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

    \section[RnSource]{Main pass of renamer}
    -}

    {-# LANGUAGE CPP #-}

    module RnTypes (
            -- Type related stuff
            rnHsType, rnLHsType, rnLHsTypes, rnContext,
            rnHsKind, rnLHsKind, rnLHsMaybeKind,
            rnHsSigType, rnLHsInstType, rnConDeclFields,
            newTyVarNameRn,

            -- Precence related stuff
            mkOpAppRn, mkNegAppRn, mkOpFormRn, mkConOpPatRn,
            checkPrecMatch, checkSectionPrec,

            -- Binding related stuff
            warnContextQuantification, warnUnusedForAlls,
            bindSigTyVarsFV, bindHsTyVars, rnHsBndrSig,
            extractHsTyRdrTyVars, extractHsTysRdrTyVars,
            extractRdrKindSigVars, extractDataDefnKindVars,
            extractWildcards, filterInScope
      ) where

    import {-# SOURCE #-} TcSplice( runQuasiQuoteType )
    import {-# SOURCE #-} RnSplice( rnSpliceType )

    import DynFlags
    import HsSyn
    import RnHsDoc          ( rnLHsDoc, rnMbLHsDoc )
    import RnEnv
    import TcRnMonad
    import RdrName
    import PrelNames
    import TysPrim          ( funTyConName )
    import Name
    import SrcLoc
    import NameSet

    import Util
    import BasicTypes       ( compareFixity, funTyFixity, negateFixity,
                              Fixity(..), FixityDirection(..) )
    import Outputable
    import FastString
    import Maybes
    import Data.List        ( nub, nubBy )
    import Control.Monad    ( unless, when )

    #include "HsVersions.h"

    {-
    These type renamers are in a separate module, rather than in (say) RnSource,
    to break several loop.

    *********************************************************
    *                                                      *
    \subsection{Renaming types}
    *                                                      *
    *********************************************************
    -}

    rnHsSigType :: SDoc -> LHsType RdrName -> RnM (LHsType Name, FreeVars)
            -- rnHsSigType is used for source-language type signatures,
            -- which use *implicit* universal quantification.
    rnHsSigType doc_str ty = rnLHsType (TypeSigCtx doc_str) ty

    rnLHsInstType :: SDoc -> LHsType RdrName -> RnM (LHsType Name, FreeVars)
    -- Rename the type in an instance or standalone deriving decl
    rnLHsInstType doc_str ty
      = do { (ty', fvs) <- rnLHsType (GenericCtx doc_str) ty
           ; unless good_inst_ty (addErrAt (getLoc ty) (badInstTy ty))
           ; return (ty', fvs) }
      where
        good_inst_ty
          | Just (_, _, L _ cls, _) <-
                            splitLHsInstDeclTy_maybe (flattenTopLevelLHsForAllTy ty)
          , isTcOcc (rdrNameOcc cls) = True
          | otherwise                = False

    badInstTy :: LHsType RdrName -> SDoc
    badInstTy ty = ptext (sLit "Malformed instance:") <+> ppr ty

    {-
    rnHsType is here because we call it from loadInstDecl, and I didn't
    want a gratuitous knot.

    Note [Context quantification]
    -----------------------------
    Variables in type signatures are implicitly quantified
    when (1) they are in a type signature not beginning
    with "forall" or (2) in any qualified type T => R.
    We are phasing out (2) since it leads to inconsistencies
    (Trac #4426):

    data A = A (a -> a)           is an error
    data A = A (Eq a => a -> a)   binds "a"
    data A = A (Eq a => a -> b)   binds "a" and "b"
    data A = A (() => a -> b)     binds "a" and "b"
    f :: forall a. a -> b         is an error
    f :: forall a. () => a -> b   is an error
    f :: forall a. a -> (() => b) binds "a" and "b"

    The -fwarn-context-quantification flag warns about
    this situation. See rnHsTyKi for case HsForAllTy Qualified.
    -}

    rnLHsTyKi  :: Bool --  True <=> renaming a type, False <=> a kind
               -> HsDocContext -> LHsType RdrName -> RnM (LHsType Name, FreeVars)
    rnLHsTyKi isType doc (L loc ty)
      = setSrcSpan loc $
        do { (ty', fvs) <- rnHsTyKi isType doc ty
           ; return (L loc ty', fvs) }

    rnLHsType  :: HsDocContext -> LHsType RdrName -> RnM (LHsType Name, FreeVars)
    rnLHsType = rnLHsTyKi True

    rnLHsKind  :: HsDocContext -> LHsKind RdrName -> RnM (LHsKind Name, FreeVars)
    rnLHsKind = rnLHsTyKi False

    rnLHsMaybeKind  :: HsDocContext -> Maybe (LHsKind RdrName)
                    -> RnM (Maybe (LHsKind Name), FreeVars)
    rnLHsMaybeKind _ Nothing
      = return (Nothing, emptyFVs)
    rnLHsMaybeKind doc (Just kind)
      = do { (kind', fvs) <- rnLHsKind doc kind
           ; return (Just kind', fvs) }

    rnHsType  :: HsDocContext -> HsType RdrName -> RnM (HsType Name, FreeVars)
    rnHsType = rnHsTyKi True
    rnHsKind  :: HsDocContext -> HsKind RdrName -> RnM (HsKind Name, FreeVars)
    rnHsKind = rnHsTyKi False

    rnHsTyKi :: Bool -> HsDocContext -> HsType RdrName -> RnM (HsType Name, FreeVars)

    rnHsTyKi isType doc ty@HsForAllTy{}
      = rnHsTyKiForAll isType doc (flattenTopLevelHsForAllTy ty)

    rnHsTyKi isType _ (HsTyVar rdr_name)
      = do { name <- rnTyVar isType rdr_name
           ; return (HsTyVar name, unitFV name) }

    -- If we see (forall a . ty), without foralls on, the forall will give
    -- a sensible error message, but we don't want to complain about the dot too
    -- Hence the jiggery pokery with ty1
    rnHsTyKi isType doc ty@(HsOpTy ty1 (wrapper, L loc op) ty2)
      = ASSERT( isType ) setSrcSpan loc $
        do  { ops_ok <- xoptM Opt_TypeOperators
            ; op' <- if ops_ok
                     then rnTyVar isType op
                     else do { addErr (opTyErr op ty)
                             ; return (mkUnboundName op) }  -- Avoid double complaint
            ; let l_op' = L loc op'
            ; fix <- lookupTyFixityRn l_op'
            ; (ty1', fvs1) <- rnLHsType doc ty1
            ; (ty2', fvs2) <- rnLHsType doc ty2
            ; res_ty <- mkHsOpTyRn (\t1 t2 -> HsOpTy t1 (wrapper, l_op') t2)
                                   op' fix ty1' ty2'
            ; return (res_ty, (fvs1 `plusFV` fvs2) `addOneFV` op') }

    rnHsTyKi isType doc (HsParTy ty)
      = do { (ty', fvs) <- rnLHsTyKi isType doc ty
           ; return (HsParTy ty', fvs) }

    rnHsTyKi isType doc (HsBangTy b ty)
      = ASSERT( isType )
        do { (ty', fvs) <- rnLHsType doc ty
           ; return (HsBangTy b ty', fvs) }

    rnHsTyKi _ doc ty@(HsRecTy flds)
      = do { addErr (hang (ptext (sLit "Record syntax is illegal here:"))
                        2 (ppr ty))
           ; (flds', fvs) <- rnConDeclFields doc flds
           ; return (HsRecTy flds', fvs) }

    rnHsTyKi isType doc (HsFunTy ty1 ty2)
      = do { (ty1', fvs1) <- rnLHsTyKi isType doc ty1
            -- Might find a for-all as the arg of a function type
           ; (ty2', fvs2) <- rnLHsTyKi isType doc ty2
            -- Or as the result.  This happens when reading Prelude.hi
            -- when we find return :: forall m. Monad m -> forall a. a -> m a

            -- Check for fixity rearrangements
           ; res_ty <- if isType
                       then mkHsOpTyRn HsFunTy funTyConName funTyFixity ty1' ty2'
                       else return (HsFunTy ty1' ty2')
           ; return (res_ty, fvs1 `plusFV` fvs2) }

    rnHsTyKi isType doc listTy@(HsListTy ty)
      = do { data_kinds <- xoptM Opt_DataKinds
           ; unless (data_kinds || isType) (addErr (dataKindsErr isType listTy))
           ; (ty', fvs) <- rnLHsTyKi isType doc ty
           ; return (HsListTy ty', fvs) }

    rnHsTyKi isType doc (HsKindSig ty k)
      = ASSERT( isType )
        do { kind_sigs_ok <- xoptM Opt_KindSignatures
           ; unless kind_sigs_ok (badSigErr False doc ty)
           ; (ty', fvs1) <- rnLHsType doc ty
           ; (k', fvs2) <- rnLHsKind doc k
           ; return (HsKindSig ty' k', fvs1 `plusFV` fvs2) }

    rnHsTyKi isType doc (HsPArrTy ty)
      = ASSERT( isType )
        do { (ty', fvs) <- rnLHsType doc ty
           ; return (HsPArrTy ty', fvs) }

    -- Unboxed tuples are allowed to have poly-typed arguments.  These
    -- sometimes crop up as a result of CPR worker-wrappering dictionaries.
    rnHsTyKi isType doc tupleTy@(HsTupleTy tup_con tys)
      = do { data_kinds <- xoptM Opt_DataKinds
           ; unless (data_kinds || isType) (addErr (dataKindsErr isType tupleTy))
           ; (tys', fvs) <- mapFvRn (rnLHsTyKi isType doc) tys
           ; return (HsTupleTy tup_con tys', fvs) }

    -- Ensure that a type-level integer is nonnegative (#8306, #8412)
    rnHsTyKi isType _ tyLit@(HsTyLit t)
      = do { data_kinds <- xoptM Opt_DataKinds
           ; unless data_kinds (addErr (dataKindsErr isType tyLit))
           ; when (negLit t) (addErr negLitErr)
           ; return (HsTyLit t, emptyFVs) }
      where
        negLit (HsStrTy _ _) = False
        negLit (HsNumTy _ i) = i < 0
        negLitErr = ptext (sLit "Illegal literal in type (type literals must not be negative):") <+> ppr tyLit

    rnHsTyKi isType doc (HsAppTy ty1 ty2)
      = do { (ty1', fvs1) <- rnLHsTyKi isType doc ty1
           ; (ty2', fvs2) <- rnLHsTyKi isType doc ty2
           ; return (HsAppTy ty1' ty2', fvs1 `plusFV` fvs2) }

    rnHsTyKi isType doc (HsIParamTy n ty)
      = ASSERT( isType )
        do { (ty', fvs) <- rnLHsType doc ty
           ; return (HsIParamTy n ty', fvs) }

    rnHsTyKi isType doc (HsEqTy ty1 ty2)
      = ASSERT( isType )
        do { (ty1', fvs1) <- rnLHsType doc ty1
           ; (ty2', fvs2) <- rnLHsType doc ty2
           ; return (HsEqTy ty1' ty2', fvs1 `plusFV` fvs2) }

    rnHsTyKi isType _ (HsSpliceTy sp k)
      = ASSERT( isType )
        rnSpliceType sp k

    rnHsTyKi isType doc (HsDocTy ty haddock_doc)
      = ASSERT( isType )
        do { (ty', fvs) <- rnLHsType doc ty
           ; haddock_doc' <- rnLHsDoc haddock_doc
           ; return (HsDocTy ty' haddock_doc', fvs) }

    rnHsTyKi isType doc (HsQuasiQuoteTy qq)
      = ASSERT( isType )
        do { ty <- runQuasiQuoteType qq
             -- Wrap the result of the quasi-quoter in parens so that we don't
             -- lose the outermost location set by runQuasiQuote (#7918)
           ; rnHsType doc (HsParTy ty) }

    rnHsTyKi isType _ (HsCoreTy ty)
      = ASSERT( isType )
        return (HsCoreTy ty, emptyFVs)
        -- The emptyFVs probably isn't quite right
        -- but I don't think it matters

    rnHsTyKi _ _ (HsWrapTy {})
      = panic "rnHsTyKi"

    rnHsTyKi isType doc ty@(HsExplicitListTy k tys)
      = ASSERT( isType )
        do { data_kinds <- xoptM Opt_DataKinds
           ; unless data_kinds (addErr (dataKindsErr isType ty))
           ; (tys', fvs) <- rnLHsTypes doc tys
           ; return (HsExplicitListTy k tys', fvs) }

    rnHsTyKi isType doc ty@(HsExplicitTupleTy kis tys)
      = ASSERT( isType )
        do { data_kinds <- xoptM Opt_DataKinds
           ; unless data_kinds (addErr (dataKindsErr isType ty))
           ; (tys', fvs) <- rnLHsTypes doc tys
           ; return (HsExplicitTupleTy kis tys', fvs) }

    rnHsTyKi _ _ HsWildcardTy = panic "rnHsTyKi HsWildcardTy"
                                -- Should be replaced by a HsNamedWildcardTy

    rnHsTyKi isType _doc (HsNamedWildcardTy rdr_name)
      = ASSERT( isType )
        do { name <- rnTyVar isType rdr_name
           ; return (HsNamedWildcardTy name, unitFV name) }

    --------------
    rnHsTyKiForAll :: Bool -> HsDocContext -> HsType RdrName
                   -> RnM (HsType Name, FreeVars)
    rnHsTyKiForAll isType doc (HsForAllTy Implicit extra _ lctxt@(L _ ctxt) ty)
      = ASSERT( isType ) do
            -- Implicit quantifiction in source code (no kinds on tyvars)
            -- Given the signature  C => T  we universally quantify
            -- over FV(T) \ {in-scope-tyvars}
        rdr_env <- getLocalRdrEnv
        loc <- getSrcSpanM
        let
            (forall_kvs, forall_tvs) = filterInScope rdr_env $
                                       extractHsTysRdrTyVars (ty:ctxt)
               -- In for-all types we don't bring in scope
               -- kind variables mentioned in kind signatures
               -- (Well, not yet anyway....)
               --    f :: Int -> T (a::k)    -- Not allowed

               -- The filterInScope is to ensure that we don't quantify over
               -- type variables that are in scope; when GlasgowExts is off,
               -- there usually won't be any, except for class signatures:
               --   class C a where { op :: a -> a }
            tyvar_bndrs = userHsTyVarBndrs loc forall_tvs

        rnForAll doc Implicit extra forall_kvs (mkHsQTvs tyvar_bndrs) lctxt ty

    rnHsTyKiForAll isType doc
                   fulltype@(HsForAllTy Qualified extra _ lctxt@(L _ ctxt) ty)
      = ASSERT( isType ) do
        rdr_env <- getLocalRdrEnv
        loc <- getSrcSpanM
        let
            (forall_kvs, forall_tvs) = filterInScope rdr_env $
                                       extractHsTysRdrTyVars (ty:ctxt)
            tyvar_bndrs = userHsTyVarBndrs loc forall_tvs
            in_type_doc = ptext (sLit "In the type") <+> quotes (ppr fulltype)

        -- See Note [Context quantification]
        warnContextQuantification (in_type_doc $$ docOfHsDocContext doc) tyvar_bndrs
        rnForAll doc Implicit extra forall_kvs (mkHsQTvs tyvar_bndrs) lctxt ty

    rnHsTyKiForAll isType doc
                   ty@(HsForAllTy Explicit extra forall_tyvars lctxt@(L _ ctxt) tau)
      = ASSERT( isType ) do {      -- Explicit quantification.
             -- Check that the forall'd tyvars are actually
             -- mentioned in the type, and produce a warning if not
             let (kvs, mentioned) = extractHsTysRdrTyVars (tau:ctxt)
                 in_type_doc = ptext (sLit "In the type") <+> quotes (ppr ty)
           ; warnUnusedForAlls (in_type_doc $$ docOfHsDocContext doc)
                               forall_tyvars mentioned
           ; traceRn (text "rnHsTyKiForAll:Exlicit" <+> vcat
                [ppr forall_tyvars, ppr lctxt,ppr tau ])
           ; rnForAll doc Explicit extra kvs forall_tyvars lctxt tau }

    -- The following should never happen but keeps the completeness checker happy
    rnHsTyKiForAll isType doc ty = rnHsTyKi isType doc ty
    --------------
    rnTyVar :: Bool -> RdrName -> RnM Name
    rnTyVar is_type rdr_name
      | is_type   = lookupTypeOccRn rdr_name
      | otherwise = lookupKindOccRn rdr_name

    --------------
    rnLHsTypes :: HsDocContext -> [LHsType RdrName]
               -> RnM ([LHsType Name], FreeVars)
    rnLHsTypes doc tys = mapFvRn (rnLHsType doc) tys

    rnForAll :: HsDocContext -> HsExplicitFlag
             -> Maybe SrcSpan           -- Location of an extra-constraints wildcard
             -> [RdrName]               -- Kind variables
             -> LHsTyVarBndrs RdrName   -- Type variables
             -> LHsContext RdrName -> LHsType RdrName
             -> RnM (HsType Name, FreeVars)

    rnForAll doc exp extra kvs forall_tyvars ctxt ty
      | null kvs, null (hsQTvBndrs forall_tyvars), null (unLoc ctxt), isNothing extra
      = rnHsType doc (unLoc ty)
            -- One reason for this case is that a type like Int#
            -- starts off as (HsForAllTy Implicit Nothing [] Int), in case
            -- there is some quantification.  Now that we have quantified
            -- and discovered there are no type variables, it's nicer to turn
            -- it into plain Int.  If it were Int# instead of Int, we'd actually
            -- get an error, because the body of a genuine for-all is
            -- of kind *.

      | otherwise
      = bindHsTyVars doc Nothing kvs forall_tyvars $ \ new_tyvars ->
        do { (new_ctxt, fvs1) <- rnContext doc ctxt
           ; (new_ty, fvs2) <- rnLHsType doc ty
           ; return (HsForAllTy exp extra new_tyvars new_ctxt new_ty, fvs1 `plusFV` fvs2) }
            -- Retain the same implicit/explicit flag as before
            -- so that we can later print it correctly

    ---------------
    bindSigTyVarsFV :: [Name]
                    -> RnM (a, FreeVars)
                    -> RnM (a, FreeVars)
    -- Used just before renaming the defn of a function
    -- with a separate type signature, to bring its tyvars into scope
    -- With no -XScopedTypeVariables, this is a no-op
    bindSigTyVarsFV tvs thing_inside
      = do  { scoped_tyvars <- xoptM Opt_ScopedTypeVariables
            ; if not scoped_tyvars then
                    thing_inside
              else
                    bindLocalNamesFV tvs thing_inside }

    ---------------
    bindHsTyVars :: HsDocContext
                 -> Maybe a                 -- Just _  => an associated type decl
                 -> [RdrName]               -- Kind variables from scope
                 -> LHsTyVarBndrs RdrName   -- Type variables
                 -> (LHsTyVarBndrs Name -> RnM (b, FreeVars))
                 -> RnM (b, FreeVars)
    -- (a) Bring kind variables into scope
    --     both (i)  passed in (kv_bndrs)
    --     and  (ii) mentioned in the kinds of tv_bndrs
    -- (b) Bring type variables into scope
    bindHsTyVars doc mb_assoc kv_bndrs tv_bndrs thing_inside
      = do { rdr_env <- getLocalRdrEnv
           ; let tvs = hsQTvBndrs tv_bndrs
                 kvs_from_tv_bndrs = [ kv | L _ (KindedTyVar _ kind) <- tvs
                                     , let (_, kvs) = extractHsTyRdrTyVars kind
                                     , kv <- kvs ]
                 all_kvs' = nub (kv_bndrs ++ kvs_from_tv_bndrs)
                 all_kvs  = filterOut (`elemLocalRdrEnv` rdr_env) all_kvs'

                 overlap_kvs = [ kv | kv <- all_kvs, any ((==) kv . hsLTyVarName) tvs ]
                    -- These variables appear both as kind and type variables
                    -- in the same declaration; eg  type family  T (x :: *) (y :: x)
                    -- We disallow this: too confusing!

           ; poly_kind <- xoptM Opt_PolyKinds
           ; unless (poly_kind || null all_kvs)
                    (addErr (badKindBndrs doc all_kvs))
           ; unless (null overlap_kvs)
                    (addErr (overlappingKindVars doc overlap_kvs))

           ; loc <- getSrcSpanM
           ; kv_names <- mapM (newLocalBndrRn . L loc) all_kvs
           ; bindLocalNamesFV kv_names $
        do { let tv_names_w_loc = hsLTyVarLocNames tv_bndrs

                 rn_tv_bndr :: LHsTyVarBndr RdrName -> RnM (LHsTyVarBndr Name, FreeVars)
                 rn_tv_bndr (L loc (UserTyVar rdr))
                   = do { nm <- newTyVarNameRn mb_assoc rdr_env loc rdr
                        ; return (L loc (UserTyVar nm), emptyFVs) }
                 rn_tv_bndr (L loc (KindedTyVar (L lv rdr) kind))
                   = do { sig_ok <- xoptM Opt_KindSignatures
                        ; unless sig_ok (badSigErr False doc kind)
                        ; nm <- newTyVarNameRn mb_assoc rdr_env loc rdr
                        ; (kind', fvs) <- rnLHsKind doc kind
                        ; return (L loc (KindedTyVar (L lv nm) kind'), fvs) }

           -- Check for duplicate or shadowed tyvar bindrs
           ; checkDupRdrNames tv_names_w_loc
           ; when (isNothing mb_assoc) (checkShadowedRdrNames tv_names_w_loc)

           ; (tv_bndrs', fvs1) <- mapFvRn rn_tv_bndr tvs
           ; (res, fvs2) <- bindLocalNamesFV (map hsLTyVarName tv_bndrs') $
                            do { inner_rdr_env <- getLocalRdrEnv
                               ; traceRn (text "bhtv" <+> vcat
                                     [ ppr tvs, ppr kv_bndrs, ppr kvs_from_tv_bndrs
                                     , ppr $ map (`elemLocalRdrEnv` rdr_env) all_kvs'
                                     , ppr $ map (getUnique . rdrNameOcc) all_kvs'
                                     , ppr all_kvs, ppr rdr_env, ppr inner_rdr_env ])
                               ; thing_inside (HsQTvs { hsq_tvs = tv_bndrs', hsq_kvs = kv_names }) }
           ; return (res, fvs1 `plusFV` fvs2) } }

    newTyVarNameRn :: Maybe a -> LocalRdrEnv -> SrcSpan -> RdrName -> RnM Name
    newTyVarNameRn mb_assoc rdr_env loc rdr
      | Just _ <- mb_assoc    -- Use the same Name as the parent class decl
      , Just n <- lookupLocalRdrEnv rdr_env rdr
      = return n
      | otherwise
      = newLocalBndrRn (L loc rdr)

    --------------------------------
    rnHsBndrSig :: HsDocContext
                -> HsWithBndrs RdrName (LHsType RdrName)
                -> (HsWithBndrs Name (LHsType Name) -> RnM (a, FreeVars))
                -> RnM (a, FreeVars)
    rnHsBndrSig doc (HsWB { hswb_cts = ty@(L loc _) }) thing_inside
      = do { sig_ok <- xoptM Opt_ScopedTypeVariables
           ; unless sig_ok (badSigErr True doc ty)
           ; let (kv_bndrs, tv_bndrs) = extractHsTyRdrTyVars ty
           ; name_env <- getLocalRdrEnv
           ; tv_names <- newLocalBndrsRn [L loc tv | tv <- tv_bndrs
                                                   , not (tv `elemLocalRdrEnv` name_env) ]
           ; kv_names <- newLocalBndrsRn [L loc kv | kv <- kv_bndrs
                                                   , not (kv `elemLocalRdrEnv` name_env) ]
           ; (wcs, ty') <- extractWildcards ty
           ; bindLocalNamesFV kv_names $
             bindLocalNamesFV tv_names $
             bindLocatedLocalsFV wcs $ \wcs_new ->
        do { (ty'', fvs1) <- rnLHsType doc ty'
           ; (res, fvs2) <- thing_inside (HsWB { hswb_cts = ty'', hswb_kvs = kv_names,
                                                 hswb_tvs = tv_names, hswb_wcs = wcs_new })
           ; return (res, fvs1 `plusFV` fvs2) } }

    overlappingKindVars :: HsDocContext -> [RdrName] -> SDoc
    overlappingKindVars doc kvs
      = vcat [ ptext (sLit "Kind variable") <> plural kvs <+>
               ptext (sLit "also used as type variable") <> plural kvs
               <> colon <+> pprQuotedList kvs
             , docOfHsDocContext doc ]

    badKindBndrs :: HsDocContext -> [RdrName] -> SDoc
    badKindBndrs doc kvs
      = vcat [ hang (ptext (sLit "Unexpected kind variable") <> plural kvs
                     <+> pprQuotedList kvs)
                  2 (ptext (sLit "Perhaps you intended to use PolyKinds"))
             , docOfHsDocContext doc ]

    badSigErr :: Bool -> HsDocContext -> LHsType RdrName -> TcM ()
    badSigErr is_type doc (L loc ty)
      = setSrcSpan loc $ addErr $
        vcat [ hang (ptext (sLit "Illegal") <+> what
                     <+> ptext (sLit "signature:") <+> quotes (ppr ty))
                  2 (ptext (sLit "Perhaps you intended to use") <+> flag)
             , docOfHsDocContext doc ]
      where
        what | is_type   = ptext (sLit "type")
             | otherwise = ptext (sLit "kind")
        flag | is_type   = ptext (sLit "ScopedTypeVariables")
             | otherwise = ptext (sLit "KindSignatures")

    dataKindsErr :: Bool -> HsType RdrName -> SDoc
    dataKindsErr is_type thing
      = hang (ptext (sLit "Illegal") <+> what <> colon <+> quotes (ppr thing))
           2 (ptext (sLit "Perhaps you intended to use DataKinds"))
      where
        what | is_type   = ptext (sLit "type")
             | otherwise = ptext (sLit "kind")

    {-
    *********************************************************
    *                                                      *
    \subsection{Contexts and predicates}
    *                                                      *
    *********************************************************
    -}

    rnConDeclFields :: HsDocContext -> [LConDeclField RdrName]
                    -> RnM ([LConDeclField Name], FreeVars)
    rnConDeclFields doc fields = mapFvRn (rnField doc) fields

    rnField :: HsDocContext -> LConDeclField RdrName
            -> RnM (LConDeclField Name, FreeVars)
    rnField doc (L l (ConDeclField names ty haddock_doc))
      = do { new_names <- mapM lookupLocatedTopBndrRn names
           ; (new_ty, fvs) <- rnLHsType doc ty
           ; new_haddock_doc <- rnMbLHsDoc haddock_doc
           ; return (L l (ConDeclField new_names new_ty new_haddock_doc), fvs) }

    rnContext :: HsDocContext -> LHsContext RdrName -> RnM (LHsContext Name, FreeVars)
    rnContext doc (L loc cxt)
      = do { (cxt', fvs) <- rnLHsTypes doc cxt
           ; return (L loc cxt', fvs) }

    {-
    ************************************************************************
    *                                                                      *
            Fixities and precedence parsing
    *                                                                      *
    ************************************************************************

    @mkOpAppRn@ deals with operator fixities.  The argument expressions
    are assumed to be already correctly arranged.  It needs the fixities
    recorded in the OpApp nodes, because fixity info applies to the things
    the programmer actually wrote, so you can't find it out from the Name.

    Furthermore, the second argument is guaranteed not to be another
    operator application.  Why? Because the parser parses all
    operator appications left-associatively, EXCEPT negation, which
    we need to handle specially.
    Infix types are read in a *right-associative* way, so that
            a `op` b `op` c
    is always read in as
            a `op` (b `op` c)

    mkHsOpTyRn rearranges where necessary.  The two arguments
    have already been renamed and rearranged.  It's made rather tiresome
    by the presence of ->, which is a separate syntactic construct.
    -}

    ---------------
    -- Building (ty1 `op1` (ty21 `op2` ty22))
    mkHsOpTyRn :: (LHsType Name -> LHsType Name -> HsType Name)
               -> Name -> Fixity -> LHsType Name -> LHsType Name
               -> RnM (HsType Name)

    mkHsOpTyRn mk1 pp_op1 fix1 ty1 (L loc2 (HsOpTy ty21 (w2, op2) ty22))
      = do  { fix2 <- lookupTyFixityRn op2
            ; mk_hs_op_ty mk1 pp_op1 fix1 ty1
                          (\t1 t2 -> HsOpTy t1 (w2, op2) t2)
                          (unLoc op2) fix2 ty21 ty22 loc2 }

    mkHsOpTyRn mk1 pp_op1 fix1 ty1 (L loc2 (HsFunTy ty21 ty22))
      = mk_hs_op_ty mk1 pp_op1 fix1 ty1
                    HsFunTy funTyConName funTyFixity ty21 ty22 loc2

    mkHsOpTyRn mk1 _ _ ty1 ty2              -- Default case, no rearrangment
      = return (mk1 ty1 ty2)

    ---------------
    mk_hs_op_ty :: (LHsType Name -> LHsType Name -> HsType Name)
                -> Name -> Fixity -> LHsType Name
                -> (LHsType Name -> LHsType Name -> HsType Name)
                -> Name -> Fixity -> LHsType Name -> LHsType Name -> SrcSpan
                -> RnM (HsType Name)
    mk_hs_op_ty mk1 op1 fix1 ty1
                mk2 op2 fix2 ty21 ty22 loc2
      | nofix_error     = do { precParseErr (op1,fix1) (op2,fix2)
                             ; return (mk1 ty1 (L loc2 (mk2 ty21 ty22))) }
      | associate_right = return (mk1 ty1 (L loc2 (mk2 ty21 ty22)))
      | otherwise       = do { -- Rearrange to ((ty1 `op1` ty21) `op2` ty22)
                               new_ty <- mkHsOpTyRn mk1 op1 fix1 ty1 ty21
                             ; return (mk2 (noLoc new_ty) ty22) }
      where
        (nofix_error, associate_right) = compareFixity fix1 fix2

    ---------------------------
    mkOpAppRn :: LHsExpr Name                       -- Left operand; already rearranged
              -> LHsExpr Name -> Fixity             -- Operator and fixity
              -> LHsExpr Name                       -- Right operand (not an OpApp, but might
                                                    -- be a NegApp)
              -> RnM (HsExpr Name)

    -- (e11 `op1` e12) `op2` e2
    mkOpAppRn e1@(L _ (OpApp e11 op1 fix1 e12)) op2 fix2 e2
      | nofix_error
      = do precParseErr (get_op op1,fix1) (get_op op2,fix2)
           return (OpApp e1 op2 fix2 e2)

      | associate_right = do
        new_e <- mkOpAppRn e12 op2 fix2 e2
        return (OpApp e11 op1 fix1 (L loc' new_e))
      where
        loc'= combineLocs e12 e2
        (nofix_error, associate_right) = compareFixity fix1 fix2

    ---------------------------
    --      (- neg_arg) `op` e2
    mkOpAppRn e1@(L _ (NegApp neg_arg neg_name)) op2 fix2 e2
      | nofix_error
      = do precParseErr (negateName,negateFixity) (get_op op2,fix2)
           return (OpApp e1 op2 fix2 e2)

      | associate_right
      = do new_e <- mkOpAppRn neg_arg op2 fix2 e2
           return (NegApp (L loc' new_e) neg_name)
      where
        loc' = combineLocs neg_arg e2
        (nofix_error, associate_right) = compareFixity negateFixity fix2

    ---------------------------
    --      e1 `op` - neg_arg
    mkOpAppRn e1 op1 fix1 e2@(L _ (NegApp _ _))     -- NegApp can occur on the right
      | not associate_right                 -- We *want* right association
      = do precParseErr (get_op op1, fix1) (negateName, negateFixity)
           return (OpApp e1 op1 fix1 e2)
      where
        (_, associate_right) = compareFixity fix1 negateFixity

    ---------------------------
    --      Default case
    mkOpAppRn e1 op fix e2                  -- Default case, no rearrangment
      = ASSERT2( right_op_ok fix (unLoc e2),
                 ppr e1 $$ text "---" $$ ppr op $$ text "---" $$ ppr fix $$ text "---" $$ ppr e2
        )
        return (OpApp e1 op fix e2)

    ----------------------------
    get_op :: LHsExpr Name -> Name
    get_op (L _ (HsVar n)) = n
    get_op other           = pprPanic "get_op" (ppr other)

    -- Parser left-associates everything, but
    -- derived instances may have correctly-associated things to
    -- in the right operarand.  So we just check that the right operand is OK
    right_op_ok :: Fixity -> HsExpr Name -> Bool
    right_op_ok fix1 (OpApp _ _ fix2 _)
      = not error_please && associate_right
      where
        (error_please, associate_right) = compareFixity fix1 fix2
    right_op_ok _ _
      = True

    -- Parser initially makes negation bind more tightly than any other operator
    -- And "deriving" code should respect this (use HsPar if not)
    mkNegAppRn :: LHsExpr id -> SyntaxExpr id -> RnM (HsExpr id)
    mkNegAppRn neg_arg neg_name
      = ASSERT( not_op_app (unLoc neg_arg) )
        return (NegApp neg_arg neg_name)

    not_op_app :: HsExpr id -> Bool
    not_op_app (OpApp _ _ _ _) = False
    not_op_app _               = True

    ---------------------------
    mkOpFormRn :: LHsCmdTop Name            -- Left operand; already rearranged
              -> LHsExpr Name -> Fixity     -- Operator and fixity
              -> LHsCmdTop Name             -- Right operand (not an infix)
              -> RnM (HsCmd Name)

    -- (e11 `op1` e12) `op2` e2
    mkOpFormRn a1@(L loc (HsCmdTop (L _ (HsCmdArrForm op1 (Just fix1) [a11,a12])) _ _ _))
            op2 fix2 a2
      | nofix_error
      = do precParseErr (get_op op1,fix1) (get_op op2,fix2)
           return (HsCmdArrForm op2 (Just fix2) [a1, a2])

      | associate_right
      = do new_c <- mkOpFormRn a12 op2 fix2 a2
           return (HsCmdArrForm op1 (Just fix1)
                   [a11, L loc (HsCmdTop (L loc new_c)
                   placeHolderType placeHolderType [])])
            -- TODO: locs are wrong
      where
        (nofix_error, associate_right) = compareFixity fix1 fix2

    --      Default case
    mkOpFormRn arg1 op fix arg2                     -- Default case, no rearrangment
      = return (HsCmdArrForm op (Just fix) [arg1, arg2])

    --------------------------------------
    mkConOpPatRn :: Located Name -> Fixity -> LPat Name -> LPat Name
                 -> RnM (Pat Name)

    mkConOpPatRn op2 fix2 p1@(L loc (ConPatIn op1 (InfixCon p11 p12))) p2
      = do  { fix1 <- lookupFixityRn (unLoc op1)
            ; let (nofix_error, associate_right) = compareFixity fix1 fix2

            ; if nofix_error then do
                    { precParseErr (unLoc op1,fix1) (unLoc op2,fix2)
                    ; return (ConPatIn op2 (InfixCon p1 p2)) }

              else if associate_right then do
                    { new_p <- mkConOpPatRn op2 fix2 p12 p2
                    ; return (ConPatIn op1 (InfixCon p11 (L loc new_p))) } -- XXX loc right?
              else return (ConPatIn op2 (InfixCon p1 p2)) }

    mkConOpPatRn op _ p1 p2                         -- Default case, no rearrangment
      = ASSERT( not_op_pat (unLoc p2) )
        return (ConPatIn op (InfixCon p1 p2))

    not_op_pat :: Pat Name -> Bool
    not_op_pat (ConPatIn _ (InfixCon _ _)) = False
    not_op_pat _                           = True

    --------------------------------------
    checkPrecMatch :: Name -> MatchGroup Name body -> RnM ()
      -- Check precedence of a function binding written infix
      --   eg  a `op` b `C` c = ...
      -- See comments with rnExpr (OpApp ...) about "deriving"

    checkPrecMatch op (MG { mg_alts = ms })
      = mapM_ check ms
      where
        check (L _ (Match _ (L l1 p1 : L l2 p2 :_) _ _))
          = setSrcSpan (combineSrcSpans l1 l2) $
            do checkPrec op p1 False
               checkPrec op p2 True

        check _ = return ()
            -- This can happen.  Consider
            --      a `op` True = ...
            --      op          = ...
            -- The infix flag comes from the first binding of the group
            -- but the second eqn has no args (an error, but not discovered
            -- until the type checker).  So we don't want to crash on the
            -- second eqn.

    checkPrec :: Name -> Pat Name -> Bool -> IOEnv (Env TcGblEnv TcLclEnv) ()
    checkPrec op (ConPatIn op1 (InfixCon _ _)) right = do
        op_fix@(Fixity op_prec  op_dir) <- lookupFixityRn op
        op1_fix@(Fixity op1_prec op1_dir) <- lookupFixityRn (unLoc op1)
        let
            inf_ok = op1_prec > op_prec ||
                     (op1_prec == op_prec &&
                      (op1_dir == InfixR && op_dir == InfixR && right ||
                       op1_dir == InfixL && op_dir == InfixL && not right))

            info  = (op,        op_fix)
            info1 = (unLoc op1, op1_fix)
            (infol, infor) = if right then (info, info1) else (info1, info)
        unless inf_ok (precParseErr infol infor)

    checkPrec _ _ _
      = return ()

    -- Check precedence of (arg op) or (op arg) respectively
    -- If arg is itself an operator application, then either
    --   (a) its precedence must be higher than that of op
    --   (b) its precedency & associativity must be the same as that of op
    checkSectionPrec :: FixityDirection -> HsExpr RdrName
            -> LHsExpr Name -> LHsExpr Name -> RnM ()
    checkSectionPrec direction section op arg
      = case unLoc arg of
            OpApp _ op fix _ -> go_for_it (get_op op) fix
            NegApp _ _       -> go_for_it negateName  negateFixity
            _                -> return ()
      where
        op_name = get_op op
        go_for_it arg_op arg_fix@(Fixity arg_prec assoc) = do
              op_fix@(Fixity op_prec _) <- lookupFixityRn op_name
              unless (op_prec < arg_prec
                      || (op_prec == arg_prec && direction == assoc))
                     (sectionPrecErr (op_name, op_fix)
                                     (arg_op, arg_fix) section)

    -- Precedence-related error messages

    precParseErr :: (Name, Fixity) -> (Name, Fixity) -> RnM ()
    precParseErr op1@(n1,_) op2@(n2,_)
      | isUnboundName n1 || isUnboundName n2
      = return ()     -- Avoid error cascade
      | otherwise
      = addErr $ hang (ptext (sLit "Precedence parsing error"))
          4 (hsep [ptext (sLit "cannot mix"), ppr_opfix op1, ptext (sLit "and"),
                   ppr_opfix op2,
                   ptext (sLit "in the same infix expression")])

    sectionPrecErr :: (Name, Fixity) -> (Name, Fixity) -> HsExpr RdrName -> RnM ()
    sectionPrecErr op@(n1,_) arg_op@(n2,_) section
      | isUnboundName n1 || isUnboundName n2
      = return ()     -- Avoid error cascade
      | otherwise
      = addErr $ vcat [ptext (sLit "The operator") <+> ppr_opfix op <+> ptext (sLit "of a section"),
             nest 4 (sep [ptext (sLit "must have lower precedence than that of the operand,"),
                          nest 2 (ptext (sLit "namely") <+> ppr_opfix arg_op)]),
             nest 4 (ptext (sLit "in the section:") <+> quotes (ppr section))]

    ppr_opfix :: (Name, Fixity) -> SDoc
    ppr_opfix (op, fixity) = pp_op <+> brackets (ppr fixity)
       where
         pp_op | op == negateName = ptext (sLit "prefix `-'")
               | otherwise        = quotes (ppr op)

    {-
    *********************************************************
    *                                                      *
    \subsection{Errors}
    *                                                      *
    *********************************************************
    -}

    warnUnusedForAlls :: SDoc -> LHsTyVarBndrs RdrName -> [RdrName] -> TcM ()
    warnUnusedForAlls in_doc bound mentioned_rdrs
      = whenWOptM Opt_WarnUnusedMatches $
        mapM_ add_warn bound_but_not_used
      where
        bound_names        = hsLTyVarLocNames bound
        bound_but_not_used = filterOut ((`elem` mentioned_rdrs) . unLoc) bound_names

        add_warn (L loc tv)
          = addWarnAt loc $
            vcat [ ptext (sLit "Unused quantified type variable") <+> quotes (ppr tv)
                 , in_doc ]

    warnContextQuantification :: SDoc -> [LHsTyVarBndr RdrName] -> TcM ()
    warnContextQuantification in_doc tvs
      = whenWOptM Opt_WarnContextQuantification $
        mapM_ add_warn tvs
      where
        add_warn (L loc tv)
          = addWarnAt loc $
            vcat [ ptext (sLit "Variable") <+> quotes (ppr tv) <+>
                   ptext (sLit "is implicitly quantified due to a context") $$
                   ptext (sLit "Use explicit forall syntax instead.") $$
                   ptext (sLit "This will become an error in GHC 7.12.")
                 , in_doc ]

    opTyErr :: RdrName -> HsType RdrName -> SDoc
    opTyErr op ty@(HsOpTy ty1 _ _)
      = hang (ptext (sLit "Illegal operator") <+> quotes (ppr op) <+> ptext (sLit "in type") <+> quotes (ppr ty))
             2 extra
      where
        extra | op == dot_tv_RDR && forall_head ty1
              = perhapsForallMsg
              | otherwise
              = ptext (sLit "Use TypeOperators to allow operators in types")

        forall_head (L _ (HsTyVar tv))   = tv == forall_tv_RDR
        forall_head (L _ (HsAppTy ty _)) = forall_head ty
        forall_head _other               = False
    opTyErr _ ty = pprPanic "opTyErr: Not an op" (ppr ty)

    {-
    ************************************************************************
    *                                                                      *
          Finding the free type variables of a (HsType RdrName)
    *                                                                    *
    ************************************************************************

    Note [Kind and type-variable binders]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    In a type signature we may implicitly bind type varaible and, more
    recently, kind variables.  For example:
      *   f :: a -> a
          f = ...
        Here we need to find the free type variables of (a -> a),
        so that we know what to quantify

      *   class C (a :: k) where ...
        This binds 'k' in ..., as well as 'a'

      *   f (x :: a -> [a]) = ....
        Here we bind 'a' in ....

      *   f (x :: T a -> T (b :: k)) = ...
        Here we bind both 'a' and the kind variable 'k'

      *   type instance F (T (a :: Maybe k)) = ...a...k...
        Here we want to constrain the kind of 'a', and bind 'k'.

    In general we want to walk over a type, and find
      * Its free type variables
      * The free kind variables of any kind signatures in the type

    Hence we returns a pair (kind-vars, type vars)
    See also Note [HsBSig binder lists] in HsTypes
    -}

    type FreeKiTyVars = ([RdrName], [RdrName])

    filterInScope :: LocalRdrEnv -> FreeKiTyVars -> FreeKiTyVars
    filterInScope rdr_env (kvs, tvs)
      = (filterOut in_scope kvs, filterOut in_scope tvs)
      where
        in_scope tv = tv `elemLocalRdrEnv` rdr_env

    extractHsTyRdrTyVars :: LHsType RdrName -> FreeKiTyVars
    -- extractHsTyRdrNames finds the free (kind, type) variables of a HsType
    --                        or the free (sort, kind) variables of a HsKind
    -- It's used when making the for-alls explicit.
    -- See Note [Kind and type-variable binders]
    extractHsTyRdrTyVars ty
      = case extract_lty ty ([],[]) of
         (kvs, tvs) -> (nub kvs, nub tvs)

    extractHsTysRdrTyVars :: [LHsType RdrName] -> FreeKiTyVars
    -- See Note [Kind and type-variable binders]
    extractHsTysRdrTyVars ty
      = case extract_ltys ty ([],[]) of
         (kvs, tvs) -> (nub kvs, nub tvs)

    extractRdrKindSigVars :: Maybe (LHsKind RdrName) -> [RdrName]
    extractRdrKindSigVars Nothing = []
    extractRdrKindSigVars (Just k) = nub (fst (extract_lkind k ([],[])))

    extractDataDefnKindVars :: HsDataDefn RdrName -> [RdrName]
    -- Get the scoped kind variables mentioned free in the constructor decls
    -- Eg    data T a = T1 (S (a :: k) | forall (b::k). T2 (S b)
    -- Here k should scope over the whole definition
    extractDataDefnKindVars (HsDataDefn { dd_ctxt = ctxt, dd_kindSig = ksig
                                        , dd_cons = cons, dd_derivs = derivs })
      = fst $ extract_lctxt ctxt $
              extract_mb extract_lkind ksig $
              extract_mb (extract_ltys . unLoc) derivs $
              foldr (extract_con . unLoc) ([],[]) cons
      where
        extract_con (ConDecl { con_res = ResTyGADT {} }) acc = acc
        extract_con (ConDecl { con_res = ResTyH98, con_qvars = qvs
                             , con_cxt = ctxt, con_details = details }) acc
          = extract_hs_tv_bndrs qvs acc $
            extract_lctxt ctxt $
            extract_ltys (hsConDeclArgTys details) ([],[])

    extract_lctxt :: LHsContext RdrName -> FreeKiTyVars -> FreeKiTyVars
    extract_lctxt ctxt = extract_ltys (unLoc ctxt)

    extract_ltys :: [LHsType RdrName] -> FreeKiTyVars -> FreeKiTyVars
    extract_ltys tys acc = foldr extract_lty acc tys

    extract_mb :: (a -> FreeKiTyVars -> FreeKiTyVars) -> Maybe a -> FreeKiTyVars -> FreeKiTyVars
    extract_mb _ Nothing  acc = acc
    extract_mb f (Just x) acc = f x acc

    extract_lkind :: LHsType RdrName -> FreeKiTyVars -> FreeKiTyVars
    extract_lkind kind (acc_kvs, acc_tvs) = case extract_lty kind ([], acc_kvs) of
                                              (_, res_kvs) -> (res_kvs, acc_tvs)
                                            -- Kinds shouldn't have sort signatures!

    extract_lty :: LHsType RdrName -> FreeKiTyVars -> FreeKiTyVars
    extract_lty (L _ ty) acc
      = case ty of
          HsTyVar tv                -> extract_tv tv acc
          HsBangTy _ ty             -> extract_lty ty acc
          HsRecTy flds              -> foldr (extract_lty . cd_fld_type . unLoc) acc
                                             flds
          HsAppTy ty1 ty2           -> extract_lty ty1 (extract_lty ty2 acc)
          HsListTy ty               -> extract_lty ty acc
          HsPArrTy ty               -> extract_lty ty acc
          HsTupleTy _ tys           -> extract_ltys tys acc
          HsFunTy ty1 ty2           -> extract_lty ty1 (extract_lty ty2 acc)
          HsIParamTy _ ty           -> extract_lty ty acc
          HsEqTy ty1 ty2            -> extract_lty ty1 (extract_lty ty2 acc)
          HsOpTy ty1 (_, (L _ tv)) ty2 -> extract_tv tv (extract_lty ty1 (extract_lty ty2 acc))
          HsParTy ty                -> extract_lty ty acc
          HsCoreTy {}               -> acc  -- The type is closed
          HsQuasiQuoteTy {}         -> acc  -- Quasi quotes mention no type variables
          HsSpliceTy {}             -> acc  -- Type splices mention no type variables
          HsDocTy ty _              -> extract_lty ty acc
          HsExplicitListTy _ tys    -> extract_ltys tys acc
          HsExplicitTupleTy _ tys   -> extract_ltys tys acc
          HsTyLit _                 -> acc
          HsWrapTy _ _              -> panic "extract_lty"
          HsKindSig ty ki           -> extract_lty ty (extract_lkind ki acc)
          HsForAllTy _ _ tvs cx ty  -> extract_hs_tv_bndrs tvs acc $
                                       extract_lctxt cx   $
                                       extract_lty ty ([],[])
          -- We deal with these to in a later stage, because they need to be
          -- replaced by fresh HsTyVars.
          HsWildcardTy              -> acc
          HsNamedWildcardTy _       -> acc

    extract_hs_tv_bndrs :: LHsTyVarBndrs RdrName -> FreeKiTyVars
                        -> FreeKiTyVars -> FreeKiTyVars
    extract_hs_tv_bndrs (HsQTvs { hsq_tvs = tvs })
                        (acc_kvs, acc_tvs)   -- Note accumulator comes first
                        (body_kvs, body_tvs)
      | null tvs
      = (body_kvs ++ acc_kvs, body_tvs ++ acc_tvs)
      | otherwise
      = (acc_kvs ++ filterOut (`elem` local_kvs) body_kvs,
         acc_tvs ++ filterOut (`elem` local_tvs) body_tvs)
      where
        local_tvs = map hsLTyVarName tvs
        (_, local_kvs) = foldr extract_lty ([], []) [k | L _ (KindedTyVar _ k) <- tvs]
           -- These kind variables are bound here if not bound further out

    extract_tv :: RdrName -> FreeKiTyVars -> FreeKiTyVars
    extract_tv tv acc
      | isRdrTyVar tv = case acc of (kvs,tvs) -> (kvs, tv : tvs)
      | otherwise     = acc

    -- | Replace all unnamed wildcards in the given type with named wildcards.
    -- These names are freshly generated, based on "_". Return a tuple of the
    -- named wildcards that weren't already in scope (amongst them the named
    -- wildcards the unnamed ones were converted into), and the type in which the
    -- unnamed wildcards are replaced by named wildcards.
    extractWildcards :: LHsType RdrName -> RnM ([Located RdrName], LHsType RdrName)
    extractWildcards ty
      = do { (nwcs, awcs, ty') <- go ty
           ; rdr_env <- getLocalRdrEnv
           -- Filter out named wildcards that are already in scope
           ; let nwcs' = nubBy eqLocated $ filterOut (flip (elemLocalRdrEnv . unLoc) rdr_env) nwcs
           ; return (nwcs' ++ awcs, ty') }
      where
        go orig@(L l ty) = case ty of
          (HsForAllTy exp extra bndrs (L locCxt cxt) ty) ->
            do (nwcs1, awcs1, cxt') <- extList cxt
               (nwcs2, awcs2, ty')  <- go ty
               return (nwcs1 ++ nwcs2, awcs1 ++ awcs2,
                       L l (HsForAllTy exp extra bndrs (L locCxt cxt') ty'))
          (HsAppTy ty1 ty2)           -> go2 HsAppTy ty1 ty2
          (HsFunTy ty1 ty2)           -> go2 HsFunTy ty1 ty2
          (HsListTy ty)               -> go1 HsListTy ty
          (HsPArrTy ty)               -> go1 HsPArrTy ty
          (HsTupleTy con tys)         -> goList (HsTupleTy con) tys
          (HsOpTy ty1 op ty2)         -> go2 (\t1 t2 -> HsOpTy t1 op t2) ty1 ty2
          (HsParTy ty)                -> go1 HsParTy ty
          (HsIParamTy n ty)           -> go1 (HsIParamTy n) ty
          (HsEqTy ty1 ty2)            -> go2 HsEqTy ty1 ty2
          (HsKindSig ty kind)         -> go2 HsKindSig ty kind
          (HsDocTy ty doc)            -> go1 (flip HsDocTy doc) ty
          (HsBangTy b ty)             -> go1 (HsBangTy b) ty
          (HsExplicitListTy ptk tys)  -> goList (HsExplicitListTy ptk) tys
          (HsExplicitTupleTy ptk tys) -> goList (HsExplicitTupleTy ptk) tys
          HsWildcardTy                -> do
            uniq <- newUnique
            let name = mkInternalName uniq (mkTyVarOcc "_") l
                rdrName = nameRdrName name
            return ([], [L l rdrName], L l $ HsNamedWildcardTy rdrName)
          (HsNamedWildcardTy name)    -> return ([L l name], [], orig)
          -- HsQuasiQuoteTy, HsSpliceTy, HsRecTy, HsCoreTy, HsTyLit, HsWrapTy
          _                           -> return ([], [], orig)
          where
            go1 f t = do (nwcs, awcs, t') <- go t
                         return (nwcs, awcs, L l $ f t')
            go2 f t1 t2 =
              do (nwcs1, awcs1, t1') <- go t1
                 (nwcs2, awcs2, t2') <- go t2
                 return (nwcs1 ++ nwcs2, awcs1 ++ awcs2, L l $ f t1' t2')
            extList l = do rec_res <- mapM go l
                           let (nwcs, awcs, tys') =
                                 foldr (\(nwcs, awcs, ty) (nwcss, awcss, tys) ->
                                         (nwcs ++ nwcss, awcs ++ awcss, ty : tys))
                                       ([], [], []) rec_res
                           return (nwcs, awcs, tys')
            goList f tys = do (nwcs, awcs, tys') <- extList tys
                              return (nwcs, awcs, L l $ f tys')