typecheck/TcTyDecls.hs

    {-
    (c) The University of Glasgow 2006
    (c) The GRASP/AQUA Project, Glasgow University, 1992-1999

    Analysis functions over data types.  Specifically, detecting recursive types.

    This stuff is only used for source-code decls; it's recorded in interface
    files for imported data types.
    -}

    {-# LANGUAGE CPP #-}

    module TcTyDecls(
            RolesInfo,
            inferRoles,
            checkSynCycles,
            checkClassCycles,

            -- * Implicits
            tcAddImplicits, mkDefaultMethodType,

            -- * Record selectors
            mkRecSelBinds, mkOneRecordSelector
        ) where

    #include "HsVersions.h"

    import TcRnMonad
    import TcEnv
    import TcBinds( tcRecSelBinds )
    import RnEnv( RoleAnnotEnv, lookupRoleAnnot )
    import TyCoRep( Type(..), Coercion(..), UnivCoProvenance(..) )
    import TcType
    import TysWiredIn( unitTy )
    import MkCore( rEC_SEL_ERROR_ID )
    import HsSyn
    import Class
    import Type
    import HscTypes
    import TyCon
    import ConLike
    import DataCon
    import Name
    import NameEnv
    import NameSet hiding (unitFV)
    import RdrName ( mkVarUnqual )
    import Id
    import IdInfo
    import VarEnv
    import VarSet
    import NameSet  ( NameSet, unitNameSet, extendNameSet, elemNameSet )
    import Coercion ( ltRole )
    import BasicTypes
    import SrcLoc
    import Unique ( mkBuiltinUnique )
    import Outputable
    import Util
    import Maybes
    import Bag
    import FastString
    import FV
    import Module

    import Control.Monad

    {-
    ************************************************************************
    *                                                                      *
            Cycles in type synonym declarations
    *                                                                      *
    ************************************************************************
    -}

    synonymTyConsOfType :: Type -> [TyCon]
    -- Does not look through type synonyms at all
    -- Return a list of synonym tycons
    -- Keep this synchronized with 'expandTypeSynonyms'
    synonymTyConsOfType ty
      = nameEnvElts (go ty)
      where
         go :: Type -> NameEnv TyCon  -- The NameEnv does duplicate elim
         go (TyConApp tc tys)         = go_tc tc `plusNameEnv` go_s tys
         go (LitTy _)                 = emptyNameEnv
         go (TyVarTy _)               = emptyNameEnv
         go (AppTy a b)               = go a `plusNameEnv` go b
         go (FunTy a b)               = go a `plusNameEnv` go b
         go (ForAllTy _ ty)           = go ty
         go (CastTy ty co)            = go ty `plusNameEnv` go_co co
         go (CoercionTy co)           = go_co co

         -- Note [TyCon cycles through coercions?!]
         -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
         -- Although, in principle, it's possible for a type synonym loop
         -- could go through a coercion (since a coercion can refer to
         -- a TyCon or Type), it doesn't seem possible to actually construct
         -- a Haskell program which tickles this case.  Here is an example
         -- program which causes a coercion:
         --
         --   type family Star where
         --       Star = Type
         --
         --   data T :: Star -> Type
         --   data S :: forall (a :: Type). T a -> Type
         --
         -- Here, the application 'T a' must first coerce a :: Type to a :: Star,
         -- witnessed by the type family.  But if we now try to make Type refer
         -- to a type synonym which in turn refers to Star, we'll run into
         -- trouble: we're trying to define and use the type constructor
         -- in the same recursive group.  Possibly this restriction will be
         -- lifted in the future but for now, this code is "just for completeness
         -- sake".
         go_co (Refl _ ty)            = go ty
         go_co (TyConAppCo _ tc cs)   = go_tc tc `plusNameEnv` go_co_s cs
         go_co (AppCo co co')         = go_co co `plusNameEnv` go_co co'
         go_co (ForAllCo _ co co')    = go_co co `plusNameEnv` go_co co'
         go_co (FunCo _ co co')       = go_co co `plusNameEnv` go_co co'
         go_co (CoVarCo _)            = emptyNameEnv
         go_co (AxiomInstCo _ _ cs)   = go_co_s cs
         go_co (UnivCo p _ ty ty')    = go_prov p `plusNameEnv` go ty `plusNameEnv` go ty'
         go_co (SymCo co)             = go_co co
         go_co (TransCo co co')       = go_co co `plusNameEnv` go_co co'
         go_co (NthCo _ co)           = go_co co
         go_co (LRCo _ co)            = go_co co
         go_co (InstCo co co')        = go_co co `plusNameEnv` go_co co'
         go_co (CoherenceCo co co')   = go_co co `plusNameEnv` go_co co'
         go_co (KindCo co)            = go_co co
         go_co (SubCo co)             = go_co co
         go_co (AxiomRuleCo _ cs)     = go_co_s cs

         go_prov UnsafeCoerceProv     = emptyNameEnv
         go_prov (PhantomProv co)     = go_co co
         go_prov (ProofIrrelProv co)  = go_co co
         go_prov (PluginProv _)       = emptyNameEnv
         go_prov (HoleProv _)         = emptyNameEnv

         go_tc tc | isTypeSynonymTyCon tc = unitNameEnv (tyConName tc) tc
                  | otherwise             = emptyNameEnv
         go_s tys = foldr (plusNameEnv . go) emptyNameEnv tys
         go_co_s cos = foldr (plusNameEnv . go_co) emptyNameEnv cos

    -- | A monad for type synonym cycle checking, which keeps
    -- track of the TyCons which are known to be acyclic, or
    -- a failure message reporting that a cycle was found.
    newtype SynCycleM a = SynCycleM {
        runSynCycleM :: SynCycleState -> Either (SrcSpan, SDoc) (a, SynCycleState) }

    type SynCycleState = NameSet

    instance Functor SynCycleM where
        fmap = liftM

    instance Applicative SynCycleM where
        pure x = SynCycleM $ \state -> Right (x, state)
        (<*>) = ap

    instance Monad SynCycleM where
        m >>= f = SynCycleM $ \state ->
            case runSynCycleM m state of
                Right (x, state') ->
                    runSynCycleM (f x) state'
                Left err -> Left err

    failSynCycleM :: SrcSpan -> SDoc -> SynCycleM ()
    failSynCycleM loc err = SynCycleM $ \_ -> Left (loc, err)

    -- | Test if a 'Name' is acyclic, short-circuiting if we've
    -- seen it already.
    checkNameIsAcyclic :: Name -> SynCycleM () -> SynCycleM ()
    checkNameIsAcyclic n m = SynCycleM $ \s ->
        if n `elemNameSet` s
            then Right ((), s) -- short circuit
            else case runSynCycleM m s of
                    Right ((), s') -> Right ((), extendNameSet s' n)
                    Left err -> Left err

    -- | Checks if any of the passed in 'TyCon's have cycles.
    -- Takes the 'UnitId' of the home package (as we can avoid
    -- checking those TyCons: cycles never go through foreign packages) and
    -- the corresponding @LTyClDecl Name@ for each 'TyCon', so we
    -- can give better error messages.
    checkSynCycles :: UnitId -> [TyCon] -> [LTyClDecl Name] -> TcM ()
    checkSynCycles this_uid tcs tyclds = do
        case runSynCycleM (mapM_ (go emptyNameSet []) tcs) emptyNameSet of
            Left (loc, err) -> setSrcSpan loc $ failWithTc err
            Right _  -> return ()
      where
        -- Try our best to print the LTyClDecl for locally defined things
        lcl_decls = mkNameEnv (zip (map tyConName tcs) tyclds)

        -- Short circuit if we've already seen this Name and concluded
        -- it was acyclic.
        go :: NameSet -> [TyCon] -> TyCon -> SynCycleM ()
        go so_far seen_tcs tc =
            checkNameIsAcyclic (tyConName tc) $ go' so_far seen_tcs tc

        -- Expand type synonyms, complaining if you find the same
        -- type synonym a second time.
        go' :: NameSet -> [TyCon] -> TyCon -> SynCycleM ()
        go' so_far seen_tcs tc
            | n `elemNameSet` so_far
                = failSynCycleM (getSrcSpan (head seen_tcs)) $
                      sep [ text "Cycle in type synonym declarations:"
                          , nest 2 (vcat (map ppr_decl seen_tcs)) ]
            -- Optimization: we don't allow cycles through external packages,
            -- so once we find a non-local name we are guaranteed to not
            -- have a cycle.
            --
            -- This won't hold once we get recursive packages with Backpack,
            -- but for now it's fine.
            | not (isHoleModule mod ||
                   moduleUnitId mod == this_uid ||
                   isInteractiveModule mod)
                = return ()
            | Just ty <- synTyConRhs_maybe tc =
                go_ty (extendNameSet so_far (tyConName tc)) (tc:seen_tcs) ty
            | otherwise = return ()
          where
            n = tyConName tc
            mod = nameModule n
            ppr_decl tc =
              case lookupNameEnv lcl_decls n of
                Just (L loc decl) -> ppr loc <> colon <+> ppr decl
                Nothing -> ppr (getSrcSpan n) <> colon <+> ppr n <+> text "from external module"
             where
              n = tyConName tc

        go_ty :: NameSet -> [TyCon] -> Type -> SynCycleM ()
        go_ty so_far seen_tcs ty =
            mapM_ (go so_far seen_tcs) (synonymTyConsOfType ty)

    {- Note [Superclass cycle check]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The superclass cycle check for C decides if we can statically
    guarantee that expanding C's superclass cycles transitively is
    guaranteed to terminate.  This is a Haskell98 requirement,
    but one that we lift with -XUndecidableSuperClasses.

    The worry is that a superclass cycle could make the type checker loop.
    More precisely, with a constraint (Given or Wanted)
        C ty1 .. tyn
    one approach is to instantiate all of C's superclasses, transitively.
    We can only do so if that set is finite.

    This potential loop occurs only through superclasses.  This, for
    example, is fine
      class C a where
        op :: C b => a -> b -> b
    even though C's full definition uses C.

    Making the check static also makes it conservative.  Eg
      type family F a
      class F a => C a
    Here an instance of (F a) might mention C:
      type instance F [a] = C a
    and now we'd have a loop.

    The static check works like this, starting with C
      * Look at C's superclass predicates
      * If any is a type-function application,
        or is headed by a type variable, fail
      * If any has C at the head, fail
      * If any has a type class D at the head,
        make the same test with D

    A tricky point is: what if there is a type variable at the head?
    Consider this:
       class f (C f) => C f
       class c       => Id c
    and now expand superclasses for constraint (C Id):
         C Id
     --> Id (C Id)
     --> C Id
     --> ....
    Each step expands superclasses one layer, and clearly does not terminate.
    -}

    checkClassCycles :: Class -> Maybe SDoc
    -- Nothing  <=> ok
    -- Just err <=> possible cycle error
    checkClassCycles cls
      = do { (definite_cycle, err) <- go (unitNameSet (getName cls))
                                         cls (mkTyVarTys (classTyVars cls))
           ; let herald | definite_cycle = text "Superclass cycle for"
                        | otherwise      = text "Potential superclass cycle for"
           ; return (vcat [ herald <+> quotes (ppr cls)
                          , nest 2 err, hint]) }
      where
        hint = text "Use UndecidableSuperClasses to accept this"

        -- Expand superclasses starting with (C a b), complaining
        -- if you find the same class a second time, or a type function
        -- or predicate headed by a type variable
        --
        -- NB: this code duplicates TcType.transSuperClasses, but
        --     with more error message generation clobber
        -- Make sure the two stay in sync.
        go :: NameSet -> Class -> [Type] -> Maybe (Bool, SDoc)
        go so_far cls tys = firstJusts $
                            map (go_pred so_far) $
                            immSuperClasses cls tys

        go_pred :: NameSet -> PredType -> Maybe (Bool, SDoc)
           -- Nothing <=> ok
           -- Just (True, err)  <=> definite cycle
           -- Just (False, err) <=> possible cycle
        go_pred so_far pred  -- NB: tcSplitTyConApp looks through synonyms
           | Just (tc, tys) <- tcSplitTyConApp_maybe pred
           = go_tc so_far pred tc tys
           | hasTyVarHead pred
           = Just (False, hang (text "one of whose superclass constraints is headed by a type variable:")
                             2 (quotes (ppr pred)))
           | otherwise
           = Nothing

        go_tc :: NameSet -> PredType -> TyCon -> [Type] -> Maybe (Bool, SDoc)
        go_tc so_far pred tc tys
          | isFamilyTyCon tc
          = Just (False, hang (text "one of whose superclass constraints is headed by a type family:")
                            2 (quotes (ppr pred)))
          | Just cls <- tyConClass_maybe tc
          = go_cls so_far cls tys
          | otherwise   -- Equality predicate, for example
          = Nothing

        go_cls :: NameSet -> Class -> [Type] -> Maybe (Bool, SDoc)
        go_cls so_far cls tys
           | cls_nm `elemNameSet` so_far
           = Just (True, text "one of whose superclasses is" <+> quotes (ppr cls))
           | isCTupleClass cls
           = go so_far cls tys
           | otherwise
           = do { (b,err) <- go  (so_far `extendNameSet` cls_nm) cls tys
              ; return (b, text "one of whose superclasses is" <+> quotes (ppr cls)
                           $$ err) }
           where
             cls_nm = getName cls

    {-
    ************************************************************************
    *                                                                      *
            Role inference
    *                                                                      *
    ************************************************************************

    Note [Role inference]
    ~~~~~~~~~~~~~~~~~~~~~
    The role inference algorithm datatype definitions to infer the roles on the
    parameters. Although these roles are stored in the tycons, we can perform this
    algorithm on the built tycons, as long as we don't peek at an as-yet-unknown
    roles field! Ah, the magic of laziness.

    First, we choose appropriate initial roles. For families and classes, roles
    (including initial roles) are N. For datatypes, we start with the role in the
    role annotation (if any), or otherwise use Phantom. This is done in
    initialRoleEnv1.

    The function irGroup then propagates role information until it reaches a
    fixpoint, preferring N over (R or P) and R over P. To aid in this, we have a
    monad RoleM, which is a combination reader and state monad. In its state are
    the current RoleEnv, which gets updated by role propagation, and an update
    bit, which we use to know whether or not we've reached the fixpoint. The
    environment of RoleM contains the tycon whose parameters we are inferring, and
    a VarEnv from parameters to their positions, so we can update the RoleEnv.
    Between tycons, this reader information is missing; it is added by
    addRoleInferenceInfo.

    There are two kinds of tycons to consider: algebraic ones (excluding classes)
    and type synonyms. (Remember, families don't participate -- all their parameters
    are N.) An algebraic tycon processes each of its datacons, in turn. Note that
    a datacon's universally quantified parameters might be different from the parent
    tycon's parameters, so we use the datacon's univ parameters in the mapping from
    vars to positions. Note also that we don't want to infer roles for existentials
    (they're all at N, too), so we put them in the set of local variables. As an
    optimisation, we skip any tycons whose roles are already all Nominal, as there
    nowhere else for them to go. For synonyms, we just analyse their right-hand sides.

    irType walks through a type, looking for uses of a variable of interest and
    propagating role information. Because anything used under a phantom position
    is at phantom and anything used under a nominal position is at nominal, the
    irType function can assume that anything it sees is at representational. (The
    other possibilities are pruned when they're encountered.)

    The rest of the code is just plumbing.

    How do we know that this algorithm is correct? It should meet the following
    specification:

    Let Z be a role context -- a mapping from variables to roles. The following
    rules define the property (Z |- t : r), where t is a type and r is a role:

    Z(a) = r'        r' <= r
    ------------------------- RCVar
    Z |- a : r

    ---------- RCConst
    Z |- T : r               -- T is a type constructor

    Z |- t1 : r
    Z |- t2 : N
    -------------- RCApp
    Z |- t1 t2 : r

    forall i<=n. (r_i is R or N) implies Z |- t_i : r_i
    roles(T) = r_1 .. r_n
    ---------------------------------------------------- RCDApp
    Z |- T t_1 .. t_n : R

    Z, a:N |- t : r
    ---------------------- RCAll
    Z |- forall a:k.t : r

    We also have the following rules:

    For all datacon_i in type T, where a_1 .. a_n are universally quantified
    and b_1 .. b_m are existentially quantified, and the arguments are t_1 .. t_p,
    then if forall j<=p, a_1 : r_1 .. a_n : r_n, b_1 : N .. b_m : N |- t_j : R,
    then roles(T) = r_1 .. r_n

    roles(->) = R, R
    roles(~#) = N, N

    With -dcore-lint on, the output of this algorithm is checked in checkValidRoles,
    called from checkValidTycon.

    Note [Role-checking data constructor arguments]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider
      data T a where
        MkT :: Eq b => F a -> (a->a) -> T (G a)

    Then we want to check the roles at which 'a' is used
    in MkT's type.  We want to work on the user-written type,
    so we need to take into account
      * the arguments:   (F a) and (a->a)
      * the context:     C a b
      * the result type: (G a)   -- this is in the eq_spec

    Note [Coercions in role inference]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Is (t |> co1) representationally equal to (t |> co2)? Of course they are! Changing
    the kind of a type is totally irrelevant to the representation of that type. So,
    we want to totally ignore coercions when doing role inference. This includes omitting
    any type variables that appear in nominal positions but only within coercions.
    -}

    type RolesInfo = Name -> [Role]

    type RoleEnv = NameEnv [Role]        -- from tycon names to roles

    -- This, and any of the functions it calls, must *not* look at the roles
    -- field of a tycon we are inferring roles about!
    -- See Note [Role inference]
    inferRoles :: HscSource -> RoleAnnotEnv -> [TyCon] -> Name -> [Role]
    inferRoles hsc_src annots tycons
      = let role_env  = initialRoleEnv hsc_src annots tycons
            role_env' = irGroup role_env tycons in
        \name -> case lookupNameEnv role_env' name of
          Just roles -> roles
          Nothing    -> pprPanic "inferRoles" (ppr name)

    initialRoleEnv :: HscSource -> RoleAnnotEnv -> [TyCon] -> RoleEnv
    initialRoleEnv hsc_src annots = extendNameEnvList emptyNameEnv .
                                    map (initialRoleEnv1 hsc_src annots)

    initialRoleEnv1 :: HscSource -> RoleAnnotEnv -> TyCon -> (Name, [Role])
    initialRoleEnv1 hsc_src annots_env tc
      | isFamilyTyCon tc      = (name, map (const Nominal) bndrs)
      | isAlgTyCon tc         = (name, default_roles)
      | isTypeSynonymTyCon tc = (name, default_roles)
      | otherwise             = pprPanic "initialRoleEnv1" (ppr tc)
      where name         = tyConName tc
            bndrs        = tyConBinders tc
            argflags     = map tyConBinderArgFlag bndrs
            num_exps     = count isVisibleArgFlag argflags

              -- if the number of annotations in the role annotation decl
              -- is wrong, just ignore it. We check this in the validity check.
            role_annots
              = case lookupRoleAnnot annots_env name of
                  Just (L _ (RoleAnnotDecl _ annots))
                    | annots `lengthIs` num_exps -> map unLoc annots
                  _                              -> replicate num_exps Nothing
            default_roles = build_default_roles argflags role_annots

            build_default_roles (argf : argfs) (m_annot : ras)
              | isVisibleArgFlag argf
              = (m_annot `orElse` default_role) : build_default_roles argfs ras
            build_default_roles (_argf : argfs) ras
              = Nominal : build_default_roles argfs ras
            build_default_roles [] [] = []
            build_default_roles _ _ = pprPanic "initialRoleEnv1 (2)"
                                               (vcat [ppr tc, ppr role_annots])

            default_role
              | isClassTyCon tc               = Nominal
              -- Note [Default roles for abstract TyCons in hs-boot/hsig]
              | HsBootFile <- hsc_src
              , isAbstractTyCon tc            = Representational
              | HsigFile   <- hsc_src
              , isAbstractTyCon tc            = Nominal
              | otherwise                     = Phantom

    -- Note [Default roles for abstract TyCons in hs-boot/hsig]
    -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    -- What should the default role for an abstract TyCon be?
    --
    -- Originally, we inferred phantom role for abstract TyCons
    -- in hs-boot files, because the type variables were never used.
    --
    -- This was silly, because the role of the abstract TyCon
    -- was required to match the implementation, and the roles of
    -- data types are almost never phantom.  Thus, in ticket #9204,
    -- the default was changed so be representational (the most common case).  If
    -- the implementing data type was actually nominal, you'd get an easy
    -- to understand error, and add the role annotation yourself.
    --
    -- Then Backpack was added, and with it we added role *subtyping*
    -- the matching judgment: if an abstract TyCon has a nominal
    -- parameter, it's OK to implement it with a representational
    -- parameter.  But now, the representational default is not a good
    -- one, because you should *only* request representational if
    -- you're planning to do coercions. To be maximally flexible
    -- with what data types you will accept, you want the default
    -- for hsig files is nominal.  We don't allow role subtyping
    -- with hs-boot files (it's good practice to give an exactly
    -- accurate role here, because any types that use the abstract
    -- type will propagate the role information.)

    irGroup :: RoleEnv -> [TyCon] -> RoleEnv
    irGroup env tcs
      = let (env', update) = runRoleM env $ mapM_ irTyCon tcs in
        if update
        then irGroup env' tcs
        else env'

    irTyCon :: TyCon -> RoleM ()
    irTyCon tc
      | isAlgTyCon tc
      = do { old_roles <- lookupRoles tc
           ; unless (all (== Nominal) old_roles) $  -- also catches data families,
                                                    -- which don't want or need role inference
             irTcTyVars tc $
             do { mapM_ (irType emptyVarSet) (tyConStupidTheta tc)  -- See #8958
                ; whenIsJust (tyConClass_maybe tc) irClass
                ; mapM_ irDataCon (visibleDataCons $ algTyConRhs tc) }}

      | Just ty <- synTyConRhs_maybe tc
      = irTcTyVars tc $
        irType emptyVarSet ty

      | otherwise
      = return ()

    -- any type variable used in an associated type must be Nominal
    irClass :: Class -> RoleM ()
    irClass cls
      = mapM_ ir_at (classATs cls)
      where
        cls_tvs    = classTyVars cls
        cls_tv_set = mkVarSet cls_tvs

        ir_at at_tc
          = mapM_ (updateRole Nominal) nvars
          where nvars = filter (`elemVarSet` cls_tv_set) $ tyConTyVars at_tc

    -- See Note [Role inference]
    irDataCon :: DataCon -> RoleM ()
    irDataCon datacon
      = setRoleInferenceVars univ_tvs $
        irExTyVars ex_tvs $ \ ex_var_set ->
        mapM_ (irType ex_var_set)
              (map tyVarKind ex_tvs ++ eqSpecPreds eq_spec ++ theta ++ arg_tys)
          -- See Note [Role-checking data constructor arguments]
      where
        (univ_tvs, ex_tvs, eq_spec, theta, arg_tys, _res_ty)
          = dataConFullSig datacon

    irType :: VarSet -> Type -> RoleM ()
    irType = go
      where
        go lcls (TyVarTy tv)       = unless (tv `elemVarSet` lcls) $
                                     updateRole Representational tv
        go lcls (AppTy t1 t2)      = go lcls t1 >> markNominal lcls t2
        go lcls (TyConApp tc tys)  = do { roles <- lookupRolesX tc
                                        ; zipWithM_ (go_app lcls) roles tys }
        go lcls (ForAllTy tvb ty)  = do { let tv = binderVar tvb
                                              lcls' = extendVarSet lcls tv
                                        ; markNominal lcls (tyVarKind tv)
                                        ; go lcls' ty }
        go lcls (FunTy arg res)    = go lcls arg >> go lcls res
        go _    (LitTy {})         = return ()
          -- See Note [Coercions in role inference]
        go lcls (CastTy ty _)      = go lcls ty
        go _    (CoercionTy _)     = return ()

        go_app _ Phantom _ = return ()                 -- nothing to do here
        go_app lcls Nominal ty = markNominal lcls ty  -- all vars below here are N
        go_app lcls Representational ty = go lcls ty

    irTcTyVars :: TyCon -> RoleM a -> RoleM a
    irTcTyVars tc thing
      = setRoleInferenceTc (tyConName tc) $ go (tyConTyVars tc)
      where
        go []       = thing
        go (tv:tvs) = do { markNominal emptyVarSet (tyVarKind tv)
                         ; addRoleInferenceVar tv $ go tvs }

    irExTyVars :: [TyVar] -> (TyVarSet -> RoleM a) -> RoleM a
    irExTyVars orig_tvs thing = go emptyVarSet orig_tvs
      where
        go lcls []       = thing lcls
        go lcls (tv:tvs) = do { markNominal lcls (tyVarKind tv)
                              ; go (extendVarSet lcls tv) tvs }

    markNominal :: TyVarSet   -- local variables
                -> Type -> RoleM ()
    markNominal lcls ty = let nvars = fvVarList (FV.delFVs lcls $ get_ty_vars ty) in
                          mapM_ (updateRole Nominal) nvars
      where
         -- get_ty_vars gets all the tyvars (no covars!) from a type *without*
         -- recurring into coercions. Recall: coercions are totally ignored during
         -- role inference. See [Coercions in role inference]
        get_ty_vars :: Type -> FV
        get_ty_vars (TyVarTy tv)      = unitFV tv
        get_ty_vars (AppTy t1 t2)     = get_ty_vars t1 `unionFV` get_ty_vars t2
        get_ty_vars (FunTy t1 t2)     = get_ty_vars t1 `unionFV` get_ty_vars t2
        get_ty_vars (TyConApp _ tys)  = mapUnionFV get_ty_vars tys
        get_ty_vars (ForAllTy tvb ty) = tyCoFVsBndr tvb (get_ty_vars ty)
        get_ty_vars (LitTy {})        = emptyFV
        get_ty_vars (CastTy ty _)     = get_ty_vars ty
        get_ty_vars (CoercionTy _)    = emptyFV

    -- like lookupRoles, but with Nominal tags at the end for oversaturated TyConApps
    lookupRolesX :: TyCon -> RoleM [Role]
    lookupRolesX tc
      = do { roles <- lookupRoles tc
           ; return $ roles ++ repeat Nominal }

    -- gets the roles either from the environment or the tycon
    lookupRoles :: TyCon -> RoleM [Role]
    lookupRoles tc
      = do { env <- getRoleEnv
           ; case lookupNameEnv env (tyConName tc) of
               Just roles -> return roles
               Nothing    -> return $ tyConRoles tc }

    -- tries to update a role; won't ever update a role "downwards"
    updateRole :: Role -> TyVar -> RoleM ()
    updateRole role tv
      = do { var_ns <- getVarNs
           ; name <- getTyConName
           ; case lookupVarEnv var_ns tv of
               Nothing -> pprPanic "updateRole" (ppr name $$ ppr tv $$ ppr var_ns)
               Just n  -> updateRoleEnv name n role }

    -- the state in the RoleM monad
    data RoleInferenceState = RIS { role_env  :: RoleEnv
                                  , update    :: Bool }

    -- the environment in the RoleM monad
    type VarPositions = VarEnv Int

    -- See [Role inference]
    newtype RoleM a = RM { unRM :: Maybe Name   -- of the tycon
                                -> VarPositions
                                -> Int          -- size of VarPositions
                                -> RoleInferenceState
                                -> (a, RoleInferenceState) }

    instance Functor RoleM where
        fmap = liftM

    instance Applicative RoleM where
        pure x = RM $ \_ _ _ state -> (x, state)
        (<*>) = ap

    instance Monad RoleM where
      a >>= f  = RM $ \m_info vps nvps state ->
                      let (a', state') = unRM a m_info vps nvps state in
                      unRM (f a') m_info vps nvps state'

    runRoleM :: RoleEnv -> RoleM () -> (RoleEnv, Bool)
    runRoleM env thing = (env', update)
      where RIS { role_env = env', update = update }
              = snd $ unRM thing Nothing emptyVarEnv 0 state
            state = RIS { role_env  = env
                        , update    = False }

    setRoleInferenceTc :: Name -> RoleM a -> RoleM a
    setRoleInferenceTc name thing = RM $ \m_name vps nvps state ->
                                    ASSERT( isNothing m_name )
                                    ASSERT( isEmptyVarEnv vps )
                                    ASSERT( nvps == 0 )
                                    unRM thing (Just name) vps nvps state

    addRoleInferenceVar :: TyVar -> RoleM a -> RoleM a
    addRoleInferenceVar tv thing
      = RM $ \m_name vps nvps state ->
        ASSERT( isJust m_name )
        unRM thing m_name (extendVarEnv vps tv nvps) (nvps+1) state

    setRoleInferenceVars :: [TyVar] -> RoleM a -> RoleM a
    setRoleInferenceVars tvs thing
      = RM $ \m_name _vps _nvps state ->
        ASSERT( isJust m_name )
        unRM thing m_name (mkVarEnv (zip tvs [0..])) (panic "setRoleInferenceVars")
             state

    getRoleEnv :: RoleM RoleEnv
    getRoleEnv = RM $ \_ _ _ state@(RIS { role_env = env }) -> (env, state)

    getVarNs :: RoleM VarPositions
    getVarNs = RM $ \_ vps _ state -> (vps, state)

    getTyConName :: RoleM Name
    getTyConName = RM $ \m_name _ _ state ->
                        case m_name of
                          Nothing   -> panic "getTyConName"
                          Just name -> (name, state)

    updateRoleEnv :: Name -> Int -> Role -> RoleM ()
    updateRoleEnv name n role
      = RM $ \_ _ _ state@(RIS { role_env = role_env }) -> ((),
             case lookupNameEnv role_env name of
               Nothing -> pprPanic "updateRoleEnv" (ppr name)
               Just roles -> let (before, old_role : after) = splitAt n roles in
                             if role `ltRole` old_role
                             then let roles' = before ++ role : after
                                      role_env' = extendNameEnv role_env name roles' in
                                  RIS { role_env = role_env', update = True }
                             else state )

    {- *********************************************************************
    *                                                                      *
                    Building implicits
    *                                                                      *
    ********************************************************************* -}

    tcAddImplicits :: [TyCon] -> TcM TcGblEnv
    -- Given a [TyCon], add to the TcGblEnv
    --   * extend the TypeEnv with their implicitTyThings
    --   * extend the TypeEnv with any default method Ids
    --   * add bindings for record selectors
    --   * add bindings for type representations for the TyThings
    tcAddImplicits tycons
      = discardWarnings $
        tcExtendGlobalEnvImplicit implicit_things  $
        tcExtendGlobalValEnv def_meth_ids          $
        do { traceTc "tcAddImplicits" $ vcat
                [ text "tycons" <+> ppr tycons
                , text "implicits" <+> ppr implicit_things ]
           ; tcRecSelBinds (mkRecSelBinds tycons) }
     where
       implicit_things = concatMap implicitTyConThings tycons
       def_meth_ids    = mkDefaultMethodIds tycons

    mkDefaultMethodIds :: [TyCon] -> [Id]
    -- We want to put the default-method Ids (both vanilla and generic)
    -- into the type environment so that they are found when we typecheck
    -- the filled-in default methods of each instance declaration
    -- See Note [Default method Ids and Template Haskell]
    mkDefaultMethodIds tycons
      = [ mkExportedVanillaId dm_name (mkDefaultMethodType cls sel_id dm_spec)
        | tc <- tycons
        , Just cls <- [tyConClass_maybe tc]
        , (sel_id, Just (dm_name, dm_spec)) <- classOpItems cls ]

    mkDefaultMethodType :: Class -> Id -> DefMethSpec Type -> Type
    -- Returns the top-level type of the default method
    mkDefaultMethodType _ sel_id VanillaDM        = idType sel_id
    mkDefaultMethodType cls _   (GenericDM dm_ty) = mkSpecSigmaTy cls_tvs [pred] dm_ty
       where
         cls_tvs = classTyVars cls
         pred    = mkClassPred cls (mkTyVarTys cls_tvs)

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

    {-
    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.
    -}

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

    mkRecSelBinds :: [TyCon] -> 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 binds sigs
      where
        (sigs, binds) = unzip rec_sels
        rec_sels = map mkRecSelBind [ (tc,fld)
                                    | tc <- tycons
                                    , fld <- tyConFieldLabels tc ]

    mkRecSelBind :: (TyCon, FieldLabel) -> (LSig Name, (RecFlag, LHsBinds Name))
    mkRecSelBind (tycon, fl)
      = mkOneRecordSelector all_cons (RecSelData tycon) fl
      where
        all_cons = map RealDataCon (tyConDataCons tycon)

    mkOneRecordSelector :: [ConLike] -> RecSelParent -> FieldLabel
                        -> (LSig Name, (RecFlag, LHsBinds Name))
    mkOneRecordSelector all_cons idDetails fl
      = (L loc (IdSig sel_id), (NonRecursive, unitBag (L loc sel_bind)))
      where
        loc    = getSrcSpan sel_name
        lbl      = flLabel fl
        sel_name = flSelector fl

        sel_id = mkExportedLocalId rec_details sel_name sel_ty
        rec_details = RecSelId { sel_tycon = idDetails, sel_naughty = is_naughty }

        -- Find a representative constructor, con1
        cons_w_field = conLikesWithFields all_cons [lbl]
        con1 = ASSERT( not (null cons_w_field) ) head cons_w_field

        -- Selector type; Note [Polymorphic selectors]
        field_ty   = conLikeFieldType con1 lbl
        data_tvs   = tyCoVarsOfTypesWellScoped inst_tys
        data_tv_set= mkVarSet data_tvs
        is_naughty = not (tyCoVarsOfType field_ty `subVarSet` data_tv_set)
        (field_tvs, field_theta, field_tau) = tcSplitSigmaTy field_ty
        sel_ty | is_naughty = unitTy  -- See Note [Naughty record selectors]
               | otherwise  = mkSpecForAllTys data_tvs          $
                              mkPhiTy (conLikeStupidTheta con1) $   -- Urgh!
                              mkFunTy data_ty                   $
                              mkSpecForAllTys field_tvs         $
                              mkPhiTy field_theta               $
                              -- req_theta is empty for normal DataCon
                              mkPhiTy req_theta                 $
                              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 (mkPrefixFunRhs sel_lname)
                                               [] unit_rhs]
                 | otherwise =  map mk_match cons_w_field ++ deflt
        mk_match con = mkSimpleMatch (mkPrefixFunRhs sel_lname)
                                     [L loc (mk_sel_pat con)]
                                     (L loc (HsVar (L loc 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
                            { hsRecFieldLbl
                               = L loc (FieldOcc (L loc $ mkVarUnqual lbl) sel_name)
                            , hsRecFieldArg = L loc (VarPat (L loc 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 CaseAlt
                                [L loc (WildPat placeHolderType)]
                                (mkHsApp (L loc (HsVar
                                                (L loc (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 :: ConLike -> Bool
        dealt_with (PatSynCon _) = False -- We can't predict overlap
        dealt_with con@(RealDataCon dc) =
          con `elem` cons_w_field || dataConCannotMatch inst_tys dc

        (univ_tvs, _, eq_spec, _, req_theta, _, data_ty) = conLikeFullSig con1

        eq_subst = mkTvSubstPrs (map eqSpecPair eq_spec)
        inst_tys = substTyVars eq_subst univ_tvs

        unit_rhs = mkLHsTupleExpr []
        msg_lit = HsStringPrim NoSourceText (fastStringToByteString lbl)

    {-
    Note [Polymorphic selectors]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We take care to build the type of a polymorphic selector in the right
    order, so that visible type application works.

      data Ord a => T a = MkT { field :: forall b. (Num a, Show b) => (a, b) }

    We want

      field :: forall a. Ord a => T a -> forall b. (Num a, Show b) => (a, b)

    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 = ()
    so that the later type-check will add them to the environment, and they'll be
    exported.  The function is never called, because the typechecker 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.

    -}