typecheck/TcTyClsDecls.hs

    {-
    (c) The University of Glasgow 2006
    (c) The AQUA Project, Glasgow University, 1996-1998

    TcTyClsDecls: Typecheck type and class declarations
    -}

    {-# LANGUAGE CPP, TupleSections #-}

    module TcTyClsDecls (
            tcTyAndClassDecls, tcAddImplicits,

            -- Functions used by TcInstDcls to check
            -- data/type family instance declarations
            kcDataDefn, tcConDecls, dataDeclChecks, checkValidTyCon,
            tcFamTyPats, tcTyFamInstEqn, famTyConShape,
            tcAddTyFamInstCtxt, tcAddDataFamInstCtxt,
            wrongKindOfFamily, dataConCtxt, badDataConTyCon
        ) where

    #include "HsVersions.h"

    import HsSyn
    import HscTypes
    import BuildTyCl
    import TcRnMonad
    import TcEnv
    import TcValidity
    import TcHsSyn
    import TcSimplify( growThetaTyVars )
    import TcBinds( tcRecSelBinds )
    import TcTyDecls
    import TcClassDcl
    import TcHsType
    import TcMType
    import TcType
    import TysWiredIn( unitTy )
    import FamInst
    import FamInstEnv( isDominatedBy, mkCoAxBranch, mkBranchedCoAxiom )
    import Coercion( pprCoAxBranch, ltRole )
    import Type
    import TypeRep   -- for checkValidRoles
    import Kind
    import Class
    import CoAxiom
    import TyCon
    import DataCon
    import Id
    import MkCore           ( rEC_SEL_ERROR_ID )
    import IdInfo
    import Var
    import VarEnv
    import VarSet
    import Module
    import Name
    import NameSet
    import NameEnv
    import Outputable
    import Maybes
    import Unify
    import Util
    import SrcLoc
    import ListSetOps
    import Digraph
    import DynFlags
    import FastString
    import Unique           ( mkBuiltinUnique )
    import BasicTypes

    import Bag
    import Control.Monad
    import Data.List

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Type checking for type and class declarations}
    *                                                                      *
    ************************************************************************

    Note [Grouping of type and class declarations]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    tcTyAndClassDecls is called on a list of `TyClGroup`s. Each group is a strongly
    connected component of mutually dependent types and classes. We kind check and
    type check each group separately to enhance kind polymorphism. Take the
    following example:

      type Id a = a
      data X = X (Id Int)

    If we were to kind check the two declarations together, we would give Id the
    kind * -> *, since we apply it to an Int in the definition of X. But we can do
    better than that, since Id really is kind polymorphic, and should get kind
    forall (k::BOX). k -> k. Since it does not depend on anything else, it can be
    kind-checked by itself, hence getting the most general kind. We then kind check
    X, which works fine because we then know the polymorphic kind of Id, and simply
    instantiate k to *.

    Note [Check role annotations in a second pass]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Role inference potentially depends on the types of all of the datacons declared
    in a mutually recursive group. The validity of a role annotation, in turn,
    depends on the result of role inference. Because the types of datacons might
    be ill-formed (see #7175 and Note [Checking GADT return types]) we must check
    *all* the tycons in a group for validity before checking *any* of the roles.
    Thus, we take two passes over the resulting tycons, first checking for general
    validity and then checking for valid role annotations.
    -}

    tcTyAndClassDecls :: ModDetails
                      -> [TyClGroup Name]   -- Mutually-recursive groups in dependency order
                      -> TcM TcGblEnv       -- Input env extended by types and classes
                                            -- and their implicit Ids,DataCons
    -- Fails if there are any errors
    tcTyAndClassDecls boot_details tyclds_s
      = checkNoErrs $       -- The code recovers internally, but if anything gave rise to
                            -- an error we'd better stop now, to avoid a cascade
        fold_env tyclds_s   -- Type check each group in dependency order folding the global env
      where
        fold_env :: [TyClGroup Name] -> TcM TcGblEnv
        fold_env [] = getGblEnv
        fold_env (tyclds:tyclds_s)
          = do { tcg_env <- tcTyClGroup boot_details tyclds
               ; setGblEnv tcg_env $ fold_env tyclds_s }
                 -- remaining groups are typecheck in the extended global env

    tcTyClGroup :: ModDetails -> TyClGroup Name -> TcM TcGblEnv
    -- Typecheck one strongly-connected component of type and class decls
    tcTyClGroup boot_details tyclds
      = do {    -- Step 1: kind-check this group and returns the final
                -- (possibly-polymorphic) kind of each TyCon and Class
                -- See Note [Kind checking for type and class decls]
             names_w_poly_kinds <- kcTyClGroup tyclds
           ; traceTc "tcTyAndCl generalized kinds" (ppr names_w_poly_kinds)

                -- Step 2: type-check all groups together, returning
                -- the final TyCons and Classes
           ; let role_annots = extractRoleAnnots tyclds
                 decls = group_tyclds tyclds
           ; tyclss <- fixM $ \ rec_tyclss -> do
               { is_boot <- tcIsHsBootOrSig
               ; let rec_flags = calcRecFlags boot_details is_boot
                                              role_annots rec_tyclss

                     -- Populate environment with knot-tied ATyCon for TyCons
                     -- NB: if the decls mention any ill-staged data cons
                     -- (see Note [Recusion and promoting data constructors]
                     -- we will have failed already in kcTyClGroup, so no worries here
               ; tcExtendRecEnv (zipRecTyClss names_w_poly_kinds rec_tyclss) $

                     -- Also extend the local type envt with bindings giving
                     -- the (polymorphic) kind of each knot-tied TyCon or Class
                     -- See Note [Type checking recursive type and class declarations]
                 tcExtendKindEnv names_w_poly_kinds              $

                     -- Kind and type check declarations for this group
                 concatMapM (tcTyClDecl rec_flags) decls }

               -- Step 3: Perform the validity check
               -- We can do this now because we are done with the recursive knot
               -- Do it before Step 4 (adding implicit things) because the latter
               -- expects well-formed TyCons
           ; tcExtendGlobalEnv tyclss $ do
           { traceTc "Starting validity check" (ppr tyclss)
           ; checkNoErrs $
             mapM_ (recoverM (return ()) . checkValidTyCl) tyclss
               -- We recover, which allows us to report multiple validity errors
               -- the checkNoErrs is necessary to fix #7175.
           ; mapM_ (recoverM (return ()) . checkValidRoleAnnots role_annots) tyclss
               -- See Note [Check role annotations in a second pass]

               -- Step 4: Add the implicit things;
               -- we want them in the environment because
               -- they may be mentioned in interface files
           ; tcExtendGlobalValEnv (mkDefaultMethodIds tyclss) $
             tcAddImplicits tyclss } }

    tcAddImplicits :: [TyThing] -> TcM TcGblEnv
    tcAddImplicits tyclss
     = tcExtendGlobalEnvImplicit implicit_things $
       tcRecSelBinds rec_sel_binds
     where
       implicit_things = concatMap implicitTyThings tyclss
       rec_sel_binds   = mkRecSelBinds tyclss

    zipRecTyClss :: [(Name, Kind)]
                 -> [TyThing]           -- Knot-tied
                 -> [(Name,TyThing)]
    -- Build a name-TyThing mapping for the things bound by decls
    -- being careful not to look at the [TyThing]
    -- The TyThings in the result list must have a visible ATyCon,
    -- because typechecking types (in, say, tcTyClDecl) looks at this outer constructor
    zipRecTyClss kind_pairs rec_things
      = [ (name, ATyCon (get name)) | (name, _kind) <- kind_pairs ]
      where
        rec_type_env :: TypeEnv
        rec_type_env = mkTypeEnv rec_things

        get name = case lookupTypeEnv rec_type_env name of
                     Just (ATyCon tc) -> tc
                     other            -> pprPanic "zipRecTyClss" (ppr name <+> ppr other)

    {-
    ************************************************************************
    *                                                                      *
                    Kind checking
    *                                                                      *
    ************************************************************************

    Note [Kind checking for type and class decls]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Kind checking is done thus:

       1. Make up a kind variable for each parameter of the *data* type, class,
          and closed type family decls, and extend the kind environment (which is
          in the TcLclEnv)

       2. Dependency-analyse the type *synonyms* (which must be non-recursive),
          and kind-check them in dependency order.  Extend the kind envt.

       3. Kind check the data type and class decls

    Synonyms are treated differently to data type and classes,
    because a type synonym can be an unboxed type
            type Foo = Int#
    and a kind variable can't unify with UnboxedTypeKind
    So we infer their kinds in dependency order

    We need to kind check all types in the mutually recursive group
    before we know the kind of the type variables.  For example:

      class C a where
         op :: D b => a -> b -> b

      class D c where
         bop :: (Monad c) => ...

    Here, the kind of the locally-polymorphic type variable "b"
    depends on *all the uses of class D*.  For example, the use of
    Monad c in bop's type signature means that D must have kind Type->Type.

    However type synonyms work differently.  They can have kinds which don't
    just involve (->) and *:
            type R = Int#           -- Kind #
            type S a = Array# a     -- Kind * -> #
            type T a b = (# a,b #)  -- Kind * -> * -> (# a,b #)
    So we must infer their kinds from their right-hand sides *first* and then
    use them, whereas for the mutually recursive data types D we bring into
    scope kind bindings D -> k, where k is a kind variable, and do inference.

    Open type families
    ~~~~~~~~~~~~~~~~~~
    This treatment of type synonyms only applies to Haskell 98-style synonyms.
    General type functions can be recursive, and hence, appear in `alg_decls'.

    The kind of an open type family is solely determinded by its kind signature;
    hence, only kind signatures participate in the construction of the initial
    kind environment (as constructed by `getInitialKind'). In fact, we ignore
    instances of families altogether in the following. However, we need to include
    the kinds of *associated* families into the construction of the initial kind
    environment. (This is handled by `allDecls').
    -}

    kcTyClGroup :: TyClGroup Name -> TcM [(Name,Kind)]
    -- Kind check this group, kind generalize, and return the resulting local env
    -- This bindds the TyCons and Classes of the group, but not the DataCons
    -- See Note [Kind checking for type and class decls]
    kcTyClGroup (TyClGroup { group_tyclds = decls })
      = do  { mod <- getModule
            ; traceTc "kcTyClGroup" (ptext (sLit "module") <+> ppr mod $$ vcat (map ppr decls))

              -- Kind checking;
              --    1. Bind kind variables for non-synonyms
              --    2. Kind-check synonyms, and bind kinds of those synonyms
              --    3. Kind-check non-synonyms
              --    4. Generalise the inferred kinds
              -- See Note [Kind checking for type and class decls]

              -- Step 1: Bind kind variables for non-synonyms
            ; let (syn_decls, non_syn_decls) = partition (isSynDecl . unLoc) decls
            ; initial_kinds <- getInitialKinds non_syn_decls
            ; traceTc "kcTyClGroup: initial kinds" (ppr initial_kinds)

            -- Step 2: Set initial envt, kind-check the synonyms
            ; lcl_env <- tcExtendKindEnv2 initial_kinds $
                         kcSynDecls (calcSynCycles syn_decls)

            -- Step 3: Set extended envt, kind-check the non-synonyms
            ; setLclEnv lcl_env $
              mapM_ kcLTyClDecl non_syn_decls

                 -- Step 4: generalisation
                 -- Kind checking done for this group
                 -- Now we have to kind generalize the flexis
            ; res <- concatMapM (generaliseTCD (tcl_env lcl_env)) decls

            ; traceTc "kcTyClGroup result" (ppr res)
            ; return res }

      where
        generalise :: TcTypeEnv -> Name -> TcM (Name, Kind)
        -- For polymorphic things this is a no-op
        generalise kind_env name
          = do { let kc_kind = case lookupNameEnv kind_env name of
                                   Just (AThing k) -> k
                                   _ -> pprPanic "kcTyClGroup" (ppr name $$ ppr kind_env)
               ; kvs <- kindGeneralize (tyVarsOfType kc_kind)
               ; kc_kind' <- zonkTcKind kc_kind    -- Make sure kc_kind' has the final,
                                                   -- skolemised kind variables
               ; traceTc "Generalise kind" (vcat [ ppr name, ppr kc_kind, ppr kvs, ppr kc_kind' ])
               ; return (name, mkForAllTys kvs kc_kind') }

        generaliseTCD :: TcTypeEnv -> LTyClDecl Name -> TcM [(Name, Kind)]
        generaliseTCD kind_env (L _ decl)
          | ClassDecl { tcdLName = (L _ name), tcdATs = ats } <- decl
          = do { first <- generalise kind_env name
               ; rest <- mapM ((generaliseFamDecl kind_env) . unLoc) ats
               ; return (first : rest) }

          | FamDecl { tcdFam = fam } <- decl
          = do { res <- generaliseFamDecl kind_env fam
               ; return [res] }

          | otherwise
          = do { res <- generalise kind_env (tcdName decl)
               ; return [res] }

        generaliseFamDecl :: TcTypeEnv -> FamilyDecl Name -> TcM (Name, Kind)
        generaliseFamDecl kind_env (FamilyDecl { fdLName = L _ name })
          = generalise kind_env name

    mk_thing_env :: [LTyClDecl Name] -> [(Name, TcTyThing)]
    mk_thing_env [] = []
    mk_thing_env (decl : decls)
      | L _ (ClassDecl { tcdLName = L _ nm, tcdATs = ats }) <- decl
      = (nm, APromotionErr ClassPE) :
        (map (, APromotionErr TyConPE) $ map (unLoc . fdLName . unLoc) ats) ++
        (mk_thing_env decls)

      | otherwise
      = (tcdName (unLoc decl), APromotionErr TyConPE) :
        (mk_thing_env decls)

    getInitialKinds :: [LTyClDecl Name] -> TcM [(Name, TcTyThing)]
    getInitialKinds decls
      = tcExtendKindEnv2 (mk_thing_env decls) $
        do { pairss <- mapM (addLocM getInitialKind) decls
           ; return (concat pairss) }

    getInitialKind :: TyClDecl Name -> TcM [(Name, TcTyThing)]
    -- Allocate a fresh kind variable for each TyCon and Class
    -- For each tycon, return   (tc, AThing k)
    --                 where k is the kind of tc, derived from the LHS
    --                       of the definition (and probably including
    --                       kind unification variables)
    --      Example: data T a b = ...
    --      return (T, kv1 -> kv2 -> kv3)
    --
    -- This pass deals with (ie incorporates into the kind it produces)
    --   * The kind signatures on type-variable binders
    --   * The result kinds signature on a TyClDecl
    --
    -- ALSO for each datacon, return (dc, APromotionErr RecDataConPE)
    -- Note [ARecDataCon: Recursion and promoting data constructors]
    --
    -- No family instances are passed to getInitialKinds

    getInitialKind decl@(ClassDecl { tcdLName = L _ name, tcdTyVars = ktvs, tcdATs = ats })
      = do { (cl_kind, inner_prs) <-
               kcHsTyVarBndrs (hsDeclHasCusk decl) ktvs $
               do { inner_prs <- getFamDeclInitialKinds ats
                  ; return (constraintKind, inner_prs) }
           ; let main_pr = (name, AThing cl_kind)
           ; return (main_pr : inner_prs) }

    getInitialKind decl@(DataDecl { tcdLName = L _ name
                                  , tcdTyVars = ktvs
                                  , tcdDataDefn = HsDataDefn { dd_kindSig = m_sig
                                                             , dd_cons = cons' } })
      = let cons = cons' -- AZ list monad coming
        in
         do { (decl_kind, _) <-
               kcHsTyVarBndrs (hsDeclHasCusk decl) ktvs $
               do { res_k <- case m_sig of
                               Just ksig -> tcLHsKind ksig
                               Nothing   -> return liftedTypeKind
                  ; return (res_k, ()) }
           ; let main_pr = (name, AThing decl_kind)
                 inner_prs = [ (unLoc con, APromotionErr RecDataConPE)
                             | L _ con' <- cons, con <- con_names con' ]
           ; return (main_pr : inner_prs) }

    getInitialKind (FamDecl { tcdFam = decl })
      = getFamDeclInitialKind decl

    getInitialKind decl@(SynDecl {})
      = pprPanic "getInitialKind" (ppr decl)

    ---------------------------------
    getFamDeclInitialKinds :: [LFamilyDecl Name] -> TcM [(Name, TcTyThing)]
    getFamDeclInitialKinds decls
      = tcExtendKindEnv2 [ (n, APromotionErr TyConPE)
                         | L _ (FamilyDecl { fdLName = L _ n }) <- decls] $
        concatMapM (addLocM getFamDeclInitialKind) decls

    getFamDeclInitialKind :: FamilyDecl Name
                          -> TcM [(Name, TcTyThing)]
    getFamDeclInitialKind decl@(FamilyDecl { fdLName = L _ name
                                           , fdTyVars = ktvs
                                           , fdKindSig = ksig })
      = do { (fam_kind, _) <-
               kcHsTyVarBndrs (famDeclHasCusk decl) ktvs $
               do { res_k <- case ksig of
                               Just k  -> tcLHsKind k
                               Nothing
                                 | famDeclHasCusk decl -> return liftedTypeKind
                                 | otherwise           -> newMetaKindVar
                  ; return (res_k, ()) }
           ; return [ (name, AThing fam_kind) ] }

    ----------------
    kcSynDecls :: [SCC (LTyClDecl Name)]
               -> TcM TcLclEnv -- Kind bindings
    kcSynDecls [] = getLclEnv
    kcSynDecls (group : groups)
      = do  { (n,k) <- kcSynDecl1 group
            ; lcl_env <- tcExtendKindEnv [(n,k)] (kcSynDecls groups)
            ; return lcl_env }

    kcSynDecl1 :: SCC (LTyClDecl Name)
               -> TcM (Name,TcKind) -- Kind bindings
    kcSynDecl1 (AcyclicSCC (L _ decl)) = kcSynDecl decl
    kcSynDecl1 (CyclicSCC decls)       = do { recSynErr decls; failM }
                                         -- Fail here to avoid error cascade
                                         -- of out-of-scope tycons

    kcSynDecl :: TyClDecl Name -> TcM (Name, TcKind)
    kcSynDecl decl@(SynDecl { tcdTyVars = hs_tvs, tcdLName = L _ name
                            , tcdRhs = rhs })
      -- Returns a possibly-unzonked kind
      = tcAddDeclCtxt decl $
        do { (syn_kind, _) <-
               kcHsTyVarBndrs (hsDeclHasCusk decl) hs_tvs $
               do { traceTc "kcd1" (ppr name <+> brackets (ppr hs_tvs))
                  ; (_, rhs_kind) <- tcLHsType rhs
                  ; traceTc "kcd2" (ppr name)
                  ; return (rhs_kind, ()) }
           ; return (name, syn_kind) }
    kcSynDecl decl = pprPanic "kcSynDecl" (ppr decl)

    ------------------------------------------------------------------------
    kcLTyClDecl :: LTyClDecl Name -> TcM ()
      -- See Note [Kind checking for type and class decls]
    kcLTyClDecl (L loc decl)
      = setSrcSpan loc $ tcAddDeclCtxt decl $ kcTyClDecl decl

    kcTyClDecl :: TyClDecl Name -> TcM ()
    -- This function is used solely for its side effect on kind variables
    -- NB kind signatures on the type variables and
    --    result kind signature have aready been dealt with
    --    by getInitialKind, so we can ignore them here.

    kcTyClDecl (DataDecl { tcdLName = L _ name, tcdTyVars = hs_tvs, tcdDataDefn = defn })
      | HsDataDefn { dd_cons = cons, dd_kindSig = Just _ } <- defn
      = mapM_ (wrapLocM kcConDecl) cons
        -- hs_tvs and dd_kindSig already dealt with in getInitialKind
        -- If dd_kindSig is Just, this must be a GADT-style decl,
        --        (see invariants of DataDefn declaration)
        -- so (a) we don't need to bring the hs_tvs into scope, because the
        --        ConDecls bind all their own variables
        --    (b) dd_ctxt is not allowed for GADT-style decls, so we can ignore it

      | HsDataDefn { dd_ctxt = ctxt, dd_cons = cons } <- defn
      = kcTyClTyVars name hs_tvs $
        do  { _ <- tcHsContext ctxt
            ; mapM_ (wrapLocM kcConDecl) cons }

    kcTyClDecl decl@(SynDecl {}) = pprPanic "kcTyClDecl" (ppr decl)

    kcTyClDecl (ClassDecl { tcdLName = L _ name, tcdTyVars = hs_tvs
                           , tcdCtxt = ctxt, tcdSigs = sigs })
      = kcTyClTyVars name hs_tvs $
        do  { _ <- tcHsContext ctxt
            ; mapM_ (wrapLocM kc_sig)     sigs }
      where
        kc_sig (TypeSig _ op_ty _)  = discardResult (tcHsLiftedType op_ty)
        kc_sig (GenericSig _ op_ty) = discardResult (tcHsLiftedType op_ty)
        kc_sig _                    = return ()

    -- closed type families look at their equations, but other families don't
    -- do anything here
    kcTyClDecl (FamDecl (FamilyDecl { fdLName  = L _ fam_tc_name
                                    , fdTyVars = hs_tvs
                                    , fdInfo   = ClosedTypeFamily eqns }))
      = do { tc_kind <- kcLookupKind fam_tc_name
           ; let fam_tc_shape = ( fam_tc_name, length (hsQTvBndrs hs_tvs), tc_kind)
           ; mapM_ (kcTyFamInstEqn fam_tc_shape) eqns }
    kcTyClDecl (FamDecl {})    = return ()

    -------------------
    kcConDecl :: ConDecl Name -> TcM ()
    kcConDecl (ConDecl { con_names = names, con_qvars = ex_tvs
                       , con_cxt = ex_ctxt, con_details = details
                       , con_res = res })
      = addErrCtxt (dataConCtxtName names) $
             -- the 'False' says that the existentials don't have a CUSK, as the
             -- concept doesn't really apply here. We just need to bring the variables
             -- into scope!
        do { _ <- kcHsTyVarBndrs False ex_tvs $
                  do { _ <- tcHsContext ex_ctxt
                     ; mapM_ (tcHsOpenType . getBangType) (hsConDeclArgTys details)
                     ; _ <- tcConRes res
                     ; return (panic "kcConDecl", ()) }
           ; return () }

    {-
    Note [Recursion and promoting data constructors]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We don't want to allow promotion in a strongly connected component
    when kind checking.

    Consider:
      data T f = K (f (K Any))

    When kind checking the `data T' declaration the local env contains the
    mappings:
      T -> AThing <some initial kind>
      K -> ARecDataCon

    ANothing is only used for DataCons, and only used during type checking
    in tcTyClGroup.

    ************************************************************************
    *                                                                      *
    \subsection{Type checking}
    *                                                                      *
    ************************************************************************

    Note [Type checking recursive type and class declarations]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    At this point we have completed *kind-checking* of a mutually
    recursive group of type/class decls (done in kcTyClGroup). However,
    we discarded the kind-checked types (eg RHSs of data type decls);
    note that kcTyClDecl returns ().  There are two reasons:

      * It's convenient, because we don't have to rebuild a
        kinded HsDecl (a fairly elaborate type)

      * It's necessary, because after kind-generalisation, the
        TyCons/Classes may now be kind-polymorphic, and hence need
        to be given kind arguments.

    Example:
           data T f a = MkT (f a) (T f a)
    During kind-checking, we give T the kind T :: k1 -> k2 -> *
    and figure out constraints on k1, k2 etc. Then we generalise
    to get   T :: forall k. (k->*) -> k -> *
    So now the (T f a) in the RHS must be elaborated to (T k f a).

    However, during tcTyClDecl of T (above) we will be in a recursive
    "knot". So we aren't allowed to look at the TyCon T itself; we are only
    allowed to put it (lazily) in the returned structures.  But when
    kind-checking the RHS of T's decl, we *do* need to know T's kind (so
    that we can correctly elaboarate (T k f a).  How can we get T's kind
    without looking at T?  Delicate answer: during tcTyClDecl, we extend

      *Global* env with T -> ATyCon (the (not yet built) TyCon for T)
      *Local*  env with T -> AThing (polymorphic kind of T)

    Then:

      * During TcHsType.kcTyVar we look in the *local* env, to get the
        known kind for T.

      * But in TcHsType.ds_type (and ds_var_app in particular) we look in
        the *global* env to get the TyCon. But we must be careful not to
        force the TyCon or we'll get a loop.

    This fancy footwork (with two bindings for T) is only necesary for the
    TyCons or Classes of this recursive group.  Earlier, finished groups,
    live in the global env only.
    -}

    tcTyClDecl :: RecTyInfo -> LTyClDecl Name -> TcM [TyThing]
    tcTyClDecl rec_info (L loc decl)
      = setSrcSpan loc $ tcAddDeclCtxt decl $
        traceTc "tcTyAndCl-x" (ppr decl) >>
        tcTyClDecl1 NoParentTyCon rec_info decl

      -- "type family" declarations
    tcTyClDecl1 :: TyConParent -> RecTyInfo -> TyClDecl Name -> TcM [TyThing]
    tcTyClDecl1 parent _rec_info (FamDecl { tcdFam = fd })
      = tcFamDecl1 parent fd

      -- "type" synonym declaration
    tcTyClDecl1 _parent rec_info
                (SynDecl { tcdLName = L _ tc_name, tcdTyVars = tvs, tcdRhs = rhs })
      = ASSERT( isNoParent _parent )
        tcTyClTyVars tc_name tvs $ \ tvs' kind ->
        tcTySynRhs rec_info tc_name tvs' kind rhs

      -- "data/newtype" declaration
    tcTyClDecl1 _parent rec_info
                (DataDecl { tcdLName = L _ tc_name, tcdTyVars = tvs, tcdDataDefn = defn })
      = ASSERT( isNoParent _parent )
        tcTyClTyVars tc_name tvs $ \ tvs' kind ->
        tcDataDefn rec_info tc_name tvs' kind defn

    tcTyClDecl1 _parent rec_info
                (ClassDecl { tcdLName = L _ class_name, tcdTyVars = tvs
                , tcdCtxt = ctxt, tcdMeths = meths
                , tcdFDs = fundeps, tcdSigs = sigs
                , tcdATs = ats, tcdATDefs = at_defs })
      = ASSERT( isNoParent _parent )
        do { (clas, tvs', gen_dm_env) <- fixM $ \ ~(clas,_,_) ->
                tcTyClTyVars class_name tvs $ \ tvs' kind ->
                do { MASSERT( isConstraintKind kind )
                     -- This little knot is just so we can get
                     -- hold of the name of the class TyCon, which we
                     -- need to look up its recursiveness
                   ; let tycon_name = tyConName (classTyCon clas)
                         tc_isrec = rti_is_rec rec_info tycon_name
                         roles = rti_roles rec_info tycon_name

                   ; ctxt' <- tcHsContext ctxt
                   ; ctxt' <- zonkTcTypeToTypes emptyZonkEnv ctxt'
                           -- Squeeze out any kind unification variables
                   ; fds'  <- mapM (addLocM tc_fundep) fundeps
                   ; (sig_stuff, gen_dm_env) <- tcClassSigs class_name sigs meths
                   ; at_stuff <- tcClassATs class_name (AssocFamilyTyCon clas) ats at_defs
                   ; mindef <- tcClassMinimalDef class_name sigs sig_stuff
                   ; clas <- buildClass
                                class_name tvs' roles ctxt' fds' at_stuff
                                sig_stuff mindef tc_isrec
                   ; traceTc "tcClassDecl" (ppr fundeps $$ ppr tvs' $$ ppr fds')
                   ; return (clas, tvs', gen_dm_env) }

           ; let { gen_dm_ids = [ AnId (mkExportedLocalId VanillaId gen_dm_name gen_dm_ty)
                                | (sel_id, GenDefMeth gen_dm_name) <- classOpItems clas
                                , let gen_dm_tau = expectJust "tcTyClDecl1" $
                                                   lookupNameEnv gen_dm_env (idName sel_id)
                                , let gen_dm_ty = mkSigmaTy tvs'
                                                          [mkClassPred clas (mkTyVarTys tvs')]
                                                          gen_dm_tau
                                ]
                 ; class_ats = map ATyCon (classATs clas) }

           ; return (ATyCon (classTyCon clas) : gen_dm_ids ++ class_ats ) }
             -- NB: Order is important due to the call to `mkGlobalThings' when
             --     tying the the type and class declaration type checking knot.
      where
        tc_fundep (tvs1, tvs2) = do { tvs1' <- mapM (tc_fd_tyvar . unLoc) tvs1 ;
                                    ; tvs2' <- mapM (tc_fd_tyvar . unLoc) tvs2 ;
                                    ; return (tvs1', tvs2') }
        tc_fd_tyvar name   -- Scoped kind variables are bound to unification variables
                           -- which are now fixed, so we can zonk
          = do { tv <- tcLookupTyVar name
               ; ty <- zonkTyVarOcc emptyZonkEnv tv
                      -- Squeeze out any kind unification variables
               ; case getTyVar_maybe ty of
                   Just tv' -> return tv'
                   Nothing  -> pprPanic "tc_fd_tyvar" (ppr name $$ ppr tv $$ ppr ty) }

    tcFamDecl1 :: TyConParent -> FamilyDecl Name -> TcM [TyThing]
    tcFamDecl1 parent
                (FamilyDecl {fdInfo = OpenTypeFamily, fdLName = L _ tc_name, fdTyVars = tvs})
      = tcTyClTyVars tc_name tvs $ \ tvs' kind -> do
      { traceTc "open type family:" (ppr tc_name)
      ; checkFamFlag tc_name
      ; tycon <- buildFamilyTyCon tc_name tvs' OpenSynFamilyTyCon kind parent
      ; return [ATyCon tycon] }

    tcFamDecl1 parent
                (FamilyDecl { fdInfo = ClosedTypeFamily eqns
                            , fdLName = lname@(L _ tc_name), fdTyVars = tvs })
    -- Closed type families are a little tricky, because they contain the definition
    -- of both the type family and the equations for a CoAxiom.
    -- Note: eqns might be empty, in a hs-boot file!
      = do { traceTc "closed type family:" (ppr tc_name)
             -- the variables in the header have no scope:
           ; (tvs', kind) <- tcTyClTyVars tc_name tvs $ \ tvs' kind ->
                             return (tvs', kind)

           ; checkFamFlag tc_name -- make sure we have -XTypeFamilies

             -- Process the equations, creating CoAxBranches
           ; tc_kind <- kcLookupKind tc_name
           ; let fam_tc_shape = (tc_name, length (hsQTvBndrs tvs), tc_kind)

           ; branches <- mapM (tcTyFamInstEqn fam_tc_shape) eqns

             -- we need the tycon that we will be creating, but it's in scope.
             -- just look it up.
           ; fam_tc <- tcLookupLocatedTyCon lname

             -- create a CoAxiom, with the correct src location. It is Vitally
             -- Important that we do not pass the branches into
             -- newFamInstAxiomName. They have types that have been zonked inside
             -- the knot and we will die if we look at them. This is OK here
             -- because there will only be one axiom, so we don't need to
             -- differentiate names.
             -- See [Zonking inside the knot] in TcHsType
           ; loc <- getSrcSpanM
           ; co_ax_name <- newFamInstAxiomName loc tc_name []

             -- mkBranchedCoAxiom will fail on an empty list of branches, but
             -- we'll never look at co_ax in this case
           ; let co_ax = mkBranchedCoAxiom co_ax_name fam_tc branches

             -- now, finally, build the TyCon
           ; let syn_rhs = if null eqns
                           then AbstractClosedSynFamilyTyCon
                           else ClosedSynFamilyTyCon co_ax
           ; tycon <- buildFamilyTyCon tc_name tvs' syn_rhs kind parent

           ; let result = if null eqns
                          then [ATyCon tycon]
                          else [ATyCon tycon, ACoAxiom co_ax]
           ; return result }
    -- We check for instance validity later, when doing validity checking for
    -- the tycon

    tcFamDecl1 parent
               (FamilyDecl {fdInfo = DataFamily, fdLName = L _ tc_name, fdTyVars = tvs})
      = tcTyClTyVars tc_name tvs $ \ tvs' kind -> do
      { traceTc "data family:" (ppr tc_name)
      ; checkFamFlag tc_name
      ; extra_tvs <- tcDataKindSig kind
      ; let final_tvs = tvs' ++ extra_tvs    -- we may not need these
            roles     = map (const Nominal) final_tvs
            tycon = buildAlgTyCon tc_name final_tvs roles Nothing []
                                  DataFamilyTyCon Recursive
                                  False   -- Not promotable to the kind level
                                  True    -- GADT syntax
                                  parent
      ; return [ATyCon tycon] }

    tcTySynRhs :: RecTyInfo
               -> Name
               -> [TyVar] -> Kind
               -> LHsType Name -> TcM [TyThing]
    tcTySynRhs rec_info tc_name tvs kind hs_ty
      = do { env <- getLclEnv
           ; traceTc "tc-syn" (ppr tc_name $$ ppr (tcl_env env))
           ; rhs_ty <- tcCheckLHsType hs_ty kind
           ; rhs_ty <- zonkTcTypeToType emptyZonkEnv rhs_ty
           ; let roles = rti_roles rec_info tc_name
           ; tycon <- buildSynonymTyCon tc_name tvs roles rhs_ty kind
           ; return [ATyCon tycon] }

    tcDataDefn :: RecTyInfo -> Name
               -> [TyVar] -> Kind
               -> HsDataDefn Name -> TcM [TyThing]
      -- NB: not used for newtype/data instances (whether associated or not)
    tcDataDefn rec_info tc_name tvs kind
             (HsDataDefn { dd_ND = new_or_data, dd_cType = cType
                         , dd_ctxt = ctxt, dd_kindSig = mb_ksig
                         , dd_cons = cons' })
     = let cons = cons' -- AZ List monad coming
       in do { extra_tvs <- tcDataKindSig kind
           ; let final_tvs  = tvs ++ extra_tvs
                 roles      = rti_roles rec_info tc_name
           ; stupid_tc_theta <- tcHsContext ctxt
           ; stupid_theta    <- zonkTcTypeToTypes emptyZonkEnv stupid_tc_theta
           ; kind_signatures <- xoptM Opt_KindSignatures
           ; is_boot         <- tcIsHsBootOrSig -- Are we compiling an hs-boot file?

                 -- Check that we don't use kind signatures without Glasgow extensions
           ; case mb_ksig of
               Nothing   -> return ()
               Just hs_k -> do { checkTc (kind_signatures) (badSigTyDecl tc_name)
                               ; tc_kind <- tcLHsKind hs_k
                               ; checkKind kind tc_kind
                               ; return () }

           ; gadt_syntax <- dataDeclChecks tc_name new_or_data stupid_theta cons

           ; tycon <- fixM $ \ tycon -> do
                 { let res_ty = mkTyConApp tycon (mkTyVarTys final_tvs)
                 ; data_cons <- tcConDecls new_or_data tycon (final_tvs, res_ty) cons
                 ; tc_rhs <-
                     if null cons && is_boot              -- In a hs-boot file, empty cons means
                     then return totallyAbstractTyConRhs  -- "don't know"; hence totally Abstract
                     else case new_or_data of
                       DataType -> return (mkDataTyConRhs data_cons)
                       NewType  -> ASSERT( not (null data_cons) )
                                        mkNewTyConRhs tc_name tycon (head data_cons)
                 ; return (buildAlgTyCon tc_name final_tvs roles (fmap unLoc cType)
                                         stupid_theta tc_rhs
                                         (rti_is_rec rec_info tc_name)
                                         (rti_promotable rec_info)
                                         gadt_syntax NoParentTyCon) }
           ; return [ATyCon tycon] }

    {-
    ************************************************************************
    *                                                                      *
                   Typechecking associated types (in class decls)
                   (including the associated-type defaults)
    *                                                                      *
    ************************************************************************

    Note [Associated type defaults]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    The following is an example of associated type defaults:
                 class C a where
                   data D a

                   type F a b :: *
                   type F a Z = [a]        -- Default
                   type F a (S n) = F a n  -- Default

    Note that:
      - We can have more than one default definition for a single associated type,
        as long as they do not overlap (same rules as for instances)
      - We can get default definitions only for type families, not data families
    -}

    tcClassATs :: Name                  -- The class name (not knot-tied)
               -> TyConParent           -- The class parent of this associated type
               -> [LFamilyDecl Name]    -- Associated types.
               -> [LTyFamDefltEqn Name] -- Associated type defaults.
               -> TcM [ClassATItem]
    tcClassATs class_name parent ats at_defs
      = do {  -- Complain about associated type defaults for non associated-types
             sequence_ [ failWithTc (badATErr class_name n)
                       | n <- map at_def_tycon at_defs
                       , not (n `elemNameSet` at_names) ]
           ; mapM tc_at ats }
      where
        at_def_tycon :: LTyFamDefltEqn Name -> Name
        at_def_tycon (L _ eqn) = unLoc (tfe_tycon eqn)

        at_fam_name :: LFamilyDecl Name -> Name
        at_fam_name (L _ decl) = unLoc (fdLName decl)

        at_names = mkNameSet (map at_fam_name ats)

        at_defs_map :: NameEnv [LTyFamDefltEqn Name]
        -- Maps an AT in 'ats' to a list of all its default defs in 'at_defs'
        at_defs_map = foldr (\at_def nenv -> extendNameEnv_C (++) nenv
                                              (at_def_tycon at_def) [at_def])
                            emptyNameEnv at_defs

        tc_at at = do { [ATyCon fam_tc] <- addLocM (tcFamDecl1 parent) at
                      ; let at_defs = lookupNameEnv at_defs_map (at_fam_name at)
                                      `orElse` []
                      ; atd <- tcDefaultAssocDecl fam_tc at_defs
                      ; return (ATI fam_tc atd) }

    -------------------------
    tcDefaultAssocDecl :: TyCon                  -- ^ Family TyCon
                       -> [LTyFamDefltEqn Name]  -- ^ Defaults
                       -> TcM (Maybe Type)       -- ^ Type checked RHS
    tcDefaultAssocDecl _ []
      = return Nothing  -- No default declaration

    tcDefaultAssocDecl _ (d1:_:_)
      = failWithTc (ptext (sLit "More than one default declaration for")
                    <+> ppr (tfe_tycon (unLoc d1)))

    tcDefaultAssocDecl fam_tc [L loc (TyFamEqn { tfe_tycon = L _ tc_name
                                               , tfe_pats = hs_tvs
                                               , tfe_rhs = rhs })]
      = setSrcSpan loc $
        tcAddFamInstCtxt (ptext (sLit "default type instance")) tc_name $
        tcTyClTyVars tc_name hs_tvs $ \ tvs rhs_kind ->
        do { traceTc "tcDefaultAssocDecl" (ppr tc_name)
           ; checkTc (isTypeFamilyTyCon fam_tc) (wrongKindOfFamily fam_tc)
           ; let (fam_name, fam_pat_arity, _) = famTyConShape fam_tc
           ; ASSERT( fam_name == tc_name )
             checkTc (length (hsQTvBndrs hs_tvs) == fam_pat_arity)
                     (wrongNumberOfParmsErr fam_pat_arity)
           ; rhs_ty <- tcCheckLHsType rhs rhs_kind
           ; rhs_ty <- zonkTcTypeToType emptyZonkEnv rhs_ty
           ; let fam_tc_tvs = tyConTyVars fam_tc
                 subst = zipTopTvSubst tvs (mkTyVarTys fam_tc_tvs)
           ; return ( ASSERT( equalLength fam_tc_tvs tvs )
                      Just (substTy subst rhs_ty) ) }
        -- We check for well-formedness and validity later, in checkValidClass

    -------------------------
    kcTyFamInstEqn :: FamTyConShape -> LTyFamInstEqn Name -> TcM ()
    kcTyFamInstEqn fam_tc_shape
        (L loc (TyFamEqn { tfe_pats = pats, tfe_rhs = hs_ty }))
      = setSrcSpan loc $
        discardResult $
        tc_fam_ty_pats fam_tc_shape pats (discardResult . (tcCheckLHsType hs_ty))

    tcTyFamInstEqn :: FamTyConShape -> LTyFamInstEqn Name -> TcM CoAxBranch
    -- Needs to be here, not in TcInstDcls, because closed families
    -- (typechecked here) have TyFamInstEqns
    tcTyFamInstEqn fam_tc_shape@(fam_tc_name,_,_)
        (L loc (TyFamEqn { tfe_tycon = L _ eqn_tc_name
                         , tfe_pats = pats
                         , tfe_rhs = hs_ty }))
      = setSrcSpan loc $
        tcFamTyPats fam_tc_shape pats (discardResult . (tcCheckLHsType hs_ty)) $
           \tvs' pats' res_kind ->
        do { checkTc (fam_tc_name == eqn_tc_name)
                     (wrongTyFamName fam_tc_name eqn_tc_name)
           ; rhs_ty <- tcCheckLHsType hs_ty res_kind
           ; rhs_ty <- zonkTcTypeToType emptyZonkEnv rhs_ty
           ; traceTc "tcTyFamInstEqn" (ppr fam_tc_name <+> ppr tvs')
              -- don't print out the pats here, as they might be zonked inside the knot
           ; return (mkCoAxBranch tvs' pats' rhs_ty loc) }

    kcDataDefn :: HsDataDefn Name -> TcKind -> TcM ()
    -- Used for 'data instance' only
    -- Ordinary 'data' is handled by kcTyClDec
    kcDataDefn (HsDataDefn { dd_ctxt = ctxt, dd_cons = cons, dd_kindSig = mb_kind }) res_k
      = do  { _ <- tcHsContext ctxt
            ; checkNoErrs $ mapM_ (wrapLocM kcConDecl) cons
              -- See Note [Failing early in kcDataDefn]
            ; kcResultKind mb_kind res_k }

    ------------------
    kcResultKind :: Maybe (LHsKind Name) -> Kind -> TcM ()
    kcResultKind Nothing res_k
      = checkKind res_k liftedTypeKind
          --             type family F a
          -- defaults to type family F a :: *
    kcResultKind (Just k) res_k
      = do { k' <- tcLHsKind k
           ; checkKind  k' res_k }

    {-
    Kind check type patterns and kind annotate the embedded type variables.
         type instance F [a] = rhs

     * Here we check that a type instance matches its kind signature, but we do
       not check whether there is a pattern for each type index; the latter
       check is only required for type synonym instances.

    Note [tc_fam_ty_pats vs tcFamTyPats]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    tc_fam_ty_pats does the type checking of the patterns, but it doesn't
    zonk or generate any desugaring. It is used when kind-checking closed
    type families.

    tcFamTyPats type checks the patterns, zonks, and then calls thing_inside
    to generate a desugaring. It is used during type-checking (not kind-checking).

    Note [Type-checking type patterns]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    When typechecking the patterns of a family instance declaration, we can't
    rely on using the family TyCon, because this is sometimes called
    from within a type-checking knot. (Specifically for closed type families.)
    The type FamTyConShape gives just enough information to do the job.

    The "arity" field of FamTyConShape is the *visible* arity of the family
    type constructor, i.e. what the users sees and writes, not including kind
    arguments.

    See also Note [tc_fam_ty_pats vs tcFamTyPats]

    Note [Failing early in kcDataDefn]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We need to use checkNoErrs when calling kcConDecl. This is because kcConDecl
    calls tcConDecl, which checks that the return type of a GADT-like constructor
    is actually an instance of the type head. Without the checkNoErrs, potentially
    two bad things could happen:

     1) Duplicate error messages, because tcConDecl will be called again during
        *type* checking (as opposed to kind checking)
     2) If we just keep blindly forging forward after both kind checking and type
        checking, we can get a panic in rejigConRes. See Trac #8368.
    -}

    -----------------
    type FamTyConShape = (Name, Arity, Kind) -- See Note [Type-checking type patterns]

    famTyConShape :: TyCon -> FamTyConShape
    famTyConShape fam_tc
      = ( tyConName fam_tc
        , length (filterOut isKindVar (tyConTyVars fam_tc))
        , tyConKind fam_tc )

    tc_fam_ty_pats :: FamTyConShape
                   -> HsWithBndrs Name [LHsType Name] -- Patterns
                   -> (TcKind -> TcM ())              -- Kind checker for RHS
                                                      -- result is ignored
                   -> TcM ([Kind], [Type], Kind)
    -- Check the type patterns of a type or data family instance
    --     type instance F <pat1> <pat2> = <type>
    -- The 'tyvars' are the free type variables of pats
    --
    -- NB: The family instance declaration may be an associated one,
    -- nested inside an instance decl, thus
    --        instance C [a] where
    --          type F [a] = ...
    -- In that case, the type variable 'a' will *already be in scope*
    -- (and, if C is poly-kinded, so will its kind parameter).

    tc_fam_ty_pats (name, arity, kind)
                   (HsWB { hswb_cts = arg_pats, hswb_kvs = kvars, hswb_tvs = tvars })
                   kind_checker
      = do { let (fam_kvs, fam_body) = splitForAllTys kind

             -- We wish to check that the pattern has the right number of arguments
             -- in checkValidFamPats (in TcValidity), so we can do the check *after*
             -- we're done with the knot. But, the splitKindFunTysN below will panic
             -- if there are *too many* patterns. So, we do a preliminary check here.
             -- Note that we don't have enough information at hand to do a full check,
             -- as that requires the full declared arity of the family, which isn't
             -- nearby.
           ; checkTc (length arg_pats == arity) $
             wrongNumberOfParmsErr arity

             -- Instantiate with meta kind vars
           ; fam_arg_kinds <- mapM (const newMetaKindVar) fam_kvs
           ; loc <- getSrcSpanM
           ; let (arg_kinds, res_kind)
                     = splitKindFunTysN (length arg_pats) $
                       substKiWith fam_kvs fam_arg_kinds fam_body
                 hs_tvs = HsQTvs { hsq_kvs = kvars
                                 , hsq_tvs = userHsTyVarBndrs loc tvars }

             -- Kind-check and quantify
             -- See Note [Quantifying over family patterns]
           ; typats <- tcHsTyVarBndrs hs_tvs $ \ _ ->
                       do { kind_checker res_kind
                          ; tcHsArgTys (quotes (ppr name)) arg_pats arg_kinds }

           ; return (fam_arg_kinds, typats, res_kind) }

    -- See Note [tc_fam_ty_pats vs tcFamTyPats]
    tcFamTyPats :: FamTyConShape
                -> HsWithBndrs Name [LHsType Name] -- patterns
                -> (TcKind -> TcM ())              -- kind-checker for RHS
                -> ([TKVar]              -- Kind and type variables
                    -> [TcType]          -- Kind and type arguments
                    -> Kind -> TcM a)
                -> TcM a
    tcFamTyPats fam_shape@(name,_,_) pats kind_checker thing_inside
      = do { (fam_arg_kinds, typats, res_kind)
                <- tc_fam_ty_pats fam_shape pats kind_checker
           ; let all_args = fam_arg_kinds ++ typats

                -- Find free variables (after zonking) and turn
                -- them into skolems, so that we don't subsequently
                -- replace a meta kind var with AnyK
                -- Very like kindGeneralize
           ; qtkvs <- quantifyTyVars emptyVarSet (tyVarsOfTypes all_args)

                -- Zonk the patterns etc into the Type world
           ; (ze, qtkvs') <- zonkTyBndrsX emptyZonkEnv qtkvs
           ; all_args'    <- zonkTcTypeToTypes ze all_args
           ; res_kind'    <- zonkTcTypeToType  ze res_kind

           ; traceTc "tcFamTyPats" (ppr name)
                -- don't print out too much, as we might be in the knot
           ; tcExtendTyVarEnv qtkvs' $
             thing_inside qtkvs' all_args' res_kind' }

    {-
    Note [Quantifying over family patterns]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We need to quantify over two different lots of kind variables:

    First, the ones that come from the kinds of the tyvar args of
    tcTyVarBndrsKindGen, as usual
      data family Dist a

      -- Proxy :: forall k. k -> *
      data instance Dist (Proxy a) = DP
      -- Generates  data DistProxy = DP
      --            ax8 k (a::k) :: Dist * (Proxy k a) ~ DistProxy k a
      -- The 'k' comes from the tcTyVarBndrsKindGen (a::k)

    Second, the ones that come from the kind argument of the type family
    which we pick up using the (tyVarsOfTypes typats) in the result of
    the thing_inside of tcHsTyvarBndrsGen.
      -- Any :: forall k. k
      data instance Dist Any = DA
      -- Generates  data DistAny k = DA
      --            ax7 k :: Dist k (Any k) ~ DistAny k
      -- The 'k' comes from kindGeneralizeKinds (Any k)

    Note [Quantified kind variables of a family pattern]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider   type family KindFam (p :: k1) (q :: k1)
               data T :: Maybe k1 -> k2 -> *
               type instance KindFam (a :: Maybe k) b = T a b -> Int
    The HsBSig for the family patterns will be ([k], [a])

    Then in the family instance we want to
      * Bring into scope [ "k" -> k:BOX, "a" -> a:k ]
      * Kind-check the RHS
      * Quantify the type instance over k and k', as well as a,b, thus
           type instance [k, k', a:Maybe k, b:k']
                         KindFam (Maybe k) k' a b = T k k' a b -> Int

    Notice that in the third step we quantify over all the visibly-mentioned
    type variables (a,b), but also over the implicitly mentioned kind variables
    (k, k').  In this case one is bound explicitly but often there will be
    none. The role of the kind signature (a :: Maybe k) is to add a constraint
    that 'a' must have that kind, and to bring 'k' into scope.

    ************************************************************************
    *                                                                      *
                   Data types
    *                                                                      *
    ************************************************************************
    -}

    dataDeclChecks :: Name -> NewOrData -> ThetaType -> [LConDecl Name] -> TcM Bool
    dataDeclChecks tc_name new_or_data stupid_theta cons
      = do {   -- Check that we don't use GADT syntax in H98 world
             gadtSyntax_ok <- xoptM Opt_GADTSyntax
           ; let gadt_syntax = consUseGadtSyntax cons
           ; checkTc (gadtSyntax_ok || not gadt_syntax) (badGadtDecl tc_name)

               -- Check that the stupid theta is empty for a GADT-style declaration
           ; checkTc (null stupid_theta || not gadt_syntax) (badStupidTheta tc_name)

             -- Check that a newtype has exactly one constructor
             -- Do this before checking for empty data decls, so that
             -- we don't suggest -XEmptyDataDecls for newtypes
           ; checkTc (new_or_data == DataType || isSingleton cons)
                    (newtypeConError tc_name (length cons))

                    -- Check that there's at least one condecl,
             -- or else we're reading an hs-boot file, or -XEmptyDataDecls
           ; empty_data_decls <- xoptM Opt_EmptyDataDecls
           ; is_boot <- tcIsHsBootOrSig  -- Are we compiling an hs-boot file?
           ; checkTc (not (null cons) || empty_data_decls || is_boot)
                     (emptyConDeclsErr tc_name)
           ; return gadt_syntax }

    -----------------------------------
    consUseGadtSyntax :: [LConDecl a] -> Bool
    consUseGadtSyntax (L _ (ConDecl { con_res = ResTyGADT _ _ }) : _) = True
    consUseGadtSyntax _                                               = False
                     -- All constructors have same shape

    -----------------------------------
    tcConDecls :: NewOrData -> TyCon -> ([TyVar], Type)
               -> [LConDecl Name] -> TcM [DataCon]
    tcConDecls new_or_data rep_tycon (tmpl_tvs, res_tmpl) cons
      = concatMapM (addLocM  $ tcConDecl new_or_data rep_tycon tmpl_tvs res_tmpl)
                   cons

    tcConDecl :: NewOrData
              -> TyCon             -- Representation tycon
              -> [TyVar] -> Type   -- Return type template (with its template tyvars)
                                   --    (tvs, T tys), where T is the family TyCon
              -> ConDecl Name
              -> TcM [DataCon]

    tcConDecl new_or_data rep_tycon tmpl_tvs res_tmpl        -- Data types
              (ConDecl { con_names = names
                       , con_qvars = hs_tvs, con_cxt = hs_ctxt
                       , con_details = hs_details, con_res = hs_res_ty })
      = addErrCtxt (dataConCtxtName names) $
        do { traceTc "tcConDecl 1" (ppr names)
           ; (ctxt, arg_tys, res_ty, field_lbls, stricts)
               <- tcHsTyVarBndrs hs_tvs $ \ _ ->
                  do { ctxt    <- tcHsContext hs_ctxt
                     ; details <- tcConArgs new_or_data hs_details
                     ; res_ty  <- tcConRes hs_res_ty
                     ; let (field_lbls, btys) = details
                           (arg_tys, stricts) = unzip btys
                     ; return (ctxt, arg_tys, res_ty, field_lbls, stricts)
                     }

                 -- Generalise the kind variables (returning quantified TcKindVars)
                 -- and quantify the type variables (substituting their kinds)
                 -- REMEMBER: 'tkvs' are:
                 --    ResTyH98:  the *existential* type variables only
                 --    ResTyGADT: *all* the quantified type variables
                 -- c.f. the comment on con_qvars in HsDecls
           ; tkvs <- case res_ty of
                       ResTyH98           -> quantifyTyVars (mkVarSet tmpl_tvs)
                                                     (tyVarsOfTypes (ctxt++arg_tys))
                       ResTyGADT _ res_ty -> quantifyTyVars emptyVarSet
                                              (tyVarsOfTypes (res_ty:ctxt++arg_tys))

                 -- Zonk to Types
           ; (ze, qtkvs) <- zonkTyBndrsX emptyZonkEnv tkvs
           ; arg_tys <- zonkTcTypeToTypes ze arg_tys
           ; ctxt    <- zonkTcTypeToTypes ze ctxt
           ; res_ty  <- case res_ty of
                          ResTyH98        -> return ResTyH98
                          ResTyGADT ls ty -> ResTyGADT ls <$> zonkTcTypeToType ze ty

           ; let (univ_tvs, ex_tvs, eq_preds, res_ty') = rejigConRes tmpl_tvs res_tmpl qtkvs res_ty

           ; fam_envs <- tcGetFamInstEnvs
           ; let
               buildOneDataCon (L _ name) = do
                 { is_infix <- tcConIsInfix name hs_details res_ty
                 ; buildDataCon fam_envs name is_infix
                                stricts field_lbls
                                univ_tvs ex_tvs eq_preds ctxt arg_tys
                                res_ty' rep_tycon
                      -- NB:  we put data_tc, the type constructor gotten from the
                      --      constructor type signature into the data constructor;
                      --      that way checkValidDataCon can complain if it's wrong.
                 }
           ; mapM buildOneDataCon names
           }

    tcConIsInfix :: Name
                 -> HsConDetails (LHsType Name) (Located [LConDeclField Name])
                 -> ResType Type
                 -> TcM Bool
    tcConIsInfix _   details ResTyH98
      = case details of
               InfixCon {}  -> return True
               _            -> return False
    tcConIsInfix con details (ResTyGADT _ _)
      = case details of
               InfixCon {}  -> return True
               RecCon {}    -> return False
               PrefixCon arg_tys           -- See Note [Infix GADT cons]
                   | isSymOcc (getOccName con)
                   , [_ty1,_ty2] <- arg_tys
                      -> do { fix_env <- getFixityEnv
                            ; return (con `elemNameEnv` fix_env) }
                   | otherwise -> return False

    tcConArgs :: NewOrData -> HsConDeclDetails Name
              -> TcM ([Name], [(TcType, HsSrcBang)])
    tcConArgs new_or_data (PrefixCon btys)
      = do { btys' <- mapM (tcConArg new_or_data) btys
           ; return ([], btys') }
    tcConArgs new_or_data (InfixCon bty1 bty2)
      = do { bty1' <- tcConArg new_or_data bty1
           ; bty2' <- tcConArg new_or_data bty2
           ; return ([], [bty1', bty2']) }
    tcConArgs new_or_data (RecCon fields)
      = do { btys' <- mapM (tcConArg new_or_data) btys
           ; return (field_names, btys') }
      where
        -- We need a one-to-one mapping from field_names to btys
        combined = map (\(L _ f) -> (cd_fld_names f,cd_fld_type f)) (unLoc fields)
        explode (ns,ty) = zip (map unLoc ns) (repeat ty)
        exploded = concatMap explode combined
        (field_names,btys) = unzip exploded

    tcConArg :: NewOrData -> LHsType Name -> TcM (TcType, HsSrcBang)
    tcConArg new_or_data bty
      = do  { traceTc "tcConArg 1" (ppr bty)
            ; arg_ty <- tcHsConArgType new_or_data bty
            ; traceTc "tcConArg 2" (ppr bty)
            ; return (arg_ty, getBangStrictness bty) }

    tcConRes :: ResType (LHsType Name) -> TcM (ResType Type)
    tcConRes ResTyH98           = return ResTyH98
    tcConRes (ResTyGADT ls res_ty) = do { res_ty' <- tcHsLiftedType res_ty
                                        ; return (ResTyGADT ls res_ty') }

    {-
    Note [Infix GADT constructors]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We do not currently have syntax to declare an infix constructor in GADT syntax,
    but it makes a (small) difference to the Show instance.  So as a slightly
    ad-hoc solution, we regard a GADT data constructor as infix if
      a) it is an operator symbol
      b) it has two arguments
      c) there is a fixity declaration for it
    For example:
       infix 6 (:--:)
       data T a where
         (:--:) :: t1 -> t2 -> T Int

    Note [Checking GADT return types]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    There is a delicacy around checking the return types of a datacon. The
    central problem is dealing with a declaration like

      data T a where
        MkT :: a -> Q a

    Note that the return type of MkT is totally bogus. When creating the T
    tycon, we also need to create the MkT datacon, which must have a "rejigged"
    return type. That is, the MkT datacon's type must be transformed to have
    a uniform return type with explicit coercions for GADT-like type parameters.
    This rejigging is what rejigConRes does. The problem is, though, that checking
    that the return type is appropriate is much easier when done over *Type*,
    not *HsType*.

    So, we want to make rejigConRes lazy and then check the validity of the return
    type in checkValidDataCon. But, if the return type is bogus, rejigConRes can't
    work -- it will have a failed pattern match. Luckily, if we run
    checkValidDataCon before ever looking at the rejigged return type
    (checkValidDataCon checks the dataConUserType, which is not rejigged!), we
    catch the error before forcing the rejigged type and panicking.
    -}

    -- Example
    --   data instance T (b,c) where
    --      TI :: forall e. e -> T (e,e)
    --
    -- The representation tycon looks like this:
    --   data :R7T b c where
    --      TI :: forall b1 c1. (b1 ~ c1) => b1 -> :R7T b1 c1
    -- In this case orig_res_ty = T (e,e)

    rejigConRes :: [TyVar] -> Type  -- Template for result type; e.g.
                                    -- data instance T [a] b c = ...
                                    --      gives template ([a,b,c], T [a] b c)
                 -> [TyVar]         -- where MkT :: forall x y z. ...
                 -> ResType Type
                 -> ([TyVar],               -- Universal
                     [TyVar],               -- Existential (distinct OccNames from univs)
                     [(TyVar,Type)],        -- Equality predicates
                     Type)          -- Typechecked return type
            -- We don't check that the TyCon given in the ResTy is
            -- the same as the parent tycon, because checkValidDataCon will do it

    rejigConRes tmpl_tvs res_ty dc_tvs ResTyH98
      = (tmpl_tvs, dc_tvs, [], res_ty)
            -- In H98 syntax the dc_tvs are the existential ones
            --      data T a b c = forall d e. MkT ...
            -- The {a,b,c} are tc_tvs, and {d,e} are dc_tvs

    rejigConRes tmpl_tvs res_tmpl dc_tvs (ResTyGADT _ res_ty)
            -- E.g.  data T [a] b c where
            --         MkT :: forall x y z. T [(x,y)] z z
            -- Then we generate
            --      Univ tyvars     Eq-spec
            --          a              a~(x,y)
            --          b              b~z
            --          z
            -- Existentials are the leftover type vars: [x,y]
            -- So we return ([a,b,z], [x,y], [a~(x,y),b~z], T [(x,y)] z z)
      = (univ_tvs, ex_tvs, eq_spec, res_ty)
      where
        Just subst = tcMatchTy (mkVarSet tmpl_tvs) res_tmpl res_ty
                    -- This 'Just' pattern is sure to match, because if not
                    -- checkValidDataCon will complain first.
                   -- See Note [Checking GADT return types]

                    -- /Lazily/ figure out the univ_tvs etc
                    -- Each univ_tv is either a dc_tv or a tmpl_tv
        (univ_tvs, eq_spec) = foldr choose ([], []) tmpl_tvs
        choose tmpl (univs, eqs)
          | Just ty <- lookupTyVar subst tmpl
          = case tcGetTyVar_maybe ty of
              Just tv | not (tv `elem` univs)
                -> (tv:univs,   eqs)
              _other  -> (new_tmpl:univs, (new_tmpl,ty):eqs)
                         where  -- see Note [Substitution in template variables kinds]
                           new_tmpl = updateTyVarKind (substTy subst) tmpl
          | otherwise = pprPanic "tcResultType" (ppr res_ty)
        ex_tvs = dc_tvs `minusList` univ_tvs

    {-
    Note [Substitution in template variables kinds]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    data List a = Nil | Cons a (List a)
    data SList s as where
      SNil :: SList s Nil

    We call tcResultType with
      tmpl_tvs = [(k :: BOX), (s :: k -> *), (as :: List k)]
      res_tmpl = SList k s as
      res_ty = ResTyGADT (SList k1 (s1 :: k1 -> *) (Nil k1))

    We get subst:
      k -> k1
      s -> s1
      as -> Nil k1

    Now we want to find out the universal variables and the equivalences
    between some of them and types (GADT).

    In this example, k and s are mapped to exactly variables which are not
    already present in the universal set, so we just add them without any
    coercion.

    But 'as' is mapped to 'Nil k1', so we add 'as' to the universal set,
    and add the equivalence with 'Nil k1' in 'eqs'.

    The problem is that with kind polymorphism, as's kind may now contain
    kind variables, and we have to apply the template substitution to it,
    which is why we create new_tmpl.

    The template substitution only maps kind variables to kind variables,
    since GADTs are not kind indexed.

    ************************************************************************
    *                                                                      *
                    Validity checking
    *                                                                      *
    ************************************************************************

    Validity checking is done once the mutually-recursive knot has been
    tied, so we can look at things freely.
    -}

    checkClassCycleErrs :: Class -> TcM ()
    checkClassCycleErrs cls = mapM_ recClsErr (calcClassCycles cls)

    checkValidTyCl :: TyThing -> TcM ()
    checkValidTyCl thing
      = setSrcSpan (getSrcSpan thing) $
        addTyThingCtxt thing $
        case thing of
          ATyCon tc -> checkValidTyCon tc
          AnId _    -> return ()  -- Generic default methods are checked
                                  -- with their parent class
          ACoAxiom _ -> return () -- Axioms checked with their parent
                                  -- closed family tycon
          _         -> pprTrace "checkValidTyCl" (ppr thing) $ return ()

    -------------------------
    -- For data types declared with record syntax, we require
    -- that each constructor that has a field 'f'
    --      (a) has the same result type
    --      (b) has the same type for 'f'
    -- module alpha conversion of the quantified type variables
    -- of the constructor.
    --
    -- Note that we allow existentials to match because the
    -- fields can never meet. E.g
    --      data T where
    --        T1 { f1 :: b, f2 :: a, f3 ::Int } :: T
    --        T2 { f1 :: c, f2 :: c, f3 ::Int } :: T
    -- Here we do not complain about f1,f2 because they are existential

    checkValidTyCon :: TyCon -> TcM ()
    checkValidTyCon tc
      | Just cl <- tyConClass_maybe tc
      = checkValidClass cl

      | Just syn_rhs <- synTyConRhs_maybe tc
      = checkValidType syn_ctxt syn_rhs

      | Just fam_flav <- famTyConFlav_maybe tc
      = case fam_flav of
        { ClosedSynFamilyTyCon ax      -> checkValidClosedCoAxiom ax
        ; AbstractClosedSynFamilyTyCon ->
          do { hsBoot <- tcIsHsBootOrSig
             ; checkTc hsBoot $
               ptext (sLit "You may omit the equations in a closed type family") $$
               ptext (sLit "only in a .hs-boot file") }
        ; OpenSynFamilyTyCon           -> return ()
        ; BuiltInSynFamTyCon _         -> return () }

      | otherwise
      = do { -- Check the context on the data decl
             traceTc "cvtc1" (ppr tc)
           ; checkValidTheta (DataTyCtxt name) (tyConStupidTheta tc)

           ; traceTc "cvtc2" (ppr tc)

           ; dflags          <- getDynFlags
           ; existential_ok  <- xoptM Opt_ExistentialQuantification
           ; gadt_ok         <- xoptM Opt_GADTs
           ; let ex_ok = existential_ok || gadt_ok  -- Data cons can have existential context
           ; mapM_ (checkValidDataCon dflags ex_ok tc) data_cons

            -- Check that fields with the same name share a type
           ; mapM_ check_fields groups }

      where
        syn_ctxt  = TySynCtxt name
        name      = tyConName tc
        data_cons = tyConDataCons tc

        groups = equivClasses cmp_fld (concatMap get_fields data_cons)
        cmp_fld (f1,_) (f2,_) = f1 `compare` f2
        get_fields con = dataConFieldLabels con `zip` repeat con
            -- dataConFieldLabels may return the empty list, which is fine

        -- See Note [GADT record selectors] in MkId.lhs
        -- We must check (a) that the named field has the same
        --                   type in each constructor
        --               (b) that those constructors have the same result type
        --
        -- However, the constructors may have differently named type variable
        -- and (worse) we don't know how the correspond to each other.  E.g.
        --     C1 :: forall a b. { f :: a, g :: b } -> T a b
        --     C2 :: forall d c. { f :: c, g :: c } -> T c d
        --
        -- So what we do is to ust Unify.tcMatchTys to compare the first candidate's
        -- result type against other candidates' types BOTH WAYS ROUND.
        -- If they magically agrees, take the substitution and
        -- apply them to the latter ones, and see if they match perfectly.
        check_fields ((label, con1) : other_fields)
            -- These fields all have the same name, but are from
            -- different constructors in the data type
            = recoverM (return ()) $ mapM_ checkOne other_fields
                    -- Check that all the fields in the group have the same type
                    -- NB: this check assumes that all the constructors of a given
                    -- data type use the same type variables
            where
            (tvs1, _, _, res1) = dataConSig con1
            ts1 = mkVarSet tvs1
            fty1 = dataConFieldType con1 label

            checkOne (_, con2)    -- Do it bothways to ensure they are structurally identical
                = do { checkFieldCompat label con1 con2 ts1 res1 res2 fty1 fty2
                     ; checkFieldCompat label con2 con1 ts2 res2 res1 fty2 fty1 }
                where
                    (tvs2, _, _, res2) = dataConSig con2
                    ts2 = mkVarSet tvs2
                    fty2 = dataConFieldType con2 label
        check_fields [] = panic "checkValidTyCon/check_fields []"

    checkValidClosedCoAxiom :: CoAxiom Branched -> TcM ()
    checkValidClosedCoAxiom (CoAxiom { co_ax_branches = branches, co_ax_tc = tc })
     = tcAddClosedTypeFamilyDeclCtxt tc $
       do { brListFoldlM_ check_accessibility [] branches
          ; void $ brListMapM (checkValidTyFamInst Nothing tc) branches }
       where
         check_accessibility :: [CoAxBranch]       -- prev branches (in reverse order)
                             -> CoAxBranch         -- cur branch
                             -> TcM [CoAxBranch]   -- cur : prev
                   -- Check whether the branch is dominated by earlier
                   -- ones and hence is inaccessible
         check_accessibility prev_branches cur_branch
           = do { when (cur_branch `isDominatedBy` prev_branches) $
                  addWarnAt (coAxBranchSpan cur_branch) $
                  inaccessibleCoAxBranch tc cur_branch
                ; return (cur_branch : prev_branches) }

    checkFieldCompat :: Name -> DataCon -> DataCon -> TyVarSet
                     -> Type -> Type -> Type -> Type -> TcM ()
    checkFieldCompat fld con1 con2 tvs1 res1 res2 fty1 fty2
      = do  { checkTc (isJust mb_subst1) (resultTypeMisMatch fld con1 con2)
            ; checkTc (isJust mb_subst2) (fieldTypeMisMatch fld con1 con2) }
      where
        mb_subst1 = tcMatchTy tvs1 res1 res2
        mb_subst2 = tcMatchTyX tvs1 (expectJust "checkFieldCompat" mb_subst1) fty1 fty2

    -------------------------------
    checkValidDataCon :: DynFlags -> Bool -> TyCon -> DataCon -> TcM ()
    checkValidDataCon dflags existential_ok tc con
      = setSrcSpan (srcLocSpan (getSrcLoc con))     $
        addErrCtxt (dataConCtxt con)                $
        do  { -- Check that the return type of the data constructor
              -- matches the type constructor; eg reject this:
              --   data T a where { MkT :: Bogus a }
              -- c.f. Note [Check role annotations in a second pass]
              --  and Note [Checking GADT return types]
              let tc_tvs = tyConTyVars tc
                  res_ty_tmpl = mkFamilyTyConApp tc (mkTyVarTys tc_tvs)
                  orig_res_ty = dataConOrigResTy con
            ; traceTc "checkValidDataCon" (vcat
                  [ ppr con, ppr tc, ppr tc_tvs
                  , ppr res_ty_tmpl <+> dcolon <+> ppr (typeKind res_ty_tmpl)
                  , ppr orig_res_ty <+> dcolon <+> ppr (typeKind orig_res_ty)])

            ; checkTc (isJust (tcMatchTy (mkVarSet tc_tvs)
                                         res_ty_tmpl
                                         orig_res_ty))
                      (badDataConTyCon con res_ty_tmpl orig_res_ty)

              -- Check that the result type is a *monotype*
              --  e.g. reject this:   MkT :: T (forall a. a->a)
              -- Reason: it's really the argument of an equality constraint
            ; checkValidMonoType orig_res_ty

              -- Check all argument types for validity
            ; checkValidType ctxt (dataConUserType con)

              -- Extra checks for newtype data constructors
            ; when (isNewTyCon tc) (checkNewDataCon con)

              -- Check that UNPACK pragmas and bangs work out
              -- E.g.  reject   data T = MkT {-# UNPACK #-} Int     -- No "!"
              --                data T = MkT {-# UNPACK #-} !a      -- Can't unpack
            ; mapM_ check_bang (zip3 (dataConSrcBangs con) (dataConImplBangs con) [1..])

              -- Check that existentials are allowed if they are used
            ; checkTc (existential_ok || isVanillaDataCon con)
                      (badExistential con)

              -- Check that we aren't doing GADT type refinement on kind variables
              -- e.g reject    data T (a::k) where
              --                  T1 :: T Int
              --                  T2 :: T Maybe
            ; checkTc (not (any (isKindVar . fst) (dataConEqSpec con)))
                      (badGadtKindCon con)

            ; traceTc "Done validity of data con" (ppr con <+> ppr (dataConRepType con))
        }
      where
        ctxt = ConArgCtxt (dataConName con)
        check_bang (HsSrcBang _ (Just want_unpack) has_bang, rep_bang, n)
          | want_unpack, not has_bang
          = addWarnTc (bad_bang n (ptext (sLit "UNPACK pragma lacks '!'")))
          | want_unpack
          , case rep_bang of { HsUnpack {} -> False; _ -> True }
          , not (gopt Opt_OmitInterfacePragmas dflags)
               -- If not optimising, se don't unpack, so don't complain!
               -- See MkId.dataConArgRep, the (HsBang True) case
          = addWarnTc (bad_bang n (ptext (sLit "Ignoring unusable UNPACK pragma")))

        check_bang _
          = return ()

        bad_bang n herald
          = hang herald 2 (ptext (sLit "on the") <+> speakNth n
                           <+> ptext (sLit "argument of") <+> quotes (ppr con))
    -------------------------------
    checkNewDataCon :: DataCon -> TcM ()
    -- Further checks for the data constructor of a newtype
    checkNewDataCon con
      = do  { checkTc (isSingleton arg_tys) (newtypeFieldErr con (length arg_tys))
                    -- One argument

            ; check_con (null eq_spec) $
              ptext (sLit "A newtype constructor must have a return type of form T a1 ... an")
                    -- Return type is (T a b c)

            ; check_con (null theta) $
              ptext (sLit "A newtype constructor cannot have a context in its type")

            ; check_con (null ex_tvs) $
              ptext (sLit "A newtype constructor cannot have existential type variables")
                    -- No existentials

            ; checkTc (not (any isBanged (dataConSrcBangs con)))
                      (newtypeStrictError con)
                    -- No strictness
        }
      where
        (_univ_tvs, ex_tvs, eq_spec, theta, arg_tys, _res_ty) = dataConFullSig con
        check_con what msg
           = checkTc what (msg $$ ppr con <+> dcolon <+> ppr (dataConUserType con))

    -------------------------------
    checkValidClass :: Class -> TcM ()
    checkValidClass cls
      = do  { constrained_class_methods <- xoptM Opt_ConstrainedClassMethods
            ; multi_param_type_classes <- xoptM Opt_MultiParamTypeClasses
            ; nullary_type_classes <- xoptM Opt_NullaryTypeClasses
            ; fundep_classes <- xoptM Opt_FunctionalDependencies

            -- Check that the class is unary, unless multiparameter type classes
            -- are enabled; also recognize deprecated nullary type classes
            -- extension (subsumed by multiparameter type classes, Trac #8993)
            ; checkTc (multi_param_type_classes || cls_arity == 1 ||
                        (nullary_type_classes && cls_arity == 0))
                      (classArityErr cls_arity cls)
            ; checkTc (fundep_classes || null fundeps) (classFunDepsErr cls)

            -- Check the super-classes
            ; checkValidTheta (ClassSCCtxt (className cls)) theta

              -- Now check for cyclic superclasses
              -- If there are superclass cycles, checkClassCycleErrs bails.
            ; checkClassCycleErrs cls

            -- Check the class operations.
            -- But only if there have been no earlier errors
            -- See Note [Abort when superclass cycle is detected]
            ; whenNoErrs $
              mapM_ (check_op constrained_class_methods) op_stuff

            -- Check the associated type defaults are well-formed and instantiated
            ; mapM_ check_at_defs at_stuff  }
      where
        (tyvars, fundeps, theta, _, at_stuff, op_stuff) = classExtraBigSig cls
        cls_arity = count isTypeVar tyvars    -- Ignore kind variables
        cls_tv_set = mkVarSet tyvars

        check_op constrained_class_methods (sel_id, dm)
          = addErrCtxt (classOpCtxt sel_id tau) $ do
            { checkValidTheta ctxt (tail theta)
                    -- The 'tail' removes the initial (C a) from the
                    -- class itself, leaving just the method type

            ; traceTc "class op type" (ppr op_ty <+> ppr tau)
            ; checkValidType ctxt tau

                    -- Check that the method type mentions a class variable
                    -- But actually check that the variables *reachable from*
                    -- the method type include a class variable.
                    -- Example: tc223
                    --   class Error e => Game b mv e | b -> mv e where
                    --      newBoard :: MonadState b m => m ()
                    -- Here, MonadState has a fundep m->b, so newBoard is fine
            ; check_mentions (growThetaTyVars theta (tyVarsOfType tau))
                             (ptext (sLit "class method") <+> quotes (ppr sel_id))

            ; case dm of
                GenDefMeth dm_name -> do { dm_id <- tcLookupId dm_name
                                         ; checkValidType (FunSigCtxt op_name) (idType dm_id) }
                _                  -> return ()
            }
            where
              ctxt    = FunSigCtxt op_name
              op_name = idName sel_id
              op_ty   = idType sel_id
              (_,theta1,tau1) = tcSplitSigmaTy op_ty
              (_,theta2,tau2)  = tcSplitSigmaTy tau1
              (theta,tau) | constrained_class_methods = (theta1 ++ theta2, tau2)
                          | otherwise = (theta1, mkPhiTy (tail theta1) tau1)
                    -- Ugh!  The function might have a type like
                    --      op :: forall a. C a => forall b. (Eq b, Eq a) => tau2
                    -- With -XConstrainedClassMethods, we want to allow this, even though the inner
                    -- forall has an (Eq a) constraint.  Whereas in general, each constraint
                    -- in the context of a for-all must mention at least one quantified
                    -- type variable.  What a mess!

        check_at_defs (ATI fam_tc _)
          = check_mentions (mkVarSet (tyConTyVars fam_tc))
                           (ptext (sLit "associated type") <+> quotes (ppr fam_tc))

        check_mentions :: TyVarSet -> SDoc -> TcM ()
           -- Check that the thing (method or associated type) mentions at least
           -- one of the class type variables
           -- The check is disabled for nullary type classes,
           -- since there is no possible ambiguity (Trac #10020)
        check_mentions thing_tvs thing_doc
          = checkTc (cls_arity == 0 || thing_tvs `intersectsVarSet` cls_tv_set)
                    (noClassTyVarErr cls thing_doc)

    checkFamFlag :: Name -> TcM ()
    -- Check that we don't use families without -XTypeFamilies
    -- The parser won't even parse them, but I suppose a GHC API
    -- client might have a go!
    checkFamFlag tc_name
      = do { idx_tys <- xoptM Opt_TypeFamilies
           ; checkTc idx_tys err_msg }
      where
        err_msg = hang (ptext (sLit "Illegal family declaration for") <+> quotes (ppr tc_name))
                     2 (ptext (sLit "Use TypeFamilies to allow indexed type families"))

    {-
    Note [Abort when superclass cycle is detected]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We must avoid doing the ambiguity check for the methods (in
    checkValidClass.check_op) when there are already errors accumulated.
    This is because one of the errors may be a superclass cycle, and
    superclass cycles cause canonicalization to loop. Here is a
    representative example:

      class D a => C a where
        meth :: D a => ()
      class C a => D a

    This fixes Trac #9415, #9739

    ************************************************************************
    *                                                                      *
                    Checking role validity
    *                                                                      *
    ************************************************************************
    -}

    checkValidRoleAnnots :: RoleAnnots -> TyThing -> TcM ()
    checkValidRoleAnnots role_annots thing
      = case thing of
        { ATyCon tc
            | isTypeSynonymTyCon tc -> check_no_roles
            | isFamilyTyCon tc      -> check_no_roles
            | isAlgTyCon tc         -> check_roles
            where
              name                   = tyConName tc

         -- Role annotations are given only on *type* variables, but a tycon stores
         -- roles for all variables. So, we drop the kind roles (which are all
         -- Nominal, anyway).
              tyvars                 = tyConTyVars tc
              roles                  = tyConRoles tc
              (kind_vars, type_vars) = span isKindVar tyvars
              type_roles             = dropList kind_vars roles
              role_annot_decl_maybe  = lookupRoleAnnots role_annots name

              check_roles
                = whenIsJust role_annot_decl_maybe $
                    \decl@(L loc (RoleAnnotDecl _ the_role_annots)) ->
                    addRoleAnnotCtxt name $
                    setSrcSpan loc $ do
                    { role_annots_ok <- xoptM Opt_RoleAnnotations
                    ; checkTc role_annots_ok $ needXRoleAnnotations tc
                    ; checkTc (type_vars `equalLength` the_role_annots)
                              (wrongNumberOfRoles type_vars decl)
                    ; _ <- zipWith3M checkRoleAnnot type_vars the_role_annots type_roles
                    -- Representational or phantom roles for class parameters
                    -- quickly lead to incoherence. So, we require
                    -- IncoherentInstances to have them. See #8773.
                    ; incoherent_roles_ok <- xoptM Opt_IncoherentInstances
                    ; checkTc (  incoherent_roles_ok
                              || (not $ isClassTyCon tc)
                              || (all (== Nominal) type_roles))
                              incoherentRoles

                    ; lint <- goptM Opt_DoCoreLinting
                    ; when lint $ checkValidRoles tc }

              check_no_roles
                = whenIsJust role_annot_decl_maybe illegalRoleAnnotDecl
        ; _ -> return () }

    checkRoleAnnot :: TyVar -> Located (Maybe Role) -> Role -> TcM ()
    checkRoleAnnot _  (L _ Nothing)   _  = return ()
    checkRoleAnnot tv (L _ (Just r1)) r2
      = when (r1 /= r2) $
        addErrTc $ badRoleAnnot (tyVarName tv) r1 r2

    -- This is a double-check on the role inference algorithm. It is only run when
    -- -dcore-lint is enabled. See Note [Role inference] in TcTyDecls
    checkValidRoles :: TyCon -> TcM ()
    -- If you edit this function, you may need to update the GHC formalism
    -- See Note [GHC Formalism] in CoreLint
    checkValidRoles tc
      | isAlgTyCon tc
        -- tyConDataCons returns an empty list for data families
      = mapM_ check_dc_roles (tyConDataCons tc)
      | Just rhs <- synTyConRhs_maybe tc
      = check_ty_roles (zipVarEnv (tyConTyVars tc) (tyConRoles tc)) Representational rhs
      | otherwise
      = return ()
      where
        check_dc_roles datacon
          = do { traceTc "check_dc_roles" (ppr datacon <+> ppr (tyConRoles tc))
               ; mapM_ (check_ty_roles role_env Representational) $
                        eqSpecPreds eq_spec ++ theta ++ arg_tys }
                        -- See Note [Role-checking data constructor arguments] in TcTyDecls
          where
            (univ_tvs, ex_tvs, eq_spec, theta, arg_tys, _res_ty) = dataConFullSig datacon
            univ_roles = zipVarEnv univ_tvs (tyConRoles tc)
                  -- zipVarEnv uses zipEqual, but we don't want that for ex_tvs
            ex_roles   = mkVarEnv (zip ex_tvs (repeat Nominal))
            role_env   = univ_roles `plusVarEnv` ex_roles

        check_ty_roles env role (TyVarTy tv)
          = case lookupVarEnv env tv of
              Just role' -> unless (role' `ltRole` role || role' == role) $
                            report_error $ ptext (sLit "type variable") <+> quotes (ppr tv) <+>
                                           ptext (sLit "cannot have role") <+> ppr role <+>
                                           ptext (sLit "because it was assigned role") <+> ppr role'
              Nothing    -> report_error $ ptext (sLit "type variable") <+> quotes (ppr tv) <+>
                                           ptext (sLit "missing in environment")

        check_ty_roles env Representational (TyConApp tc tys)
          = let roles' = tyConRoles tc in
            zipWithM_ (maybe_check_ty_roles env) roles' tys

        check_ty_roles env Nominal (TyConApp _ tys)
          = mapM_ (check_ty_roles env Nominal) tys

        check_ty_roles _   Phantom ty@(TyConApp {})
          = pprPanic "check_ty_roles" (ppr ty)

        check_ty_roles env role (AppTy ty1 ty2)
          =  check_ty_roles env role    ty1
          >> check_ty_roles env Nominal ty2

        check_ty_roles env role (FunTy ty1 ty2)
          =  check_ty_roles env role ty1
          >> check_ty_roles env role ty2

        check_ty_roles env role (ForAllTy tv ty)
          = check_ty_roles (extendVarEnv env tv Nominal) role ty

        check_ty_roles _   _    (LitTy {}) = return ()

        maybe_check_ty_roles env role ty
          = when (role == Nominal || role == Representational) $
            check_ty_roles env role ty

        report_error doc
          = addErrTc $ vcat [ptext (sLit "Internal error in role inference:"),
                             doc,
                             ptext (sLit "Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug")]

    {-
    ************************************************************************
    *                                                                      *
                    Building record selectors
    *                                                                      *
    ************************************************************************
    -}

    mkDefaultMethodIds :: [TyThing] -> [Id]
    -- See Note [Default method Ids and Template Haskell]
    mkDefaultMethodIds things
      = [ mkExportedLocalId VanillaId dm_name (idType sel_id)
        | ATyCon tc <- things
        , Just cls <- [tyConClass_maybe tc]
        , (sel_id, DefMeth dm_name) <- classOpItems cls ]

    {-
    Note [Default method Ids and Template Haskell]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider this (Trac #4169):
       class Numeric a where
         fromIntegerNum :: a
         fromIntegerNum = ...

       ast :: Q [Dec]
       ast = [d| instance Numeric Int |]

    When we typecheck 'ast' we have done the first pass over the class decl
    (in tcTyClDecls), but we have not yet typechecked the default-method
    declarations (because they can mention value declarations).  So we
    must bring the default method Ids into scope first (so they can be seen
    when typechecking the [d| .. |] quote, and typecheck them later.
    -}

    mkRecSelBinds :: [TyThing] -> HsValBinds Name
    -- NB We produce *un-typechecked* bindings, rather like 'deriving'
    --    This makes life easier, because the later type checking will add
    --    all necessary type abstractions and applications
    mkRecSelBinds tycons
      = ValBindsOut [(NonRecursive, b) | b <- binds] sigs
      where
        (sigs, binds) = unzip rec_sels
        rec_sels = map mkRecSelBind [ (tc,fld)
                                    | ATyCon tc <- tycons
                                    , fld <- tyConFields tc ]

    mkRecSelBind :: (TyCon, FieldLabel) -> (LSig Name, LHsBinds Name)
    mkRecSelBind (tycon, sel_name)
      = (L loc (IdSig sel_id), unitBag (L loc sel_bind))
      where
        loc    = getSrcSpan sel_name
        sel_id = mkExportedLocalId rec_details sel_name sel_ty
        rec_details = RecSelId { sel_tycon = tycon, sel_naughty = is_naughty }

        -- Find a representative constructor, con1
        all_cons     = tyConDataCons tycon
        cons_w_field = [ con | con <- all_cons
                       , sel_name `elem` dataConFieldLabels con ]
        con1 = ASSERT( not (null cons_w_field) ) head cons_w_field

        -- Selector type; Note [Polymorphic selectors]
        field_ty   = dataConFieldType con1 sel_name
        data_ty    = dataConOrigResTy con1
        data_tvs   = tyVarsOfType data_ty
        is_naughty = not (tyVarsOfType field_ty `subVarSet` data_tvs)
        (field_tvs, field_theta, field_tau) = tcSplitSigmaTy field_ty
        sel_ty | is_naughty = unitTy  -- See Note [Naughty record selectors]
               | otherwise  = mkForAllTys (varSetElemsKvsFirst $
                                           data_tvs `extendVarSetList` field_tvs) $
                              mkPhiTy (dataConStupidTheta con1) $   -- Urgh!
                              mkPhiTy field_theta               $   -- Urgh!
                              mkFunTy data_ty field_tau

        -- Make the binding: sel (C2 { fld = x }) = x
        --                   sel (C7 { fld = x }) = x
        --    where cons_w_field = [C2,C7]
        sel_bind = mkTopFunBind Generated sel_lname alts
          where
            alts | is_naughty = [mkSimpleMatch [] unit_rhs]
                 | otherwise =  map mk_match cons_w_field ++ deflt
        mk_match con = mkSimpleMatch [L loc (mk_sel_pat con)]
                                     (L loc (HsVar field_var))
        mk_sel_pat con = ConPatIn (L loc (getName con)) (RecCon rec_fields)
        rec_fields = HsRecFields { rec_flds = [rec_field], rec_dotdot = Nothing }
        rec_field  = noLoc (HsRecField { hsRecFieldId = sel_lname
                                       , hsRecFieldArg = L loc (VarPat field_var)
                                       , hsRecPun = False })
        sel_lname = L loc sel_name
        field_var = mkInternalName (mkBuiltinUnique 1) (getOccName sel_name) loc

        -- Add catch-all default case unless the case is exhaustive
        -- We do this explicitly so that we get a nice error message that
        -- mentions this particular record selector
        deflt | all dealt_with all_cons = []
              | otherwise = [mkSimpleMatch [L loc (WildPat placeHolderType)]
                                (mkHsApp (L loc (HsVar (getName rEC_SEL_ERROR_ID)))
                                         (L loc (HsLit msg_lit)))]

            -- Do not add a default case unless there are unmatched
            -- constructors.  We must take account of GADTs, else we
            -- get overlap warning messages from the pattern-match checker
            -- NB: we need to pass type args for the *representation* TyCon
            --     to dataConCannotMatch, hence the calculation of inst_tys
            --     This matters in data families
            --              data instance T Int a where
            --                 A :: { fld :: Int } -> T Int Bool
            --                 B :: { fld :: Int } -> T Int Char
        dealt_with con = con `elem` cons_w_field || dataConCannotMatch inst_tys con
        inst_tys = substTyVars (mkTopTvSubst (dataConEqSpec con1)) (dataConUnivTyVars con1)

        unit_rhs = mkLHsTupleExpr []
        msg_lit = HsStringPrim "" $ unsafeMkByteString $
                  occNameString (getOccName sel_name)

    ---------------
    tyConFields :: TyCon -> [FieldLabel]
    tyConFields tc
      | isAlgTyCon tc = nub (concatMap dataConFieldLabels (tyConDataCons tc))
      | otherwise     = []

    {-
    Note [Polymorphic selectors]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    When a record has a polymorphic field, we pull the foralls out to the front.
       data T = MkT { f :: forall a. [a] -> a }
    Then f :: forall a. T -> [a] -> a
    NOT  f :: T -> forall a. [a] -> a

    This is horrid.  It's only needed in deeply obscure cases, which I hate.
    The only case I know is test tc163, which is worth looking at.  It's far
    from clear that this test should succeed at all!

    Note [Naughty record selectors]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    A "naughty" field is one for which we can't define a record
    selector, because an existential type variable would escape.  For example:
            data T = forall a. MkT { x,y::a }
    We obviously can't define
            x (MkT v _) = v
    Nevertheless we *do* put a RecSelId into the type environment
    so that if the user tries to use 'x' as a selector we can bleat
    helpfully, rather than saying unhelpfully that 'x' is not in scope.
    Hence the sel_naughty flag, to identify record selectors that don't really exist.

    In general, a field is "naughty" if its type mentions a type variable that
    isn't in the result type of the constructor.  Note that this *allows*
    GADT record selectors (Note [GADT record selectors]) whose types may look
    like     sel :: T [a] -> a

    For naughty selectors we make a dummy binding
       sel = ()
    for naughty selectors, so that the later type-check will add them to the
    environment, and they'll be exported.  The function is never called, because
    the tyepchecker spots the sel_naughty field.

    Note [GADT record selectors]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    For GADTs, we require that all constructors with a common field 'f' have the same
    result type (modulo alpha conversion).  [Checked in TcTyClsDecls.checkValidTyCon]
    E.g.
            data T where
              T1 { f :: Maybe a } :: T [a]
              T2 { f :: Maybe a, y :: b  } :: T [a]
              T3 :: T Int

    and now the selector takes that result type as its argument:
       f :: forall a. T [a] -> Maybe a

    Details: the "real" types of T1,T2 are:
       T1 :: forall r a.   (r~[a]) => a -> T r
       T2 :: forall r a b. (r~[a]) => a -> b -> T r

    So the selector loooks like this:
       f :: forall a. T [a] -> Maybe a
       f (a:*) (t:T [a])
         = case t of
             T1 c   (g:[a]~[c]) (v:Maybe c)       -> v `cast` Maybe (right (sym g))
             T2 c d (g:[a]~[c]) (v:Maybe c) (w:d) -> v `cast` Maybe (right (sym g))
             T3 -> error "T3 does not have field f"

    Note the forall'd tyvars of the selector are just the free tyvars
    of the result type; there may be other tyvars in the constructor's
    type (e.g. 'b' in T2).

    Note the need for casts in the result!

    Note [Selector running example]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    It's OK to combine GADTs and type families.  Here's a running example:

            data instance T [a] where
              T1 { fld :: b } :: T [Maybe b]

    The representation type looks like this
            data :R7T a where
              T1 { fld :: b } :: :R7T (Maybe b)

    and there's coercion from the family type to the representation type
            :CoR7T a :: T [a] ~ :R7T a

    The selector we want for fld looks like this:

            fld :: forall b. T [Maybe b] -> b
            fld = /\b. \(d::T [Maybe b]).
                  case d `cast` :CoR7T (Maybe b) of
                    T1 (x::b) -> x

    The scrutinee of the case has type :R7T (Maybe b), which can be
    gotten by appying the eq_spec to the univ_tvs of the data con.

    ************************************************************************
    *                                                                      *
                    Error messages
    *                                                                      *
    ************************************************************************
    -}

    tcAddTyFamInstCtxt :: TyFamInstDecl Name -> TcM a -> TcM a
    tcAddTyFamInstCtxt decl
      = tcAddFamInstCtxt (ptext (sLit "type instance")) (tyFamInstDeclName decl)

    tcAddDataFamInstCtxt :: DataFamInstDecl Name -> TcM a -> TcM a
    tcAddDataFamInstCtxt decl
      = tcAddFamInstCtxt (pprDataFamInstFlavour decl <+> ptext (sLit "instance"))
                         (unLoc (dfid_tycon decl))

    tcAddFamInstCtxt :: SDoc -> Name -> TcM a -> TcM a
    tcAddFamInstCtxt flavour tycon thing_inside
      = addErrCtxt ctxt thing_inside
      where
         ctxt = hsep [ptext (sLit "In the") <+> flavour
                      <+> ptext (sLit "declaration for"),
                      quotes (ppr tycon)]

    tcAddClosedTypeFamilyDeclCtxt :: TyCon -> TcM a -> TcM a
    tcAddClosedTypeFamilyDeclCtxt tc
      = addErrCtxt ctxt
      where
        ctxt = ptext (sLit "In the equations for closed type family") <+>
               quotes (ppr tc)

    resultTypeMisMatch :: Name -> DataCon -> DataCon -> SDoc
    resultTypeMisMatch field_name con1 con2
      = vcat [sep [ptext (sLit "Constructors") <+> ppr con1 <+> ptext (sLit "and") <+> ppr con2,
                    ptext (sLit "have a common field") <+> quotes (ppr field_name) <> comma],
              nest 2 $ ptext (sLit "but have different result types")]

    fieldTypeMisMatch :: Name -> DataCon -> DataCon -> SDoc
    fieldTypeMisMatch field_name con1 con2
      = sep [ptext (sLit "Constructors") <+> ppr con1 <+> ptext (sLit "and") <+> ppr con2,
             ptext (sLit "give different types for field"), quotes (ppr field_name)]

    dataConCtxtName :: [Located Name] -> SDoc
    dataConCtxtName [con]
       = ptext (sLit "In the definition of data constructor") <+> quotes (ppr con)
    dataConCtxtName con
       = ptext (sLit "In the definition of data constructors") <+> interpp'SP con

    dataConCtxt :: Outputable a => a -> SDoc
    dataConCtxt con = ptext (sLit "In the definition of data constructor") <+> quotes (ppr con)

    classOpCtxt :: Var -> Type -> SDoc
    classOpCtxt sel_id tau = sep [ptext (sLit "When checking the class method:"),
                                  nest 2 (pprPrefixOcc sel_id <+> dcolon <+> ppr tau)]

    classArityErr :: Int -> Class -> SDoc
    classArityErr n cls
        | n == 0 = mkErr "No" "no-parameter"
        | otherwise = mkErr "Too many" "multi-parameter"
      where
        mkErr howMany allowWhat =
            vcat [ptext (sLit $ howMany ++ " parameters for class") <+> quotes (ppr cls),
                  parens (ptext (sLit $ "Use MultiParamTypeClasses to allow "
                                        ++ allowWhat ++ " classes"))]

    classFunDepsErr :: Class -> SDoc
    classFunDepsErr cls
      = vcat [ptext (sLit "Fundeps in class") <+> quotes (ppr cls),
              parens (ptext (sLit "Use FunctionalDependencies to allow fundeps"))]

    noClassTyVarErr :: Class -> SDoc -> SDoc
    noClassTyVarErr clas what
      = sep [ptext (sLit "The") <+> what,
             ptext (sLit "mentions none of the type or kind variables of the class") <+>
                    quotes (ppr clas <+> hsep (map ppr (classTyVars clas)))]

    recSynErr :: [LTyClDecl Name] -> TcRn ()
    recSynErr syn_decls
      = setSrcSpan (getLoc (head sorted_decls)) $
        addErr (sep [ptext (sLit "Cycle in type synonym declarations:"),
                     nest 2 (vcat (map ppr_decl sorted_decls))])
      where
        sorted_decls = sortLocated syn_decls
        ppr_decl (L loc decl) = ppr loc <> colon <+> ppr decl

    recClsErr :: [TyCon] -> TcRn ()
    recClsErr cycles
      = addErr (sep [ptext (sLit "Cycle in class declaration (via superclasses):"),
                     nest 2 (hsep (intersperse (text "->") (map ppr cycles)))])

    badDataConTyCon :: DataCon -> Type -> Type -> SDoc
    badDataConTyCon data_con res_ty_tmpl actual_res_ty
      = hang (ptext (sLit "Data constructor") <+> quotes (ppr data_con) <+>
                    ptext (sLit "returns type") <+> quotes (ppr actual_res_ty))
           2 (ptext (sLit "instead of an instance of its parent type") <+> quotes (ppr res_ty_tmpl))

    badGadtKindCon :: DataCon -> SDoc
    badGadtKindCon data_con
      = hang (ptext (sLit "Data constructor") <+> quotes (ppr data_con)
              <+> ptext (sLit "cannot be GADT-like in its *kind* arguments"))
           2 (ppr data_con <+> dcolon <+> ppr (dataConUserType data_con))

    badGadtDecl :: Name -> SDoc
    badGadtDecl tc_name
      = vcat [ ptext (sLit "Illegal generalised algebraic data declaration for") <+> quotes (ppr tc_name)
             , nest 2 (parens $ ptext (sLit "Use GADTs to allow GADTs")) ]

    badExistential :: DataCon -> SDoc
    badExistential con
      = hang (ptext (sLit "Data constructor") <+> quotes (ppr con) <+>
                    ptext (sLit "has existential type variables, a context, or a specialised result type"))
           2 (vcat [ ppr con <+> dcolon <+> ppr (dataConUserType con)
                   , parens $ ptext (sLit "Use ExistentialQuantification or GADTs to allow this") ])

    badStupidTheta :: Name -> SDoc
    badStupidTheta tc_name
      = ptext (sLit "A data type declared in GADT style cannot have a context:") <+> quotes (ppr tc_name)

    newtypeConError :: Name -> Int -> SDoc
    newtypeConError tycon n
      = sep [ptext (sLit "A newtype must have exactly one constructor,"),
             nest 2 $ ptext (sLit "but") <+> quotes (ppr tycon) <+> ptext (sLit "has") <+> speakN n ]

    newtypeStrictError :: DataCon -> SDoc
    newtypeStrictError con
      = sep [ptext (sLit "A newtype constructor cannot have a strictness annotation,"),
             nest 2 $ ptext (sLit "but") <+> quotes (ppr con) <+> ptext (sLit "does")]

    newtypeFieldErr :: DataCon -> Int -> SDoc
    newtypeFieldErr con_name n_flds
      = sep [ptext (sLit "The constructor of a newtype must have exactly one field"),
             nest 2 $ ptext (sLit "but") <+> quotes (ppr con_name) <+> ptext (sLit "has") <+> speakN n_flds]

    badSigTyDecl :: Name -> SDoc
    badSigTyDecl tc_name
      = vcat [ ptext (sLit "Illegal kind signature") <+>
               quotes (ppr tc_name)
             , nest 2 (parens $ ptext (sLit "Use KindSignatures to allow kind signatures")) ]

    emptyConDeclsErr :: Name -> SDoc
    emptyConDeclsErr tycon
      = sep [quotes (ppr tycon) <+> ptext (sLit "has no constructors"),
             nest 2 $ ptext (sLit "(EmptyDataDecls permits this)")]

    wrongKindOfFamily :: TyCon -> SDoc
    wrongKindOfFamily family
      = ptext (sLit "Wrong category of family instance; declaration was for a")
        <+> kindOfFamily
      where
        kindOfFamily | isTypeFamilyTyCon family = text "type family"
                     | isDataFamilyTyCon family = text "data family"
                     | otherwise = pprPanic "wrongKindOfFamily" (ppr family)

    wrongNumberOfParmsErr :: Arity -> SDoc
    wrongNumberOfParmsErr max_args
      = ptext (sLit "Number of parameters must match family declaration; expected")
        <+> ppr max_args

    wrongTyFamName :: Name -> Name -> SDoc
    wrongTyFamName fam_tc_name eqn_tc_name
      = hang (ptext (sLit "Mismatched type name in type family instance."))
           2 (vcat [ ptext (sLit "Expected:") <+> ppr fam_tc_name
                   , ptext (sLit "  Actual:") <+> ppr eqn_tc_name ])

    inaccessibleCoAxBranch :: TyCon -> CoAxBranch -> SDoc
    inaccessibleCoAxBranch tc fi
      = ptext (sLit "Overlapped type family instance equation:") $$
          (pprCoAxBranch tc fi)

    badRoleAnnot :: Name -> Role -> Role -> SDoc
    badRoleAnnot var annot inferred
      = hang (ptext (sLit "Role mismatch on variable") <+> ppr var <> colon)
           2 (sep [ ptext (sLit "Annotation says"), ppr annot
                  , ptext (sLit "but role"), ppr inferred
                  , ptext (sLit "is required") ])

    wrongNumberOfRoles :: [a] -> LRoleAnnotDecl Name -> SDoc
    wrongNumberOfRoles tyvars d@(L _ (RoleAnnotDecl _ annots))
      = hang (ptext (sLit "Wrong number of roles listed in role annotation;") $$
              ptext (sLit "Expected") <+> (ppr $ length tyvars) <> comma <+>
              ptext (sLit "got") <+> (ppr $ length annots) <> colon)
           2 (ppr d)

    illegalRoleAnnotDecl :: LRoleAnnotDecl Name -> TcM ()
    illegalRoleAnnotDecl (L loc (RoleAnnotDecl tycon _))
      = setErrCtxt [] $
        setSrcSpan loc $
        addErrTc (ptext (sLit "Illegal role annotation for") <+> ppr tycon <> char ';' $$
                  ptext (sLit "they are allowed only for datatypes and classes."))

    needXRoleAnnotations :: TyCon -> SDoc
    needXRoleAnnotations tc
      = ptext (sLit "Illegal role annotation for") <+> ppr tc <> char ';' $$
        ptext (sLit "did you intend to use RoleAnnotations?")

    incoherentRoles :: SDoc
    incoherentRoles = (text "Roles other than" <+> quotes (text "nominal") <+>
                       text "for class parameters can lead to incoherence.") $$
                      (text "Use IncoherentInstances to allow this; bad role found")

    addTyThingCtxt :: TyThing -> TcM a -> TcM a
    addTyThingCtxt thing
      = addErrCtxt ctxt
      where
        name = getName thing
        flav = case thing of
                 ATyCon tc
                    | isClassTyCon tc       -> ptext (sLit "class")
                    | isTypeFamilyTyCon tc  -> ptext (sLit "type family")
                    | isDataFamilyTyCon tc  -> ptext (sLit "data family")
                    | isTypeSynonymTyCon tc -> ptext (sLit "type")
                    | isNewTyCon tc         -> ptext (sLit "newtype")
                    | isDataTyCon tc        -> ptext (sLit "data")

                 _ -> pprTrace "addTyThingCtxt strange" (ppr thing)
                      Outputable.empty

        ctxt = hsep [ ptext (sLit "In the"), flav
                    , ptext (sLit "declaration for"), quotes (ppr name) ]

    addRoleAnnotCtxt :: Name -> TcM a -> TcM a
    addRoleAnnotCtxt name
      = addErrCtxt $
        text "while checking a role annotation for" <+> quotes (ppr name)