the same rule as for value bindings:

 * We always kind-generalise if the type signature is closed
 * Additionally, we attempt to generalise if we have NoMonoLocalBinds

Trac #13337 shows the problem if we kind-generalise an open type (i.e.
one that mentions in-scope tpe variable
  foo :: forall k (a :: k) proxy. (Typeable k, Typeable a)
      => proxy a -> String
  foo _ = case eqT :: Maybe (k :~: Type) of
            Nothing   -> ...
            Just Refl -> case eqT :: Maybe (a :~: Int) of ...

In the expression type sig on the last line, we have (a :: k)
but (Int :: Type).  Since (:~:) is kind-homogeneous, this requires
k ~ *, which is true in the Refl branch of the outer case.

That equality will be solved if we allow it to float out to the
implication constraint for the Refl match, bnot not if we aggressively
attempt to solve all equalities the moment they occur; that is, when
checking (Maybe (a :~: Int)).   (NB: solveEqualities fails unless it
solves all the kind equalities, which is the right thing at top level.)

So here the right thing is simply not to do kind generalisation!

*                                                                      *
      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.
data TcTyMode
  = TcTyMode { mode_level :: TypeOrKind
             , mode_unsat :: Bool        -- True <=> allow unsaturated type families
 -- The mode_unsat field is solely so that type families/synonyms can be unsaturated
 -- in GHCi :kind calls

typeLevelMode :: TcTyMode
typeLevelMode = TcTyMode { mode_level = TypeLevel, mode_unsat = False }

kindLevelMode :: TcTyMode
kindLevelMode = TcTyMode { mode_level = KindLevel, mode_unsat = False }

allowUnsaturated :: TcTyMode -> TcTyMode
allowUnsaturated mode = mode { mode_unsat = True }

-- 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

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.

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 GhcRn -> TcM (TcType, TcKind)
tc_infer_lhs_type mode (L span ty)
  = setSrcSpan span $
    do { (ty', kind) <- tc_infer_hs_type mode ty
       ; return (ty', kind) }

-- | 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 GhcRn -> 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
       ; tcTyApps 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 loc_op op) rhs)
  | not (op `hasKey` funTyConKey)
  = do { (op', op_kind) <- tcTyVar mode op
       ; op_kind' <- zonkTcType op_kind
       ; tcTyApps mode (noLoc $ HsTyVar NotPromoted (L loc_op 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') }
-- HsSpliced is an annotation produced by 'RnSplice.rnSpliceType' to communicate
-- the splice location to the typechecker. Here we skip over it in order to have
-- the same kind inferred for a given expression whether it was produced from
-- splices or not.
-- See Note [Delaying modFinalizers in untyped splices].
tc_infer_hs_type mode (HsSpliceTy (HsSpliced _ (HsSplicedTy ty)) _)
  = tc_infer_hs_type mode ty
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 GhcRn -> TcKind -> TcM TcType
tc_lhs_type mode (L span ty) exp_kind
  = setSrcSpan span $
    do { ty' <- tc_hs_type mode ty exp_kind
       ; return ty' }

tc_fun_type :: TcTyMode -> LHsType GhcRn -> LHsType GhcRn -> TcKind
            -> TcM TcType
tc_fun_type mode ty1 ty2 exp_kind = case mode_level mode of
  TypeLevel ->
    do { arg_k <- newOpenTypeKind
       ; res_k <- newOpenTypeKind
       ; ty1' <- tc_lhs_type mode ty1 arg_k
       ; ty2' <- tc_lhs_type mode ty2 res_k
       ; checkExpectedKind (HsFunTy ty1 ty2) (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 (HsFunTy ty1 ty2) (mkFunTy ty1' ty2') liftedTypeKind exp_kind }

-- See also Note [Bidirectional type checking]
tc_hs_type :: TcTyMode -> HsType GhcRn -> 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)

-- HsSpliced is an annotation produced by 'RnSplice.rnSpliceType'.
-- Here we get rid of it and add the finalizers to the global environment
-- while capturing the local environment.
-- See Note [Delaying modFinalizers in untyped splices].
tc_hs_type mode (HsSpliceTy (HsSpliced mod_finalizers (HsSplicedTy ty))
  = do addModFinalizersWithLclEnv mod_finalizers
       tc_hs_type mode ty exp_kind

-- 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      = mkTyVarBinders 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 <- newOpenTypeKind
                                -- The body kind (result of the function)
                                -- can be * or #, hence newOpenTypeKind
                        ; ty' <- tc_lhs_type mode ty ek
                        ; checkExpectedKind (unLoc ty) ty' liftedTypeKind exp_kind }

       ; return (mkPhiTy ctxt' ty') }

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

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

-- See Note [Distinguishing tuple kinds] in HsTypes
-- See Note [Inferring tuple kinds]
tc_hs_type mode rn_ty@(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 rn_ty 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 hs_ty ty kind arg_kind
                          | ((L loc hs_ty),ty,kind) <- zip3 hs_tys tys kinds ]

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

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

tc_hs_type mode rn_ty@(HsSumTy hs_tys) exp_kind
  = do { let arity = length hs_tys
       ; arg_kinds <- mapM (\_ -> newOpenTypeKind) hs_tys
       ; tau_tys   <- zipWithM (tc_lhs_type mode) hs_tys arg_kinds
       ; let arg_reps = map getRuntimeRepFromKind arg_kinds
             arg_tys  = arg_reps ++ tau_tys
       ; checkExpectedKind rn_ty
                           (mkTyConApp (sumTyCon arity) arg_tys)
                           (unboxedSumKind arg_reps)

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

tc_hs_type mode rn_ty@(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 rn_ty (mkTyConApp ty_con (ks ++ taus)) tup_k exp_kind }
    arity = length tys

--------- Constraint types
tc_hs_type mode rn_ty@(HsIParamTy (L _ 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 rn_ty (mkClassPred ipClass [n',ty'])
           constraintKind exp_kind }

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

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

tc_hs_type _ rn_ty@(HsTyLit (HsStrTy _ s)) exp_kind
  = do { checkWiredInTyCon typeSymbolKindCon
       ; checkExpectedKind rn_ty (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 { wc_tv <- tcWildCardOcc wc exp_kind
       ; return (mkTyVarTy wc_tv) }

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

tcWildCardOcc :: HsWildCardInfo GhcRn -> Kind -> TcM TcTyVar
tcWildCardOcc wc_info exp_kind
  = do { wc_tv <- tcLookupTyVar (wildCardName wc_info)
          -- The wildcard's kind should be an un-filled-in meta tyvar
       ; let Just wc_kind_var = tcGetTyVar_maybe (tyVarKind wc_tv)
       ; writeMetaTyVar wc_kind_var exp_kind
       ; return wc_tv }

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

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

tc_tuple :: HsType GhcRn -> TcTyMode -> TupleSort -> [LHsType GhcRn] -> TcKind -> TcM TcType
tc_tuple rn_ty mode tup_sort tys exp_kind
  = do { arg_kinds <- case tup_sort of
           BoxedTuple      -> return (nOfThem arity liftedTypeKind)
           UnboxedTuple    -> mapM (\_ -> newOpenTypeKind) tys
           ConstraintTuple -> return (nOfThem arity constraintKind)
       ; tau_tys <- zipWithM (tc_lhs_type mode) tys arg_kinds
       ; finish_tuple rn_ty tup_sort tau_tys arg_kinds exp_kind }
    arity   = length tys

finish_tuple :: HsType GhcRn
             -> TupleSort
             -> [TcType]    -- ^ argument types
             -> [TcKind]    -- ^ of these kinds
             -> TcKind      -- ^ expected kind of the whole tuple
             -> TcM TcType
finish_tuple rn_ty 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    -> tau_reps ++ tau_tys
                 BoxedTuple      -> tau_tys
                 ConstraintTuple -> tau_tys
       ; tycon <- case tup_sort of
             | 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 rn_ty (mkTyConApp tycon arg_tys) res_kind exp_kind }
    arity = length tau_tys
    tau_reps = map getRuntimeRepFromKind tau_kinds
    res_kind = case tup_sort of
                 UnboxedTuple    -> unboxedTupleKind tau_reps
                 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. Always consumes all the arguments,
-- using matchExpectedFunKind as necessary.
-- This takes an optional @VarEnv [email protected] 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.
tcInferApps :: TcTyMode
            -> Maybe (VarEnv Kind)  -- ^ Possibly, kind info (see above)
            -> LHsType GhcRn        -- ^ Function (for printing only)
            -> TcType               -- ^ Function (could be knot-tied)
            -> TcKind               -- ^ Function kind (zonked)
            -> [LHsType GhcRn]      -- ^ Args
            -> TcM (TcType, [TcType], TcKind) -- ^ (f args, args, result kind)
tcInferApps mode mb_kind_info orig_hs_ty fun_ty fun_ki orig_hs_args
  = do { traceTc "tcInferApps" (ppr orig_hs_ty $$ ppr orig_hs_args $$ ppr fun_ki)
       ; go 1 [] empty_subst fun_ty orig_ki_binders orig_inner_ki orig_hs_args }
    empty_subst                      = mkEmptyTCvSubst $ mkInScopeSet $
                                       tyCoVarsOfType fun_ki
    (orig_ki_binders, orig_inner_ki) = tcSplitPiTys fun_ki

    go :: Int             -- the # of the next argument
       -> [TcType]        -- already type-checked args, in reverse order
       -> TCvSubst        -- instantiating substitution
       -> TcType          -- function applied to some args, could be knot-tied
       -> [TyBinder]      -- binders in function kind (both vis. and invis.)
       -> TcKind          -- function kind body (not a Pi-type)
       -> [LHsType GhcRn] -- un-type-checked args
       -> TcM (TcType, [TcType], TcKind)  -- same as overall return type

      -- no user-written args left. We're done!
    go _ acc_args subst fun ki_binders inner_ki []
      = return (fun, reverse acc_args, substTy subst $ mkPiTys ki_binders inner_ki)

      -- The function's kind has a binder. Is it visible or invisible?
    go n acc_args subst fun (ki_binder:ki_binders) inner_ki
      | isInvisibleBinder ki_binder
        -- It's invisible. Instantiate.
      = do { traceTc "tcInferApps (invis)" (ppr ki_binder $$ ppr subst)
           ; (subst', arg') <- tcInstBinder mb_kind_info subst ki_binder
           ; go n (arg' : acc_args) subst' (mkNakedAppTy fun arg')
                ki_binders inner_ki all_args }

      | otherwise
        -- It's visible. Check the next user-written argument
      = do { traceTc "tcInferApps (vis)" (vcat [ ppr ki_binder, ppr arg
                                               , ppr (tyBinderType ki_binder)
                                               , ppr subst ])
           ; arg' <- addErrCtxt (funAppCtxt orig_hs_ty arg n) $
                     tc_lhs_type mode arg (substTy subst $ tyBinderType ki_binder)
           ; let subst' = extendTvSubstBinderAndInScope subst ki_binder arg'
           ; go (n+1) (arg' : acc_args) subst' (mkNakedAppTy fun arg')
                ki_binders inner_ki args }

       -- We've run out of known binders in the functions's kind.
    go n acc_args subst fun [] inner_ki all_args
      | not (null new_ki_binders)
         -- But, after substituting, we have more binders.
      = go n acc_args zapped_subst fun new_ki_binders new_inner_ki all_args

      | otherwise
         -- Even after substituting, still no binders. Use matchExpectedFunKind
      = do { traceTc "tcInferApps (no binder)" (ppr new_inner_ki $$ ppr zapped_subst)
           ; (co, arg_k, res_k)
               <- matchExpectedFunKind (mkHsAppTys orig_hs_ty (take (n-1) orig_hs_args))
           ; let subst' = zapped_subst `extendTCvInScopeSet` tyCoVarsOfTypes [arg_k, res_k]
           ; go n acc_args subst' (fun `mkNakedCastTy` co)
                [mkAnonBinder arg_k] res_k all_args }
        substed_inner_ki               = substTy subst inner_ki
        (new_ki_binders, new_inner_ki) = tcSplitPiTys substed_inner_ki
        zapped_subst                   = zapTCvSubst subst

-- | Applies a type to a list of arguments.
-- Always consumes all the arguments, using 'matchExpectedFunKind' as
-- necessary. If you wish to apply a type to a list of HsTypes, this is
-- your function.
-- Used for type-checking types only.
tcTyApps :: TcTyMode
         -> LHsType GhcRn        -- ^ Function (for printing only)
         -> TcType               -- ^ Function (could be knot-tied)
         -> TcKind               -- ^ Function kind (zonked)
         -> [LHsType GhcRn]      -- ^ Args
         -> TcM (TcType, TcKind) -- ^ (f args, result kind)
tcTyApps mode orig_hs_ty ty ki args
  = do { (ty', _args, ki') <- tcInferApps mode Nothing orig_hs_ty ty ki args
       ; return (ty', ki') }

-- like checkExpectedKindX, but returns only the final type; convenient wrapper
checkExpectedKind :: HsType GhcRn
                  -> TcType
                  -> TcKind
                  -> TcKind
                  -> TcM TcType
checkExpectedKind hs_ty ty act exp
  = fstOf3 <$> checkExpectedKindX Nothing (ppr hs_ty) ty act exp

checkExpectedKindX :: Maybe (VarEnv Kind)  -- Possibly, instantiations for kind vars
                   -> SDoc                 -- HsType whose kind we're checking
                   -> TcType               -- the type whose kind we're checking
                   -> TcKind               -- the known kind of that type, k
                   -> TcKind               -- the expected kind, exp_kind
                   -> TcM (TcType, [TcType], TcCoercionN)
    -- (an possibly-inst'ed, casted type :: exp_kind, the new args, the coercion)
-- 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
checkExpectedKindX mb_kind_env pp_hs_ty ty act_kind exp_kind
 = do { (ty', new_args, act_kind') <- instantiate ty act_kind exp_kind
      ; let origin = TypeEqOrigin { uo_actual   = act_kind'
                                  , uo_expected = exp_kind
                                  , uo_thing    = Just pp_hs_ty
                                  , uo_visible  = True } -- the hs_ty is visible
      ; co_k <- uType KindLevel origin 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, new_args, co_k) }
    -- 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
                       , [TcType] -- the new args
                       , TcKind ) -- its new kind
    instantiate ty act_ki exp_ki
      = let (exp_bndrs, _) = splitPiTysInvisible exp_ki in
        instantiateTyUntilN mb_kind_env (length exp_bndrs) ty act_ki

-- | Instantiate @[email protected] invisible arguments to a type. If @n <= [email protected], no instantiation
-- occurs. If @[email protected] is too big, then all available invisible arguments are instantiated.
-- (In other words, this function is very forgiving about bad values of @[email protected])
instantiateTyN :: Maybe (VarEnv Kind)              -- ^ Predetermined instantiations
                                                   -- (for assoc. type patterns)
               -> Int                              -- ^ @[email protected]
               -> TcType                           -- ^ the type
               -> [TyBinder] -> TcKind             -- ^ its kind
               -> TcM (TcType, [TcType], TcKind)   -- ^ The inst'ed type, new args, kind
instantiateTyN mb_kind_env n ty bndrs inner_ki
  = let   -- NB: splitAt is forgiving with invalid numbers
        (inst_bndrs, leftover_bndrs) = splitAt n bndrs
        ki          = mkPiTys bndrs inner_ki
        empty_subst = mkEmptyTCvSubst (mkInScopeSet (tyCoVarsOfType ki))
    if n <= 0 then return (ty, [], ki) else
    do { (subst, inst_args) <- tcInstBinders empty_subst mb_kind_env inst_bndrs
       ; let rebuilt_ki = mkPiTys leftover_bndrs inner_ki
             ki'        = substTy subst rebuilt_ki
       ; traceTc "instantiateTyN" (vcat [ ppr ki
                                        , ppr n
                                        , ppr subst
                                        , ppr rebuilt_ki
                                        , ppr ki' ])
       ; return (mkNakedAppTys ty inst_args, inst_args, ki') }

-- | Instantiate a type to have at most @[email protected] invisible arguments.
instantiateTyUntilN :: Maybe (VarEnv Kind)   -- ^ Possibly, instantiations for vars
                    -> Int         -- ^ @[email protected]
                    -> TcType      -- ^ the type
                    -> TcKind      -- ^ its kind
                    -> TcM (TcType, [TcType], TcKind)   -- ^ The inst'ed type, new args,
                                                        -- final kind
instantiateTyUntilN mb_kind_env n ty ki
  = let (bndrs, inner_ki) = splitPiTysInvisible ki
        num_to_inst       = length bndrs - n
    instantiateTyN mb_kind_env num_to_inst ty bndrs inner_ki

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

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

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

tc_lhs_pred :: TcTyMode -> LHsType GhcRn -> 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 { -- See Note [GADT kind self-reference]
                                    (isTypeLevel (mode_level mode))
                                    (promotionErr name TyConPE)
                                ; 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 }
    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)
                  -> TcTyCon   -- a non-loopy version of the tycon
                  -> TcM (TcType, TcKind)
    handle_tyfams tc tc_tc
      | mightBeUnsaturatedTyCon tc_tc || mode_unsat mode
                                         -- This is where mode_unsat is used
      = do { traceTc "tcTyVar2a" (ppr tc_tc $$ ppr tc_kind)
           ; return (ty, tc_kind) }

      | otherwise
      = do { (tc_ty, _, kind) <- instantiateTyN Nothing (length (tyConBinders tc_tc))
                                                ty tc_kind_bndrs tc_inner_ki
           -- 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) }
        ty                           = mkNakedTyConApp tc []
        tc_kind                      = tyConKind tc_tc
        (tc_kind_bndrs, tc_inner_ki) = splitPiTysInvisible tc_kind

    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 (FunTy 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

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

Note [GADT kind self-reference]

A promoted type cannot be used in the body of that type's declaration.
Trac #11554 shows this example, which made GHC loop:

  import Data.Kind
  data P (x :: k) = Q
  data A :: Type where
    B :: forall (a :: A). P a -> A

In order to check the constructor B, we need to have the promoted type A, but in
order to get that promoted type, B must first be checked. To prevent looping, a
TyConPE promotion error is given when tcTyVar checks an ATcTyCon in kind mode.
Any ATcTyCon is a TyCon being defined in the current recursive group (see data
type decl for TcTyThing), and all such TyCons are illegal in kinds.

Trac #11962 proposes checking the head of a data declaration separately from
its constructors. This would allow the example above to pass.

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 analysis; see

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 ...
    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.

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

*                                                                      *
                Type-variable binders
%*                                                                      *

Note [Scope-check inferred kinds]

  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 kcLHsQTyVars
can produce the right TyConBinders, and tell Anon vs. Required.

Example   data T k1 (a:k1) (b:k2) c
               = MkT (Proxy a) (Proxy b) (Proxy c)

       are Anon     (explicitly specified as a binder, not used
                     in the kind of any other binder
  k1   is Required  (explicitly specifed as a binder, but used
                     in the kind of another binder i.e. dependently)
  k2   is Specified (not explicitly bound, but used in the kind
                     of another binder)
  k3   in Inferred  (not lexically in scope at all, but inferred
                     by kind inference)
  T :: forall {k3} k1. forall k3 -> k1 -> k2 -> k3 -> *

See Note [TyVarBndrs, TyVarBinders, TyConBinders, and visibility]
in TyCoRep.

kcLHsQTyVars uses the hsq_dependent field to decide whether
k1, a, b, c should be Required or Anon.

Earlier, thought it would work simply to do a free-variable check
during kcLHsQTyVars, 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
kcLHsQTyVars is called.  (Recall that kcLHsQTyVars 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.

Note [Kind generalisation and SigTvs]
  data T (a :: k1) x = MkT (S a ())
  data S (b :: k2) y = MkS (T b ())

While we are doing kind inference for the mutually-recursive S,T,
we will end up unifying k1 and k2 together. So they can't be skolems.
We therefore make them SigTvs, which can unify with type variables,
but not with general types.  All this is very similar at the level
of terms: see Note [Quantified variables in partial type signatures]
in TcBinds.

There are some wrinkles

* We always want to kind-generalise over SigTvs, and /not/ default
  them to Type.  Another way to say this is: a SigTV should /never/
  stand for a type, even via defaulting. Hence the check in
  TcSimplify.defaultTyVarTcS, and TcMType.defaultTyVar.  Here's
  another example (Trac #14555):
     data Exp :: [TYPE rep] -> TYPE rep -> Type where
        Lam :: Exp (a:xs) b -> Exp xs (a -> b)
  We want to kind-generalise over the 'rep' variable.
  Trac #14563 is another example.

* Consider Trac #11203
    data SameKind :: k -> k -> *
    data Q (a :: k1) (b :: k2) c = MkQ (SameKind a b)
  Here we will unify k1 with k2, but this time doing so is an error,
  because k1 and k2 are bound in the same delcaration.

  We sort this out using findDupSigTvs, in TcTyClTyVars; very much
  as we do with partial type signatures in mk_psig_qtvs in

tcWildCardBinders :: [Name]
                  -> ([(Name, TcTyVar)] -> TcM a)
                  -> TcM a
tcWildCardBinders = tcWildCardBindersX new_tv
    new_tv name = do { kind <- newMetaKindVar
                     ; newSkolemTyVar name kind }

tcWildCardBindersX :: (Name -> TcM TcTyVar)
                   -> [Name]
                   -> ([(Name, TcTyVar)] -> TcM a)
                   -> TcM a
tcWildCardBindersX new_wc wc_names thing_inside
  = do { wcs <- mapM new_wc wc_names
       ; let wc_prs = wc_names `zip` wcs
       ; tcExtendTyVarEnv2 wc_prs $
         thing_inside wc_prs }

-- | 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.
kcLHsQTyVars :: Name    -- ^ of the thing being checked
             -> TyConFlavour -- ^ What sort of 'TyCon' is being checked
             -> Bool    -- ^ True <=> the decl being checked has a CUSK
             -> Bool    -- ^ True <=> all the hsq_implicit are *kind* vars
                        -- (will give these kind * if -XNoTypeInType)
             -> LHsQTyVars GhcRn
             -> TcM (Kind, r)     -- ^ The result kind, possibly with other info
             -> TcM (TcTyCon, r)  -- ^ A suitably-kinded TcTyCon
kcLHsQTyVars name flav cusk 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
       ; lvl <- getTcLevel
       ; let scoped_kvs    = zipWith (mk_skolem_tv lvl) kv_ns kv_kinds
       ; tcExtendTyVarEnv scoped_kvs $
    do { (tc_tvs, (res_kind, stuff))
              <- solveEqualities $
                 kcLHsTyVarBndrs open_fam hs_tvs thing_inside

           -- Now, because we're in a CUSK, quantify over the mentioned
           -- kind vars, in dependency order.
       ; tc_tvs  <- mapM zonkTcTyVarToTyVar tc_tvs
       ; res_kind <- zonkTcType res_kind
       ; let tc_binders = zipWith mk_tc_binder hs_tvs tc_tvs
             qkvs = tyCoVarsOfTypeWellScoped (mkTyConKind tc_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 ++ tc_tvs)

          -- If any of the scoped_kvs aren't actually mentioned in a binder's
          -- kind (or the return kind), then we're in the CUSK case from
          -- Note [Free-floating kind vars]
       ; let all_tc_tvs        = good_tvs ++ tc_tvs
             all_mentioned_tvs = mapUnionVarSet (tyCoVarsOfType . tyVarKind)
                                 `unionVarSet` tyCoVarsOfType res_kind
             unmentioned_kvs   = filterOut (`elemVarSet` all_mentioned_tvs)
       ; reportFloatingKvs name flav all_tc_tvs unmentioned_kvs

       ; let final_binders = map (mkNamedTyConBinder Specified) good_tvs
                            ++ tc_binders
             tycon = mkTcTyCon name final_binders res_kind
                               (mkTyVarNamePairs (scoped_kvs ++ tc_tvs))
                           -- the tvs contain the binders already
                           -- in scope from an enclosing class, but
                           -- re-adding tvs to the env't doesn't cause
                           -- harm

       ; traceTc "kcLHsQTyVars: cusk" $
         vcat [ ppr name, ppr kv_ns, ppr hs_tvs, ppr dep_names
              , ppr tc_tvs, ppr (mkTyConKind final_binders res_kind)
              , ppr qkvs, ppr meta_tvs, ppr good_tvs, ppr final_binders ]

       ; return (tycon, stuff) }}

  | otherwise
  = do { kv_kinds <- mk_kv_kinds
       ; scoped_kvs <- zipWithM newSigTyVar kv_ns kv_kinds
           -- Why newSigTyVar?  See Note [Kind generalisation and sigTvs]
       ; (tc_tvs, (res_kind, stuff))
           <- tcExtendTyVarEnv2 (kv_ns `zip` scoped_kvs) $
              kcLHsTyVarBndrs open_fam hs_tvs thing_inside
       ; let   -- NB: Don't add scoped_kvs to tyConTyVars, because they
               -- must remain lined up with the binders
             tc_binders = zipWith mk_tc_binder hs_tvs tc_tvs
             tycon = mkTcTyCon name tc_binders res_kind
                               (mkTyVarNamePairs (scoped_kvs ++ tc_tvs))

       ; traceTc "kcLHsQTyVars: not-cusk" $
         vcat [ ppr name, ppr kv_ns, ppr hs_tvs, ppr dep_names
              , ppr tc_tvs, ppr (mkTyConKind tc_binders res_kind) ]
       ; return (tycon, stuff) }
    open_fam = tcFlavourIsOpen flav

    mk_tc_binder :: LHsTyVarBndr GhcRn -> TyVar -> TyConBinder
    -- See Note [Dependent LHsQTyVars]
    mk_tc_binder hs_tv tv
       | hsLTyVarName hs_tv `elemNameSet` dep_names
       = mkNamedTyConBinder Required tv
       | otherwise
       = mkAnonTyConBinder tv

      -- 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 }

    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)) ] }
        pp_tv tv = ppr tv <+> dcolon <+> ppr (tyVarKind tv)

kcLHsTyVarBndrs :: Bool   -- True <=> Default un-annotated tyvar
                          --          binders to kind *
                -> [LHsTyVarBndr GhcRn]
                -> TcM r
                -> TcM ([TyVar], r)
-- There may be dependency between the explicit "ty" vars.
-- So, we have to handle them one at a time.
kcLHsTyVarBndrs _ [] thing
  = do { stuff <- thing; return ([], stuff) }

kcLHsTyVarBndrs open_fam (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, stuff) <- bind_unless_scoped tv_pair $
                         kcLHsTyVarBndrs open_fam hs_tvs $

       ; return ( tv : tvs, 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 GhcRn -> TcM (TcTyVar, Bool)
    kc_hs_tv (UserTyVar lname@(L _ name))
      = do { tv_pair@(tv, in_scope) <- tcHsTyVarName Nothing name

             -- Open type/data families default their variables to kind *.
             -- But don't default in-scope class tyvars, of course
           ; when (open_fam && not in_scope) $
             discardResult $ unifyKind (Just (HsTyVar NotPromoted lname)) liftedTypeKind
                                       (tyVarKind tv)

           ; return tv_pair }

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

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,
-- but may be in different order to the original [Name]
--   (because of sorting to respect dependency)
-- Returned TcTyVars have zonked kinds
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 $

         -- Check that the implicitly-bound kind variable
         -- really can go at the beginning.
         -- e.g.   forall (a :: k) (b :: *). ...(forces k :: b)...
       ; tkvs <- mapM zonkTyCoVarKind tkvs
                 -- NB: /not/ zonkTcTyVarToTyVar. tcImplicitTKBndrsX
                 -- guarantees to return TcTyVars with the same Names
                 -- as the var_ns.  See [Pattern signature binders]

       ; 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 GhcRn]
                  -> ([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
  = tcExplicitTKBndrsX newSkolemTyVar orig_hs_tvs thing_inside

tcExplicitTKBndrsX :: (Name -> Kind -> TcM TyVar)
                   -> [LHsTyVarBndr GhcRn]
                   -> ([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
-- See also Note [Associated type tyvar names] in Class
tcExplicitTKBndrsX new_tv 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 pprTyVar tvs) ]

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

tcHsTyVarBndr :: (Name -> Kind -> TcM TyVar)
              -> HsTyVarBndr GhcRn -> 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 new_tv (UserTyVar (L _ name))
  = do { kind <- newMetaKindVar
       ; new_tv name kind }

tcHsTyVarBndr new_tv (KindedTyVar (L _ name) kind)
  = do { kind <- tcLHsKindSig kind
       ; new_tv name kind }

newWildTyVar :: Name -> TcM TcTyVar
-- ^ New unification variable for a wildcard
newWildTyVar _name
  = do { kind <- newMetaKindVar
       ; uniq <- newUnique
       ; details <- newMetaDetails TauTv
       ; let name = mkSysTvName uniq (fsLit "w")
       ; return (mkTcTyVar name kind details) }

-- | 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 (HsTyVar NotPromoted (noLoc name))) kind (tyVarKind tv)
                   ; return (tv, True) }
           _ -> do { kind <- case m_kind of
                               Just kind -> return kind
                               Nothing   -> newMetaKindVar
                   ; tv <- newSkolemTyVar name kind
                   ; return (tv, False) }}

-- makes a new skolem tv
newSkolemTyVar :: Name -> Kind -> TcM TcTyVar
newSkolemTyVar name kind = do { lvl <- getTcLevel
                              ; return (mk_skolem_tv lvl name kind) }

mk_skolem_tv :: TcLevel -> Name -> Kind -> TcTyVar
mk_skolem_tv lvl n k = mkTcTyVar n k (SkolemTv lvl False)

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

kindGeneralize :: TcType -> TcM [KindVar]
-- Quantify the free kind variables of a kind or type
-- In the latter case the type is closed, so it has no free
-- type variables.  So in both cases, all the free vars are kind vars
kindGeneralize kind_or_type
  = do { kvs <- zonkTcTypeAndFV kind_or_type
       ; let dvs = DV { dv_kvs = kvs, dv_tvs = emptyDVarSet }
       ; gbl_tvs <- tcGetGlobalTyCoVars -- Already zonked
       ; quantifyTyVars gbl_tvs dvs }

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 [Free-floating kind vars]

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

This is not an existential datatype, but a higher-rank one (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, in both
cases those k's scope over the whole declaration. 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.

Our approach depends on whether or not the datatype has a CUSK.

Non-CUSK: In the first pass (kcTyClTyVars) we just bring
k into scope. In the second pass (tcTyClTyVars),
we check to make sure that k has been unified with some other variable
(or generalized over, making k into a skolem). If it hasn't been, then
it must be a free-floating kind var. Error.

CUSK: When we determine the tycon's final, never-to-be-changed kind
in kcLHsQTyVars, we check to make sure all implicitly-bound kind
vars are indeed mentioned in a kind somewhere. If not, error.

We also perform free-floating kind var analysis for type family instances
(see #13985). Here is an interesting example:

    type family   T :: k
    type instance T = (Nothing :: Maybe a)

Upon a cursory glance, it may appear that the kind variable `a` is
free-floating above, since there are no (visible) LHS patterns in `T`. However,
there is an *invisible* pattern due to the return kind, so inside of GHC, the
instance looks closer to this:

    type family T @k :: k
    type instance T @(Maybe a) = (Nothing :: Maybe a)

Here, we can see that `a` really is bound by a LHS type pattern, so `a` is in
fact not free-floating. Contrast that with this example:

    type instance T = Proxy (Nothing :: Maybe a)

This would looks like this inside of GHC:

    type instance T @(*) = Proxy (Nothing :: Maybe a)

So this time, `a` is neither bound by a visible nor invisible type pattern on
the LHS, so it would be reported as free-floating.

Finally, here's one more brain-teaser (from #9574). In the example below:

    class Funct f where
      type Codomain f :: *
    instance Funct ('KProxy :: KProxy o) where
      type Codomain 'KProxy = NatTr (Proxy :: o -> *)

As it turns out, `o` is not free-floating in this example. That is because `o`
bound by the kind signature of the LHS type pattern 'KProxy. To make this more
obvious, one can also write the instance like so:

    instance Funct ('KProxy :: KProxy o) where
      type Codomain ('KProxy :: KProxy o) = NatTr (Proxy :: o -> *)


-- getInitialKind has made a suitably-shaped kind for the type or class
-- Look it up in the local environment. This is used only for tycons
-- that we're currently type-checking, so we're sure to find a TcTyCon.
kcLookupTcTyCon :: Name -> TcM TcTyCon
kcLookupTcTyCon nm
  = do { tc_ty_thing <- tcLookup nm
       ; return $ case tc_ty_thing of
           ATcTyCon tc -> tc
           _           -> pprPanic "kcLookupTcTyCon" (ppr tc_ty_thing) }

-- | Bring tycon tyvars into scope. This is used during the "kind-checking"
-- pass in TcTyClsDecls. (Never in getInitialKind, never in the
-- "type-checking"/desugaring pass.)
-- Never emits constraints, though the thing_inside might.
kcTyClTyVars :: Name -> TcM a -> TcM a
kcTyClTyVars tycon_name thing_inside
  = do { tycon <- kcLookupTcTyCon tycon_name
       ; tcExtendTyVarEnv2 (tcTyConScopedTyVars tycon) $ thing_inside }

tcTyClTyVars :: Name
             -> ([TyConBinder] -> 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:*, Anon (k1 -> *), Anon 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_name thing_inside
  = do { tycon <- kcLookupTcTyCon tycon_name

       -- Do checks on scoped tyvars
       -- See Note [Free-floating kind vars]
       ; let scoped_prs = tcTyConScopedTyVars tycon
             scoped_tvs = map snd scoped_prs
             still_sig_tvs = filter isSigTyVar scoped_tvs

       ; mapM_ report_sig_tv_err (findDupSigTvs scoped_prs)

       ; checkNoErrs $ reportFloatingKvs tycon_name (tyConFlavour tycon)
                                         scoped_tvs still_sig_tvs

       ; let res_kind   = tyConResKind tycon
             binders    = correct_binders (tyConBinders tycon) res_kind
       ; traceTc "tcTyClTyVars" (ppr tycon_name <+> ppr binders)

       ; tcExtendTyVarEnv2 scoped_prs $
         thing_inside binders res_kind }
    report_sig_tv_err (n1, n2)
      = setSrcSpan (getSrcSpan n2) $
        addErrTc (text "Couldn't match" <+> quotes (ppr n1)
                        <+> text "with" <+> quotes (ppr n2))

    -- Given some TyConBinders and a TyCon's result kind, make sure that the
    -- correct any wrong Named/Anon choices. For example, consider
    --   type Syn k = forall (a :: k). Proxy a
    -- At first, it looks like k should be named -- after all, it appears on the RHS.
    -- However, the correct kind for Syn is (* -> *).
    -- (Why? Because k is the kind of a type, so k's kind is *. And the RHS also has
    -- kind *.) See also #13963.
    correct_binders :: [TyConBinder] -> Kind -> [TyConBinder]
    correct_binders binders kind
      = binders'
        (_, binders') = mapAccumR go (tyCoVarsOfType kind) binders

        go :: TyCoVarSet -> TyConBinder -> (TyCoVarSet, TyConBinder)
        go fvs binder
          | isNamedTyConBinder binder
          , not (tv `elemVarSet` fvs)
          = (new_fvs, mkAnonTyConBinder tv)

          | not (isNamedTyConBinder binder)
          , tv `elemVarSet` fvs
          = (new_fvs, mkNamedTyConBinder Required tv)
             -- always Required, because it was anonymous (i.e. visible) previously

          | otherwise
          = (new_fvs, binder)

            tv      = binderVar binder
            new_fvs = fvs `delVarSet` tv `unionVarSet` tyCoVarsOfType (tyVarKind tv)

tcDataKindSig :: [TyConBinder]
              -> Kind
              -> TcM ([TyConBinder], Kind)
-- GADT decls can have a (perhaps partial) kind signature
--      e.g.  data T a :: * -> * -> * where ...
-- This function makes up suitable (kinded) TyConBinders for the
-- argument kinds.  E.g. in this case it might return
--   ([b::*, c::*], *)
-- Never emits constraints.
-- It's a little trickier than you might think: see
-- Note [TyConBinders for the result kind signature of a data type]
tcDataKindSig tc_bndrs kind
  = do  { loc     <- getSrcSpanM
        ; uniqs   <- newUniqueSupply
        ; rdr_env <- getLocalRdrEnv
        ; let new_occs = [ occ
                         | str <- allNameStrings
                         , let occ = mkOccName tvName str
                         , isNothing (lookupLocalRdrOcc rdr_env occ)
                         -- Note [Avoid name clashes for associated data types]
                         , not (occ `elem` lhs_occs) ]
              new_uniqs = uniqsFromSupply uniqs
              subst = mkEmptyTCvSubst (mkInScopeSet (mkVarSet lhs_tvs))
        ; return (go loc new_occs new_uniqs subst [] kind) }
    lhs_tvs  = map binderVar tc_bndrs
    lhs_occs = map getOccName lhs_tvs

    go loc occs uniqs subst acc kind
      = case splitPiTy_maybe kind of
          Nothing -> (reverse acc, substTy subst kind)

          Just (Anon arg, kind')
            -> go loc occs' uniqs' subst' (tcb : acc) kind'
              arg'   = substTy subst arg
              tv     = mkTyVar (mkInternalName uniq occ loc) arg'
              subst' = extendTCvInScope subst tv
              tcb    = TvBndr tv AnonTCB
              (uniq:uniqs') = uniqs
              (occ:occs')   = occs

          Just (Named (TvBndr tv vis), kind')
            -> go loc occs uniqs subst' (tcb : acc) kind'
              (subst', tv') = substTyVarBndr subst tv
              tcb = TvBndr tv' (NamedTCB vis)

badKindSig :: Bool -> Kind -> SDoc
badKindSig check_for_type kind
 = hang (sep [ text "Kind signature on data type declaration has non-*"
             , (if check_for_type then empty else text "and non-variable") <+>
               text "return kind" ])
        2 (ppr kind)

{- Note [TyConBinders for the result kind signature of a data type]
  data T (a::*) :: * -> forall k. k -> *
we want to generate the extra TyConBinders for T, so we finally get
  (a::*) (b::*) (k::*) (c::k)
The function tcDataKindSig generates these extra TyConBinders from
the result kind signature.

We need to take care to give the TyConBinders
  (a) OccNames that are fresh (because the TyConBinders of a TyCon
      must have distinct OccNames

  (b) Uniques that are fresh (obviously)

For (a) we need to avoid clashes with the tyvars declared by
the user before the "::"; in the above example that is 'a'.
And also see Note [Avoid name clashes for associated data types].

For (b) suppose we have
   data T :: forall k. k -> forall k. k -> *
where the two k's are identical even up to their uniques.  Surprisingly,
this can happen: see Trac #14515.

It's reasonably easy to solve all this; just run down the list with a
substitution; hence the recursive 'go' function.  But it has to be

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.

*                                                                      *
             Partial signatures
*                                                                      *


  :: UserTypeCtxt
  -> LHsSigWcType GhcRn       -- The type signature
  -> TcM ( [(Name, TcTyVar)]  -- Wildcards
         , Maybe TcTyVar      -- Extra-constraints wildcard
         , [TcTyVar]          -- Implicitly and explicitly bound type variables
         , TcThetaType        -- Theta part
         , TcType )           -- Tau part
tcHsPartialSigType ctxt sig_ty
  | HsWC { hswc_wcs  = sig_wcs,         hswc_body = ib_ty } <- sig_ty
  , HsIB { hsib_vars = implicit_hs_tvs, hsib_body = hs_ty } <- ib_ty
  , (explicit_hs_tvs, L _ hs_ctxt, hs_tau) <- splitLHsSigmaTy hs_ty
  = addSigCtxt ctxt hs_ty $
    do { (implicit_tvs, (wcs, wcx, explicit_tvs, theta, tau))
            <- tcWildCardBindersX newWildTyVar sig_wcs        $ \ wcs ->
               tcImplicitTKBndrsX new_implicit_tv implicit_hs_tvs $
               tcExplicitTKBndrs explicit_hs_tvs $ \ explicit_tvs ->
             do {   -- Instantiate the type-class context; but if there
                    -- is an extra-constraints wildcard, just discard it here
                    (theta, wcx) <- tcPartialContext hs_ctxt

                  ; tau <- tcHsOpenType hs_tau

                  ; let bound_tvs = unionVarSets [ allBoundVariables tau
                                                 , mkVarSet explicit_tvs
                                                 , mkVarSet (map snd wcs) ]

                  ; return ( (wcs, wcx, explicit_tvs, theta, tau)
                           , bound_tvs) }

        -- Spit out the wildcards (including the extra-constraints one)
        -- as "hole" constraints, so that they'll be reported if necessary
        -- See Note [Extra-constraint holes in partial type signatures]
        ; emitWildCardHoleConstraints wcs

        ; explicit_tvs <- mapM zonkTyCoVarKind explicit_tvs
        ; let all_tvs = implicit_tvs ++ explicit_tvs
                        -- The implicit_tvs already have zonked kinds

        ; theta   <- mapM zonkTcType theta
        ; tau     <- zonkTcType tau

        ; checkValidType ctxt (mkSpecForAllTys all_tvs $ mkPhiTy theta tau)

        ; traceTc "tcHsPartialSigType" (ppr all_tvs)
        ; return (wcs, wcx, all_tvs, theta, tau) }
    new_implicit_tv name
      = do { kind <- newMetaKindVar
           ; tv <- newSkolemTyVar name kind
           ; return (tv, False) }

tcPartialContext :: HsContext GhcRn -> TcM (TcThetaType, Maybe TcTyVar)
tcPartialContext hs_theta
  | Just (hs_theta1, hs_ctxt_last) <- snocView hs_theta
  , L _ (HsWildCardTy wc) <- ignoreParens hs_ctxt_last
  = do { wc_tv <- tcWildCardOcc wc constraintKind
       ; theta <- mapM tcLHsPredType hs_theta1
       ; return (theta, Just wc_tv) }
  | otherwise
  = do { theta <- mapM tcLHsPredType hs_theta
       ; return (theta, Nothing) }

{- Note [Extra-constraint holes in partial type signatures]
  f :: (_) => a -> a
  f x = ...

* The renamer makes a wildcard name for the "_", and puts it in
  the hswc_wcs field.

* Then, in tcHsPartialSigType, we make a new hole TcTyVar, in

* TcBinds.chooseInferredQuantifiers fills in that hole TcTyVar
  with the inferred constraints, e.g. (Eq a, Show a)

* TcErrors.mkHoleError finally reports the error.

An annoying difficulty happens if there are more than 62 inferred
constraints. Then we need to fill in the TcTyVar with (say) a 70-tuple.
Where do we find the TyCon?  For good reasons we only have constraint
tuples up to 62 (see Note [How tuples work] in TysWiredIn).  So how
can we make a 70-tuple?  This was the root cause of Trac #14217.

It's incredibly tiresome, because we only need this type to fill
in the hole, to communicate to the error reporting machinery.  Nothing
more.  So I use a HACK:

* I make an /ordinary/ tuple of the constraints, in
  TcBinds.chooseInferredQuantifiers. This is ill-kinded because
  ordinary tuples can't contain constraints, but it works fine. And for
  ordinary tuples we don't have the same limit as for constraint
  tuples (which need selectors and an assocated class).

* Because it is ill-kinded, it trips an assert in writeMetaTyVar,
  so now I disable the assertion if we are writing a type of
  kind Constraint.  (That seldom/never normally happens so we aren't
  losing much.)

Result works fine, but it may eventually bite us.

*                                                                      *
      Pattern signatures (i.e signatures that occur in patterns)
*                                                                      *
********************************************************************* -}

tcHsPatSigType :: UserTypeCtxt
               -> LHsSigWcType GhcRn          -- The type signature
               -> TcM ( [(Name, TcTyVar)]     -- Wildcards
                      , [(Name, TcTyVar)]     -- The new bit of type environment, binding
                                              -- the scoped type variables
                      , TcType)       -- The type
-- 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
  | HsWC { hswc_wcs = sig_wcs,   hswc_body = ib_ty } <- sig_ty
  , HsIB { hsib_vars = sig_vars, hsib_body = hs_ty } <- ib_ty
  = addSigCtxt ctxt hs_ty $
    do { (implicit_tvs, (wcs, sig_ty))
            <- tcWildCardBindersX newWildTyVar    sig_wcs  $ \ wcs ->
               tcImplicitTKBndrsX new_implicit_tv sig_vars $
               do { sig_ty <- tcHsOpenType hs_ty
                  ; return ((wcs, sig_ty), allBoundVariables sig_ty) }

        ; emitWildCardHoleConstraints wcs

        ; sig_ty <- zonkTcType sig_ty
        ; checkValidType ctxt sig_ty

        ; tv_pairs <- mapM mk_tv_pair implicit_tvs

        ; traceTc "tcHsPatSigType" (ppr sig_vars)
        ; return (wcs, tv_pairs, sig_ty) }
    new_implicit_tv name = do { kind <- newMetaKindVar
                              ; tv <- new_tv name kind
                              ; return (tv, False) }
       -- "False" means that these tyvars aren't yet in scope
    new_tv = case ctxt of
               RuleSigCtxt {} -> newSkolemTyVar
               _              -> newSigTyVar
      -- See Note [Pattern signature binders]
      -- See Note [Unifying SigTvs]

    mk_tv_pair tv = do { tv' <- zonkTcTyVarToTyVar tv
                       ; return (tyVarName tv, tv') }
         -- The Name is one of sig_vars, the lexically scoped name
         -- But if it's a SigTyVar, it might have been unified
         -- with an existing in-scope skolem, so we must zonk
         -- here.  See Note [Pattern signature binders]

tcPatSig :: Bool                    -- True <=> pattern binding
         -> LHsSigWcType GhcRn
         -> ExpSigmaType
         -> TcM (TcType,            -- The type to use for "inside" the signature
                 [(Name,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_wcs, sig_tvs, sig_ty) <- 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 PatSigOrigin 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 PatSigOrigin PatSigCtxt res_ty sig_ty

        -- Phew!
        ; return (sig_ty, sig_tvs, sig_wcs, wrap)
        } }
    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 :: [(Name,TcTyVar)] -> SDoc
patBindSigErr sig_tvs
  = hang (text "You cannot bind scoped type variable" <> plural sig_tvs
          <+> pprQuotedList (map fst sig_tvs))
       2 (text "in a pattern binding signature")

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

 * 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 :: b->Int) makes a fresh meta-tyvar b_sig
   (a SigTv), and binds "b" :-> b_sig in the envt

 * Then unification makes b_sig := a_sk
   That's why we must make b_sig a MetaTv (albeit a SigTv),
   not a SkolemTv, so that it can unify to a_sk.

 * Finally, in 'blah' we must have the envt "b" :-> a_sk.  The pair
   ("b" :-> a_sk) is returned by tcHsPatSigType, constructed by
   mk_tv_pair in that function.

Another example (Trac #13881):
   fl :: forall (l :: [a]). Sing l -> Sing l
   fl (SNil :: Sing (l :: [y])) = SNil
When we reach the pattern signature, 'l' is in scope from the
outer 'forall':
   "a" :-> a_sk :: *
   "l" :-> l_sk :: [a_sk]
We make up a fresh meta-SigTv, y_sig, for 'y', and kind-check
the pattern signature
   Sing (l :: [y])
That unifies y_sig := a_sk.  We return from tcHsPatSigType with
the pair ("y" :-> 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

Note [Unifying SigTvs]
ALAS we have no decent way of avoiding two SigTvs getting unified.
  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
*                                                                      *


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

*                                                                      *
        Sort checking kinds
*                                                                      *

tcLHsKindSig 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.

tcLHsKindSig :: LHsKind GhcRn -> TcM Kind
tcLHsKindSig hs_kind
  = do { kind <- tc_lhs_kind kindLevelMode hs_kind
       ; zonkTcType kind }
         -- This zonk is very important in the case of higher rank kinds
         -- E.g. Trac #13879    f :: forall (p :: forall z (y::z). <blah>).
         --                          <more blah>
         --      When instantiating p's kind at occurrences of p in <more blah>
         --      it's crucial that the kind we instantiate is fully zonked,
         --      else we may fail to substitute properly

tc_lhs_kind :: TcTyMode -> LHsKind GhcRn -> 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))
    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))

-- See Note [Free-floating kind vars]
reportFloatingKvs :: Name         -- of the tycon
                  -> TyConFlavour -- What sort of TyCon it is
                  -> [TcTyVar]    -- all tyvars, not necessarily zonked
                  -> [TcTyVar]    -- floating tyvars
                  -> TcM ()
reportFloatingKvs tycon_name flav all_tvs bad_tvs
  = unless (null bad_tvs) $  -- don't bother zonking if there's no error
    do { all_tvs <- mapM zonkTcTyVarToTyVar all_tvs
       ; bad_tvs <- mapM zonkTcTyVarToTyVar bad_tvs
       ; let (tidy_env, tidy_all_tvs) = tidyOpenTyCoVars emptyTidyEnv all_tvs
             tidy_bad_tvs             = map (tidyTyVarOcc tidy_env) bad_tvs
       ; typeintype <- xoptM LangExt.TypeInType
       ; mapM_ (report typeintype tidy_all_tvs) tidy_bad_tvs }
    report typeintype tidy_all_tvs tidy_bad_tv
      = addErr $
        vcat [ text "Kind variable" <+> quotes (ppr tidy_bad_tv) <+>
               text "is implicitly bound in" <+> ppr flav
             , quotes (ppr tycon_name) <> comma <+>
               text "but does not appear as the kind of any"
             , text "of its type variables. Perhaps you meant"
             , text "to bind it" <+> ppWhen (not typeintype)
                                            (text "(with TypeInType)") <+>
                                 text "explicitly somewhere?"
             , ppWhen (not (null tidy_all_tvs)) $
                 hang (text "Type variables with inferred kinds:")
                 2 (ppr_tv_bndrs tidy_all_tvs) ]

    ppr_tv_bndrs tvs = sep (map pp_tv tvs)
    pp_tv tv         = parens (ppr tv <+> dcolon <+> ppr (tyVarKind tv))