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

-}

{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE CPP #-}
{-# LANGUAGE ViewPatterns #-}
{-# LANGUAGE TypeFamilies #-}

module GHC.Rename.HsType (
        -- Type related stuff
        rnHsType, rnLHsType, rnLHsTypes, rnContext,
        rnHsKind, rnLHsKind, rnLHsTypeArgs,
        rnHsSigType, rnHsWcType,
        HsSigWcTypeScoping(..), rnHsSigWcType, rnHsPatSigType,
        newTyVarNameRn,
        rnConDeclFields,
        rnLTyVar,

        rnScaledLHsType,

        -- Precence related stuff
        NegationHandling(..),
        mkOpAppRn, mkNegAppRn, mkOpFormRn, mkConOpPatRn,
        checkPrecMatch, checkSectionPrec,

        -- Binding related stuff
        bindHsForAllTelescope,
        bindLHsTyVarBndr, bindLHsTyVarBndrs, WarnUnusedForalls(..),
        rnImplicitBndrs, bindSigTyVarsFV, bindHsQTyVars,
        FreeKiTyVars,
        extractHsTyRdrTyVars, extractHsTyRdrTyVarsKindVars,
        extractHsTysRdrTyVars, extractRdrKindSigVars, extractDataDefnKindVars,
        extractHsTvBndrs, extractHsTyArgRdrKiTyVars,
        extractHsScaledTysRdrTyVars,
        forAllOrNothing, nubL
  ) where

import GHC.Prelude

import {-# SOURCE #-} GHC.Rename.Splice( rnSpliceType )

import GHC.Driver.Session
import GHC.Hs
import GHC.Rename.Doc    ( rnLHsDoc, rnMbLHsDoc )
import GHC.Rename.Env
import GHC.Rename.Utils  ( HsDocContext(..), inHsDocContext, withHsDocContext
                         , mapFvRn, pprHsDocContext, bindLocalNamesFV
                         , typeAppErr, newLocalBndrRn, checkDupRdrNames
                         , checkShadowedRdrNames )
import GHC.Rename.Fixity ( lookupFieldFixityRn, lookupFixityRn
                         , lookupTyFixityRn )
import GHC.Tc.Utils.Monad
import GHC.Types.Name.Reader
import GHC.Builtin.Names
import GHC.Builtin.Types.Prim ( funTyConName )
import GHC.Types.Name
import GHC.Types.SrcLoc
import GHC.Types.Name.Set
import GHC.Types.FieldLabel

import GHC.Utils.Misc
import GHC.Types.Basic  ( compareFixity, funTyFixity, negateFixity
                        , Fixity(..), FixityDirection(..), LexicalFixity(..)
                        , TypeOrKind(..) )
import GHC.Utils.Outputable
import GHC.Data.FastString
import GHC.Data.Maybe
import qualified GHC.LanguageExtensions as LangExt

import Data.List          ( nubBy, partition )
import Control.Monad      ( unless, when )

#include "HsVersions.h"

{-
These type renamers are in a separate module, rather than in (say) GHC.Rename.Module,
to break several loops.

*********************************************************
*                                                       *
    HsSigWcType and HsPatSigType (i.e with wildcards)
*                                                       *
*********************************************************
-}

data HsSigWcTypeScoping
  = AlwaysBind
    -- ^ Always bind any free tyvars of the given type, regardless of whether we
    -- have a forall at the top.
    --
    -- For pattern type sigs, we /do/ want to bring those type
    -- variables into scope, even if there's a forall at the top which usually
    -- stops that happening, e.g:
    --
    -- > \ (x :: forall a. a -> b) -> e
    --
    -- Here we do bring 'b' into scope.
    --
    -- RULES can also use 'AlwaysBind', such as in the following example:
    --
    -- > {-# RULES \"f\" forall (x :: forall a. a -> b). f x = ... b ... #-}
    --
    -- This only applies to RULES that do not explicitly bind their type
    -- variables. If a RULE explicitly quantifies its type variables, then
    -- 'NeverBind' is used instead. See also
    -- @Note [Pattern signature binders and scoping]@ in "GHC.Hs.Type".
  | BindUnlessForall
    -- ^ Unless there's forall at the top, do the same thing as 'AlwaysBind'.
    -- This is only ever used in places where the \"@forall@-or-nothing\" rule
    -- is in effect. See @Note [forall-or-nothing rule]@.
  | NeverBind
    -- ^ Never bind any free tyvars. This is used for RULES that have both
    -- explicit type and term variable binders, e.g.:
    --
    -- > {-# RULES \"const\" forall a. forall (x :: a) y. const x y = x #-}
    --
    -- The presence of the type variable binder @forall a.@ implies that the
    -- free variables in the types of the term variable binders @x@ and @y@
    -- are /not/ bound. In the example above, there are no such free variables,
    -- but if the user had written @(y :: b)@ instead of @y@ in the term
    -- variable binders, then @b@ would be rejected for being out of scope.
    -- See also @Note [Pattern signature binders and scoping]@ in
    -- "GHC.Hs.Type".

rnHsSigWcType :: HsDocContext
              -> LHsSigWcType GhcPs
              -> RnM (LHsSigWcType GhcRn, FreeVars)
rnHsSigWcType doc (HsWC { hswc_body = HsIB { hsib_body = hs_ty }})
  = rn_hs_sig_wc_type BindUnlessForall doc hs_ty $ \nwcs imp_tvs body ->
    let ib_ty = HsIB { hsib_ext = imp_tvs, hsib_body = body  }
        wc_ty = HsWC { hswc_ext = nwcs,    hswc_body = ib_ty } in
    pure (wc_ty, emptyFVs)

rnHsPatSigType :: HsSigWcTypeScoping
               -> HsDocContext
               -> HsPatSigType GhcPs
               -> (HsPatSigType GhcRn -> RnM (a, FreeVars))
               -> RnM (a, FreeVars)
-- Used for
--   - Pattern type signatures, which are only allowed with ScopedTypeVariables
--   - Signatures on binders in a RULE, which are allowed even if
--     ScopedTypeVariables isn't enabled
-- Wildcards are allowed
--
-- See Note [Pattern signature binders and scoping] in GHC.Hs.Type
rnHsPatSigType scoping ctx sig_ty thing_inside
  = do { ty_sig_okay <- xoptM LangExt.ScopedTypeVariables
       ; checkErr ty_sig_okay (unexpectedPatSigTypeErr sig_ty)
       ; rn_hs_sig_wc_type scoping ctx (hsPatSigType sig_ty) $
         \nwcs imp_tvs body ->
    do { let sig_names = HsPSRn { hsps_nwcs = nwcs, hsps_imp_tvs = imp_tvs }
             sig_ty'   = HsPS { hsps_ext = sig_names, hsps_body = body }
       ; thing_inside sig_ty'
       } }

-- The workhorse for rnHsSigWcType and rnHsPatSigType.
rn_hs_sig_wc_type :: HsSigWcTypeScoping -> HsDocContext
                  -> LHsType GhcPs
                  -> ([Name]    -- Wildcard names
                      -> [Name] -- Implicitly bound type variable names
                      -> LHsType GhcRn
                      -> RnM (a, FreeVars))
                  -> RnM (a, FreeVars)
rn_hs_sig_wc_type scoping ctxt hs_ty thing_inside
  = do { free_vars <- filterInScopeM (extractHsTyRdrTyVars hs_ty)
       ; (nwc_rdrs', tv_rdrs) <- partition_nwcs free_vars
       ; let nwc_rdrs = nubL nwc_rdrs'
       ; implicit_bndrs <- case scoping of
           AlwaysBind       -> pure tv_rdrs
           BindUnlessForall -> forAllOrNothing (isLHsForAllTy hs_ty) tv_rdrs
           NeverBind        -> pure []
       ; rnImplicitBndrs Nothing implicit_bndrs $ \ vars ->
    do { (wcs, hs_ty', fvs1) <- rnWcBody ctxt nwc_rdrs hs_ty
       ; (res, fvs2) <- thing_inside wcs vars hs_ty'
       ; return (res, fvs1 `plusFV` fvs2) } }

rnHsWcType :: HsDocContext -> LHsWcType GhcPs -> RnM (LHsWcType GhcRn, FreeVars)
rnHsWcType ctxt (HsWC { hswc_body = hs_ty })
  = do { free_vars <- filterInScopeM (extractHsTyRdrTyVars hs_ty)
       ; (nwc_rdrs', _) <- partition_nwcs free_vars
       ; let nwc_rdrs = nubL nwc_rdrs'
       ; (wcs, hs_ty', fvs) <- rnWcBody ctxt nwc_rdrs hs_ty
       ; let sig_ty' = HsWC { hswc_ext = wcs, hswc_body = hs_ty' }
       ; return (sig_ty', fvs) }

rnWcBody :: HsDocContext -> [Located RdrName] -> LHsType GhcPs
         -> RnM ([Name], LHsType GhcRn, FreeVars)
rnWcBody ctxt nwc_rdrs hs_ty
  = do { nwcs <- mapM newLocalBndrRn nwc_rdrs
       ; let env = RTKE { rtke_level = TypeLevel
                        , rtke_what  = RnTypeBody
                        , rtke_nwcs  = mkNameSet nwcs
                        , rtke_ctxt  = ctxt }
       ; (hs_ty', fvs) <- bindLocalNamesFV nwcs $
                          rn_lty env hs_ty
       ; return (nwcs, hs_ty', fvs) }
  where
    rn_lty env (L loc hs_ty)
      = setSrcSpan loc $
        do { (hs_ty', fvs) <- rn_ty env hs_ty
           ; return (L loc hs_ty', fvs) }

    rn_ty :: RnTyKiEnv -> HsType GhcPs -> RnM (HsType GhcRn, FreeVars)
    -- A lot of faff just to allow the extra-constraints wildcard to appear
    rn_ty env (HsForAllTy { hst_tele = tele, hst_body = hs_body })
      = bindHsForAllTelescope (rtke_ctxt env) tele $ \ tele' ->
        do { (hs_body', fvs) <- rn_lty env hs_body
           ; return (HsForAllTy { hst_xforall = noExtField
                                , hst_tele = tele', hst_body = hs_body' }
                    , fvs) }

    rn_ty env (HsQualTy { hst_ctxt = L cx hs_ctxt
                        , hst_body = hs_ty })
      | Just (hs_ctxt1, hs_ctxt_last) <- snocView hs_ctxt
      , L lx (HsWildCardTy _)  <- ignoreParens hs_ctxt_last
      = do { (hs_ctxt1', fvs1) <- mapFvRn (rn_top_constraint env) hs_ctxt1
           ; setSrcSpan lx $ checkExtraConstraintWildCard env hs_ctxt1
           ; let hs_ctxt' = hs_ctxt1' ++ [L lx (HsWildCardTy noExtField)]
           ; (hs_ty', fvs2) <- rnLHsTyKi env hs_ty
           ; return (HsQualTy { hst_xqual = noExtField
                              , hst_ctxt = L cx hs_ctxt', hst_body = hs_ty' }
                    , fvs1 `plusFV` fvs2) }

      | otherwise
      = do { (hs_ctxt', fvs1) <- mapFvRn (rn_top_constraint env) hs_ctxt
           ; (hs_ty', fvs2)   <- rnLHsTyKi env hs_ty
           ; return (HsQualTy { hst_xqual = noExtField
                              , hst_ctxt = L cx hs_ctxt'
                              , hst_body = hs_ty' }
                    , fvs1 `plusFV` fvs2) }

    rn_ty env hs_ty = rnHsTyKi env hs_ty

    rn_top_constraint env = rnLHsTyKi (env { rtke_what = RnTopConstraint })


checkExtraConstraintWildCard :: RnTyKiEnv -> HsContext GhcPs -> RnM ()
-- Rename the extra-constraint spot in a type signature
--    (blah, _) => type
-- Check that extra-constraints are allowed at all, and
-- if so that it's an anonymous wildcard
checkExtraConstraintWildCard env hs_ctxt
  = checkWildCard env mb_bad
  where
    mb_bad | not (extraConstraintWildCardsAllowed env)
           = Just base_msg
             -- Currently, we do not allow wildcards in their full glory in
             -- standalone deriving declarations. We only allow a single
             -- extra-constraints wildcard à la:
             --
             --   deriving instance _ => Eq (Foo a)
             --
             -- i.e., we don't support things like
             --
             --   deriving instance (Eq a, _) => Eq (Foo a)
           | DerivDeclCtx {} <- rtke_ctxt env
           , not (null hs_ctxt)
           = Just deriv_decl_msg
           | otherwise
           = Nothing

    base_msg = text "Extra-constraint wildcard" <+> quotes pprAnonWildCard
                   <+> text "not allowed"

    deriv_decl_msg
      = hang base_msg
           2 (vcat [ text "except as the sole constraint"
                   , nest 2 (text "e.g., deriving instance _ => Eq (Foo a)") ])

extraConstraintWildCardsAllowed :: RnTyKiEnv -> Bool
extraConstraintWildCardsAllowed env
  = case rtke_ctxt env of
      TypeSigCtx {}       -> True
      ExprWithTySigCtx {} -> True
      DerivDeclCtx {}     -> True
      StandaloneKindSigCtx {} -> False  -- See Note [Wildcards in standalone kind signatures] in "GHC.Hs.Decls"
      _                   -> False

-- | When the NamedWildCards extension is enabled, partition_nwcs
-- removes type variables that start with an underscore from the
-- FreeKiTyVars in the argument and returns them in a separate list.
-- When the extension is disabled, the function returns the argument
-- and empty list.  See Note [Renaming named wild cards]
partition_nwcs :: FreeKiTyVars -> RnM ([Located RdrName], FreeKiTyVars)
partition_nwcs free_vars
  = do { wildcards_enabled <- xoptM LangExt.NamedWildCards
       ; return $
           if wildcards_enabled
           then partition is_wildcard free_vars
           else ([], free_vars) }
  where
     is_wildcard :: Located RdrName -> Bool
     is_wildcard rdr = startsWithUnderscore (rdrNameOcc (unLoc rdr))

{- Note [Renaming named wild cards]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Identifiers starting with an underscore are always parsed as type variables.
It is only here in the renamer that we give the special treatment.
See Note [The wildcard story for types] in GHC.Hs.Type.

It's easy!  When we collect the implicitly bound type variables, ready
to bring them into scope, and NamedWildCards is on, we partition the
variables into the ones that start with an underscore (the named
wildcards) and the rest. Then we just add them to the hswc_wcs field
of the HsWildCardBndrs structure, and we are done.


*********************************************************
*                                                       *
           HsSigtype (i.e. no wildcards)
*                                                       *
****************************************************** -}

rnHsSigType :: HsDocContext
            -> TypeOrKind
            -> LHsSigType GhcPs
            -> RnM (LHsSigType GhcRn, FreeVars)
-- Used for source-language type signatures
-- that cannot have wildcards
rnHsSigType ctx level (HsIB { hsib_body = hs_ty })
  = do { traceRn "rnHsSigType" (ppr hs_ty)
       ; rdr_env <- getLocalRdrEnv
       ; vars0 <- forAllOrNothing (isLHsForAllTy hs_ty)
           $ filterInScope rdr_env
           $ extractHsTyRdrTyVars hs_ty
       ; rnImplicitBndrs Nothing vars0 $ \ vars ->
    do { (body', fvs) <- rnLHsTyKi (mkTyKiEnv ctx level RnTypeBody) hs_ty

       ; return ( HsIB { hsib_ext = vars
                       , hsib_body = body' }
                , fvs ) } }

-- Note [forall-or-nothing rule]
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-- Free variables in signatures are usually bound in an implicit
-- 'forall' at the beginning of user-written signatures. However, if the
-- signature has an explicit forall at the beginning, this is disabled.
--
-- The idea is nested foralls express something which is only
-- expressible explicitly, while a top level forall could (usually) be
-- replaced with an implicit binding. Top-level foralls alone ("forall.") are
-- therefore an indication that the user is trying to be fastidious, so
-- we don't implicitly bind any variables.

-- | See @Note [forall-or-nothing rule]@. This tiny little function is used
-- (rather than its small body inlined) to indicate that we are implementing
-- that rule.
forAllOrNothing :: Bool
                -- ^ True <=> explicit forall
                -- E.g.  f :: forall a. a->b
                --  we do not want to bring 'b' into scope, hence True
                -- But   f :: a -> b
                --  we want to bring both 'a' and 'b' into scope, hence False
                -> FreeKiTyVars
                -- ^ Free vars of the type
                -> RnM FreeKiTyVars
forAllOrNothing has_outer_forall fvs = case has_outer_forall of
  True -> do
    traceRn "forAllOrNothing" $ text "has explicit outer forall"
    pure []
  False -> do
    traceRn "forAllOrNothing" $ text "no explicit forall. implicit binders:" <+> ppr fvs
    pure fvs

rnImplicitBndrs :: Maybe assoc
                -- ^ @'Just' _@ => an associated type decl
                -> FreeKiTyVars
                -- ^ Surface-syntax free vars that we will implicitly bind.
                -- May have duplicates, which are removed here.
                -> ([Name] -> RnM (a, FreeVars))
                -> RnM (a, FreeVars)
rnImplicitBndrs mb_assoc implicit_vs_with_dups thing_inside
  = do { let implicit_vs = nubL implicit_vs_with_dups

       ; traceRn "rnImplicitBndrs" $
         vcat [ ppr implicit_vs_with_dups, ppr implicit_vs ]

         -- Use the currently set SrcSpan as the new source location for each Name.
         -- See Note [Source locations for implicitly bound type variables].
       ; loc <- getSrcSpanM
       ; vars <- mapM (newTyVarNameRn mb_assoc . L loc . unLoc) implicit_vs

       ; bindLocalNamesFV vars $
         thing_inside vars }

{-
Note [Source locations for implicitly bound type variables]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When bringing implicitly bound type variables into scope (in rnImplicitBndrs),
we do something peculiar: we drop the original SrcSpan attached to each
variable and replace it with the currently set SrcSpan. Moreover, this new
SrcSpan is usually /less/ precise than the original one, and that's OK. To see
why this is done, consider the following example:

  f :: a -> b -> a

Suppose that a warning or error message needs to point to the SrcSpans of the
binding sites for `a` and `b`. But where /are/ they bound, anyway? Technically,
they're bound by an unwritten `forall` at the front of the type signature, but
there is no SrcSpan for that. We could point to the first occurrence of `a` as
the binding site for `a`, but that would make the first occurrence of `a`
special. Moreover, we don't want IDEs to confuse binding sites and occurrences.

As a result, we make the `SrcSpan`s for `a` and `b` span the entirety of the
type signature, since the type signature implicitly carries their binding
sites. This is less precise, but more accurate.
-}

{- ******************************************************
*                                                       *
           LHsType and HsType
*                                                       *
****************************************************** -}

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

Note [QualTy in kinds]
~~~~~~~~~~~~~~~~~~~~~~
I was wondering whether QualTy could occur only at TypeLevel.  But no,
we can have a qualified type in a kind too. Here is an example:

  type family F a where
    F Bool = Nat
    F Nat  = Type

  type family G a where
    G Type = Type -> Type
    G ()   = Nat

  data X :: forall k1 k2. (F k1 ~ G k2) => k1 -> k2 -> Type where
    MkX :: X 'True '()

See that k1 becomes Bool and k2 becomes (), so the equality is
satisfied. If I write MkX :: X 'True 'False, compilation fails with a
suitable message:

  MkX :: X 'True '()
    • Couldn't match kind ‘G Bool’ with ‘Nat’
      Expected kind: G Bool
        Actual kind: F Bool

However: in a kind, the constraints in the QualTy must all be
equalities; or at least, any kinds with a class constraint are
uninhabited.
-}

data RnTyKiEnv
  = RTKE { rtke_ctxt  :: HsDocContext
         , rtke_level :: TypeOrKind  -- Am I renaming a type or a kind?
         , rtke_what  :: RnTyKiWhat  -- And within that what am I renaming?
         , rtke_nwcs  :: NameSet     -- These are the in-scope named wildcards
    }

data RnTyKiWhat = RnTypeBody
                | RnTopConstraint   -- Top-level context of HsSigWcTypes
                | RnConstraint      -- All other constraints

instance Outputable RnTyKiEnv where
  ppr (RTKE { rtke_level = lev, rtke_what = what
            , rtke_nwcs = wcs, rtke_ctxt = ctxt })
    = text "RTKE"
      <+> braces (sep [ ppr lev, ppr what, ppr wcs
                      , pprHsDocContext ctxt ])

instance Outputable RnTyKiWhat where
  ppr RnTypeBody      = text "RnTypeBody"
  ppr RnTopConstraint = text "RnTopConstraint"
  ppr RnConstraint    = text "RnConstraint"

mkTyKiEnv :: HsDocContext -> TypeOrKind -> RnTyKiWhat -> RnTyKiEnv
mkTyKiEnv cxt level what
 = RTKE { rtke_level = level, rtke_nwcs = emptyNameSet
        , rtke_what = what, rtke_ctxt = cxt }

isRnKindLevel :: RnTyKiEnv -> Bool
isRnKindLevel (RTKE { rtke_level = KindLevel }) = True
isRnKindLevel _                                 = False

--------------
rnLHsType  :: HsDocContext -> LHsType GhcPs -> RnM (LHsType GhcRn, FreeVars)
rnLHsType ctxt ty = rnLHsTyKi (mkTyKiEnv ctxt TypeLevel RnTypeBody) ty

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

rnScaledLHsType :: HsDocContext -> HsScaled GhcPs (LHsType GhcPs)
                                  -> RnM (HsScaled GhcRn (LHsType GhcRn), FreeVars)
rnScaledLHsType doc (HsScaled w ty) = do
  (w' , fvs_w) <- rnHsArrow (mkTyKiEnv doc TypeLevel RnTypeBody) w
  (ty', fvs) <- rnLHsType doc ty
  return (HsScaled w' ty', fvs `plusFV` fvs_w)


rnHsType  :: HsDocContext -> HsType GhcPs -> RnM (HsType GhcRn, FreeVars)
rnHsType ctxt ty = rnHsTyKi (mkTyKiEnv ctxt TypeLevel RnTypeBody) ty

rnLHsKind  :: HsDocContext -> LHsKind GhcPs -> RnM (LHsKind GhcRn, FreeVars)
rnLHsKind ctxt kind = rnLHsTyKi (mkTyKiEnv ctxt KindLevel RnTypeBody) kind

rnHsKind  :: HsDocContext -> HsKind GhcPs -> RnM (HsKind GhcRn, FreeVars)
rnHsKind ctxt kind = rnHsTyKi  (mkTyKiEnv ctxt KindLevel RnTypeBody) kind

-- renaming a type only, not a kind
rnLHsTypeArg :: HsDocContext -> LHsTypeArg GhcPs
                -> RnM (LHsTypeArg GhcRn, FreeVars)
rnLHsTypeArg ctxt (HsValArg ty)
   = do { (tys_rn, fvs) <- rnLHsType ctxt ty
        ; return (HsValArg tys_rn, fvs) }
rnLHsTypeArg ctxt (HsTypeArg l ki)
   = do { (kis_rn, fvs) <- rnLHsKind ctxt ki
        ; return (HsTypeArg l kis_rn, fvs) }
rnLHsTypeArg _ (HsArgPar sp)
   = return (HsArgPar sp, emptyFVs)

rnLHsTypeArgs :: HsDocContext -> [LHsTypeArg GhcPs]
                 -> RnM ([LHsTypeArg GhcRn], FreeVars)
rnLHsTypeArgs doc args = mapFvRn (rnLHsTypeArg doc) args

--------------
rnTyKiContext :: RnTyKiEnv -> LHsContext GhcPs
              -> RnM (LHsContext GhcRn, FreeVars)
rnTyKiContext env (L loc cxt)
  = do { traceRn "rncontext" (ppr cxt)
       ; let env' = env { rtke_what = RnConstraint }
       ; (cxt', fvs) <- mapFvRn (rnLHsTyKi env') cxt
       ; return (L loc cxt', fvs) }

rnContext :: HsDocContext -> LHsContext GhcPs
          -> RnM (LHsContext GhcRn, FreeVars)
rnContext doc theta = rnTyKiContext (mkTyKiEnv doc TypeLevel RnConstraint) theta

--------------
rnLHsTyKi  :: RnTyKiEnv -> LHsType GhcPs -> RnM (LHsType GhcRn, FreeVars)
rnLHsTyKi env (L loc ty)
  = setSrcSpan loc $
    do { (ty', fvs) <- rnHsTyKi env ty
       ; return (L loc ty', fvs) }

rnHsTyKi :: RnTyKiEnv -> HsType GhcPs -> RnM (HsType GhcRn, FreeVars)

rnHsTyKi env ty@(HsForAllTy { hst_tele = tele, hst_body = tau })
  = do { checkPolyKinds env ty
       ; bindHsForAllTelescope (rtke_ctxt env) tele $ \ tele' ->
    do { (tau',  fvs) <- rnLHsTyKi env tau
       ; return ( HsForAllTy { hst_xforall = noExtField
                             , hst_tele = tele' , hst_body =  tau' }
                , fvs) } }

rnHsTyKi env ty@(HsQualTy { hst_ctxt = lctxt, hst_body = tau })
  = do { checkPolyKinds env ty  -- See Note [QualTy in kinds]
       ; (ctxt', fvs1) <- rnTyKiContext env lctxt
       ; (tau',  fvs2) <- rnLHsTyKi env tau
       ; return (HsQualTy { hst_xqual = noExtField, hst_ctxt = ctxt'
                          , hst_body =  tau' }
                , fvs1 `plusFV` fvs2) }

rnHsTyKi env (HsTyVar _ ip (L loc rdr_name))
  = do { when (isRnKindLevel env && isRdrTyVar rdr_name) $
         unlessXOptM LangExt.PolyKinds $ addErr $
         withHsDocContext (rtke_ctxt env) $
         vcat [ text "Unexpected kind variable" <+> quotes (ppr rdr_name)
              , text "Perhaps you intended to use PolyKinds" ]
           -- Any type variable at the kind level is illegal without the use
           -- of PolyKinds (see #14710)
       ; name <- rnTyVar env rdr_name
       ; return (HsTyVar noExtField ip (L loc name), unitFV name) }

rnHsTyKi env ty@(HsOpTy _ ty1 l_op ty2)
  = setSrcSpan (getLoc l_op) $
    do  { (l_op', fvs1) <- rnHsTyOp env ty l_op
        ; fix   <- lookupTyFixityRn l_op'
        ; (ty1', fvs2) <- rnLHsTyKi env ty1
        ; (ty2', fvs3) <- rnLHsTyKi env ty2
        ; res_ty <- mkHsOpTyRn (\t1 t2 -> HsOpTy noExtField t1 l_op' t2)
                               (unLoc l_op') fix ty1' ty2'
        ; return (res_ty, plusFVs [fvs1, fvs2, fvs3]) }

rnHsTyKi env (HsParTy _ ty)
  = do { (ty', fvs) <- rnLHsTyKi env ty
       ; return (HsParTy noExtField ty', fvs) }

rnHsTyKi env (HsBangTy _ b ty)
  = do { (ty', fvs) <- rnLHsTyKi env ty
       ; return (HsBangTy noExtField b ty', fvs) }

rnHsTyKi env ty@(HsRecTy _ flds)
  = do { let ctxt = rtke_ctxt env
       ; fls          <- get_fields ctxt
       ; (flds', fvs) <- rnConDeclFields ctxt fls flds
       ; return (HsRecTy noExtField flds', fvs) }
  where
    get_fields (ConDeclCtx names)
      = concatMapM (lookupConstructorFields . unLoc) names
    get_fields _
      = do { addErr (hang (text "Record syntax is illegal here:")
                                   2 (ppr ty))
           ; return [] }

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

        -- Check for fixity rearrangements
       ; (mult', w_fvs) <- rnHsArrow env mult
       ; res_ty <- mkHsOpTyRn (hs_fun_ty mult') funTyConName funTyFixity ty1' ty2'
       ; return (res_ty, fvs1 `plusFV` fvs2 `plusFV` w_fvs) }
  where
    hs_fun_ty w a b = HsFunTy u w a b

rnHsTyKi env listTy@(HsListTy _ ty)
  = do { data_kinds <- xoptM LangExt.DataKinds
       ; when (not data_kinds && isRnKindLevel env)
              (addErr (dataKindsErr env listTy))
       ; (ty', fvs) <- rnLHsTyKi env ty
       ; return (HsListTy noExtField ty', fvs) }

rnHsTyKi env t@(HsKindSig _ ty k)
  = do { checkPolyKinds env t
       ; kind_sigs_ok <- xoptM LangExt.KindSignatures
       ; unless kind_sigs_ok (badKindSigErr (rtke_ctxt env) ty)
       ; (ty', lhs_fvs) <- rnLHsTyKi env ty
       ; (k', sig_fvs)  <- rnLHsTyKi (env { rtke_level = KindLevel }) k
       ; return (HsKindSig noExtField ty' k', lhs_fvs `plusFV` sig_fvs) }

-- Unboxed tuples are allowed to have poly-typed arguments.  These
-- sometimes crop up as a result of CPR worker-wrappering dictionaries.
rnHsTyKi env tupleTy@(HsTupleTy _ tup_con tys)
  = do { data_kinds <- xoptM LangExt.DataKinds
       ; when (not data_kinds && isRnKindLevel env)
              (addErr (dataKindsErr env tupleTy))
       ; (tys', fvs) <- mapFvRn (rnLHsTyKi env) tys
       ; return (HsTupleTy noExtField tup_con tys', fvs) }

rnHsTyKi env sumTy@(HsSumTy _ tys)
  = do { data_kinds <- xoptM LangExt.DataKinds
       ; when (not data_kinds && isRnKindLevel env)
              (addErr (dataKindsErr env sumTy))
       ; (tys', fvs) <- mapFvRn (rnLHsTyKi env) tys
       ; return (HsSumTy noExtField tys', fvs) }

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

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

rnHsTyKi env (HsAppKindTy l ty k)
  = do { kind_app <- xoptM LangExt.TypeApplications
       ; unless kind_app (addErr (typeAppErr "kind" k))
       ; (ty', fvs1) <- rnLHsTyKi env ty
       ; (k', fvs2) <- rnLHsTyKi (env {rtke_level = KindLevel }) k
       ; return (HsAppKindTy l ty' k', fvs1 `plusFV` fvs2) }

rnHsTyKi env t@(HsIParamTy _ n ty)
  = do { notInKinds env t
       ; (ty', fvs) <- rnLHsTyKi env ty
       ; return (HsIParamTy noExtField n ty', fvs) }

rnHsTyKi _ (HsStarTy _ isUni)
  = return (HsStarTy noExtField isUni, emptyFVs)

rnHsTyKi _ (HsSpliceTy _ sp)
  = rnSpliceType sp

rnHsTyKi env (HsDocTy _ ty haddock_doc)
  = do { (ty', fvs) <- rnLHsTyKi env ty
       ; haddock_doc' <- rnLHsDoc haddock_doc
       ; return (HsDocTy noExtField ty' haddock_doc', fvs) }

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

rnHsTyKi env ty@(HsExplicitListTy _ ip tys)
  = do { checkPolyKinds env ty
       ; data_kinds <- xoptM LangExt.DataKinds
       ; unless data_kinds (addErr (dataKindsErr env ty))
       ; (tys', fvs) <- mapFvRn (rnLHsTyKi env) tys
       ; return (HsExplicitListTy noExtField ip tys', fvs) }

rnHsTyKi env ty@(HsExplicitTupleTy _ tys)
  = do { checkPolyKinds env ty
       ; data_kinds <- xoptM LangExt.DataKinds
       ; unless data_kinds (addErr (dataKindsErr env ty))
       ; (tys', fvs) <- mapFvRn (rnLHsTyKi env) tys
       ; return (HsExplicitTupleTy noExtField tys', fvs) }

rnHsTyKi env (HsWildCardTy _)
  = do { checkAnonWildCard env
       ; return (HsWildCardTy noExtField, emptyFVs) }

rnHsArrow :: RnTyKiEnv -> HsArrow GhcPs -> RnM (HsArrow GhcRn, FreeVars)
rnHsArrow _env (HsUnrestrictedArrow u) = return (HsUnrestrictedArrow u, emptyFVs)
rnHsArrow _env (HsLinearArrow u) = return (HsLinearArrow u, emptyFVs)
rnHsArrow env (HsExplicitMult u p)
  = (\(mult, fvs) -> (HsExplicitMult u mult, fvs)) <$> rnLHsTyKi env p

--------------
rnTyVar :: RnTyKiEnv -> RdrName -> RnM Name
rnTyVar env rdr_name
  = do { name <- lookupTypeOccRn rdr_name
       ; checkNamedWildCard env name
       ; return name }

rnLTyVar :: Located RdrName -> RnM (Located Name)
-- Called externally; does not deal with wildcards
rnLTyVar (L loc rdr_name)
  = do { tyvar <- lookupTypeOccRn rdr_name
       ; return (L loc tyvar) }

--------------
rnHsTyOp :: Outputable a
         => RnTyKiEnv -> a -> Located RdrName
         -> RnM (Located Name, FreeVars)
rnHsTyOp env overall_ty (L loc op)
  = do { ops_ok <- xoptM LangExt.TypeOperators
       ; op' <- rnTyVar env op
       ; unless (ops_ok || op' `hasKey` eqTyConKey) $
           addErr (opTyErr op overall_ty)
       ; let l_op' = L loc op'
       ; return (l_op', unitFV op') }

--------------
notAllowed :: SDoc -> SDoc
notAllowed doc
  = text "Wildcard" <+> quotes doc <+> ptext (sLit "not allowed")

checkWildCard :: RnTyKiEnv -> Maybe SDoc -> RnM ()
checkWildCard env (Just doc)
  = addErr $ vcat [doc, nest 2 (text "in" <+> pprHsDocContext (rtke_ctxt env))]
checkWildCard _ Nothing
  = return ()

checkAnonWildCard :: RnTyKiEnv -> RnM ()
-- Report an error if an anonymous wildcard is illegal here
checkAnonWildCard env
  = checkWildCard env mb_bad
  where
    mb_bad :: Maybe SDoc
    mb_bad | not (wildCardsAllowed env)
           = Just (notAllowed pprAnonWildCard)
           | otherwise
           = case rtke_what env of
               RnTypeBody      -> Nothing
               RnTopConstraint -> Just constraint_msg
               RnConstraint    -> Just constraint_msg

    constraint_msg = hang
                         (notAllowed pprAnonWildCard <+> text "in a constraint")
                        2 hint_msg
    hint_msg = vcat [ text "except as the last top-level constraint of a type signature"
                    , nest 2 (text "e.g  f :: (Eq a, _) => blah") ]

checkNamedWildCard :: RnTyKiEnv -> Name -> RnM ()
-- Report an error if a named wildcard is illegal here
checkNamedWildCard env name
  = checkWildCard env mb_bad
  where
    mb_bad | not (name `elemNameSet` rtke_nwcs env)
           = Nothing  -- Not a wildcard
           | not (wildCardsAllowed env)
           = Just (notAllowed (ppr name))
           | otherwise
           = case rtke_what env of
               RnTypeBody      -> Nothing   -- Allowed
               RnTopConstraint -> Nothing   -- Allowed; e.g.
                  -- f :: (Eq _a) => _a -> Int
                  -- g :: (_a, _b) => T _a _b -> Int
                  -- The named tyvars get filled in from elsewhere
               RnConstraint    -> Just constraint_msg
    constraint_msg = notAllowed (ppr name) <+> text "in a constraint"

wildCardsAllowed :: RnTyKiEnv -> Bool
-- ^ In what contexts are wildcards permitted
wildCardsAllowed env
   = case rtke_ctxt env of
       TypeSigCtx {}       -> True
       TypBrCtx {}         -> True   -- Template Haskell quoted type
       SpliceTypeCtx {}    -> True   -- Result of a Template Haskell splice
       ExprWithTySigCtx {} -> True
       PatCtx {}           -> True
       RuleCtx {}          -> True
       FamPatCtx {}        -> True   -- Not named wildcards though
       GHCiCtx {}          -> True
       HsTypeCtx {}        -> True
       StandaloneKindSigCtx {} -> False  -- See Note [Wildcards in standalone kind signatures] in "GHC.Hs.Decls"
       _                   -> False



---------------
-- | Ensures either that we're in a type or that -XPolyKinds is set
checkPolyKinds :: Outputable ty
                => RnTyKiEnv
                -> ty      -- ^ type
                -> RnM ()
checkPolyKinds env ty
  | isRnKindLevel env
  = do { polykinds <- xoptM LangExt.PolyKinds
       ; unless polykinds $
         addErr (text "Illegal kind:" <+> ppr ty $$
                 text "Did you mean to enable PolyKinds?") }
checkPolyKinds _ _ = return ()

notInKinds :: Outputable ty
           => RnTyKiEnv
           -> ty
           -> RnM ()
notInKinds env ty
  | isRnKindLevel env
  = addErr (text "Illegal kind:" <+> ppr ty)
notInKinds _ _ = return ()

{- *****************************************************
*                                                      *
          Binding type variables
*                                                      *
***************************************************** -}

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

---------------
bindHsQTyVars :: forall a b.
                 HsDocContext
              -> Maybe a            -- Just _  => an associated type decl
              -> FreeKiTyVars       -- Kind variables from scope
              -> LHsQTyVars GhcPs
              -> (LHsQTyVars GhcRn -> Bool -> RnM (b, FreeVars))
                  -- The Bool is True <=> all kind variables used in the
                  -- kind signature are bound on the left.  Reason:
                  -- the last clause of Note [CUSKs: Complete user-supplied
                  -- kind signatures] in GHC.Hs.Decls
              -> RnM (b, FreeVars)

-- See Note [bindHsQTyVars examples]
-- (a) Bring kind variables into scope
--     both (i)  passed in body_kv_occs
--     and  (ii) mentioned in the kinds of hsq_bndrs
-- (b) Bring type variables into scope
--
bindHsQTyVars doc mb_assoc body_kv_occs hsq_bndrs thing_inside
  = do { let bndr_kv_occs = extractHsTyVarBndrsKVs hs_tv_bndrs

       ; let -- See Note [bindHsQTyVars examples] for what
             -- all these various things are doing
             bndrs, implicit_kvs :: [Located RdrName]
             bndrs        = map hsLTyVarLocName hs_tv_bndrs
             implicit_kvs = filterFreeVarsToBind bndrs $
               bndr_kv_occs ++ body_kv_occs
             body_remaining = filterFreeVarsToBind bndr_kv_occs $
              filterFreeVarsToBind bndrs body_kv_occs
             all_bound_on_lhs = null body_remaining

       ; traceRn "checkMixedVars3" $
           vcat [ text "bndrs"   <+> ppr hs_tv_bndrs
                , text "bndr_kv_occs"   <+> ppr bndr_kv_occs
                , text "body_kv_occs"   <+> ppr body_kv_occs
                , text "implicit_kvs"   <+> ppr implicit_kvs
                , text "body_remaining" <+> ppr body_remaining
                ]

       ; rnImplicitBndrs mb_assoc implicit_kvs $ \ implicit_kv_nms' ->
         bindLHsTyVarBndrs doc NoWarnUnusedForalls mb_assoc hs_tv_bndrs $ \ rn_bndrs ->
           -- This is the only call site for bindLHsTyVarBndrs where we pass
           -- NoWarnUnusedForalls, which suppresses -Wunused-foralls warnings.
           -- See Note [Suppress -Wunused-foralls when binding LHsQTyVars].
    do { let -- The SrcSpan that rnImplicitBndrs will attach to each Name will
             -- span the entire declaration to which the LHsQTyVars belongs,
             -- which will be reflected in warning and error messages. We can
             -- be a little more precise than that by pointing to the location
             -- of the LHsQTyVars instead, which is what bndrs_loc
             -- corresponds to.
             implicit_kv_nms = map (`setNameLoc` bndrs_loc) implicit_kv_nms'

       ; traceRn "bindHsQTyVars" (ppr hsq_bndrs $$ ppr implicit_kv_nms $$ ppr rn_bndrs)
       ; thing_inside (HsQTvs { hsq_ext = implicit_kv_nms
                              , hsq_explicit  = rn_bndrs })
                      all_bound_on_lhs } }
  where
    hs_tv_bndrs = hsQTvExplicit hsq_bndrs

    -- The SrcSpan of the LHsQTyVars. For example, bndrs_loc would be the
    -- highlighted part in the class below:
    --
    --   class C (a :: j) (b :: k) where
    --            ^^^^^^^^^^^^^^^
    bndrs_loc = case map getLoc hs_tv_bndrs ++ map getLoc body_kv_occs of
      []         -> panic "bindHsQTyVars.bndrs_loc"
      [loc]      -> loc
      (loc:locs) -> loc `combineSrcSpans` last locs

{- Note [bindHsQTyVars examples]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Suppose we have
   data T k (a::k1) (b::k) :: k2 -> k1 -> *

Then:
  hs_tv_bndrs = [k, a::k1, b::k], the explicitly-bound variables
  bndrs       = [k,a,b]

  bndr_kv_occs = [k,k1], kind variables free in kind signatures
                         of hs_tv_bndrs

  body_kv_occs = [k2,k1], kind variables free in the
                          result kind signature

  implicit_kvs = [k1,k2,k1], kind variables free in kind signatures
                             of hs_tv_bndrs, and not bound by bndrs

* We want to quantify add implicit bindings for implicit_kvs

* If body_kv_occs is non-empty, then there is a kind variable
  mentioned in the kind signature that is not bound "on the left".
  That's one of the rules for a CUSK, so we pass that info on
  as the second argument to thing_inside.

* Order is not important in these lists.  All we are doing is
  bring Names into scope.

* bndr_kv_occs, body_kv_occs, and implicit_kvs can contain duplicates. All
  duplicate occurrences are removed when we bind them with rnImplicitBndrs.

Finally, you may wonder why filterFreeVarsToBind removes in-scope variables
from bndr/body_kv_occs.  How can anything be in scope?  Answer:
HsQTyVars is /also/ used (slightly oddly) for Haskell-98 syntax
ConDecls
   data T a = forall (b::k). MkT a b
The ConDecl has a LHsQTyVars in it; but 'a' scopes over the entire
ConDecl.  Hence the local RdrEnv may be non-empty and we must filter
out 'a' from the free vars.  (Mind you, in this situation all the
implicit kind variables are bound at the data type level, so there
are none to bind in the ConDecl, so there are no implicitly bound
variables at all.

Note [Kind variable scoping]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If we have
  data T (a :: k) k = ...
we report "k is out of scope" for (a::k).  Reason: k is not brought
into scope until the explicit k-binding that follows.  It would be
terribly confusing to bring into scope an /implicit/ k for a's kind
and a distinct, shadowing explicit k that follows, something like
  data T {k1} (a :: k1) k = ...

So the rule is:

   the implicit binders never include any
   of the explicit binders in the group

Note that in the denerate case
  data T (a :: a) = blah
we get a complaint the second 'a' is not in scope.

That applies to foralls too: e.g.
   forall (a :: k) k . blah

But if the foralls are split, we treat the two groups separately:
   forall (a :: k). forall k. blah
Here we bring into scope an implicit k, which is later shadowed
by the explicit k.

In implementation terms

* In bindHsQTyVars 'k' is free in bndr_kv_occs; then we delete
  the binders {a,k}, and so end with no implicit binders.  Then we
  rename the binders left-to-right, and hence see that 'k' is out of
  scope in the kind of 'a'.

* Similarly in extract_hs_tv_bndrs

Note [Variables used as both types and kinds]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We bind the type variables tvs, and kvs is the set of free variables of the
kinds in the scope of the binding. Here is one typical example:

   forall a b. a -> (b::k) -> (c::a)

Here, tvs will be {a,b}, and kvs {k,a}.

We must make sure that kvs includes all of variables in the kinds of type
variable bindings. For instance:

   forall k (a :: k). Proxy a

If we only look in the body of the `forall` type, we will mistakenly conclude
that kvs is {}. But in fact, the type variable `k` is also used as a kind
variable in (a :: k), later in the binding. (This mistake lead to #14710.)
So tvs is {k,a} and kvs is {k}.

NB: we do this only at the binding site of 'tvs'.

Note [Suppress -Wunused-foralls when binding LHsQTyVars]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The WarnUnusedForalls flag controls whether bindLHsTyVarBndrs should warn about
explicit type variable binders that go unused (e.g., the `a` in
`forall a. Int`). We almost always want to warn about these, since unused type
variables can usually be deleted without any repercussions. There is one
exception to this rule, however: binding LHsQTyVars. Consider this example:

  data Proxy a = Proxy

The `a` in `Proxy a` is bound by an LHsQTyVars, and the code which brings it
into scope, bindHsQTyVars, will invoke bindLHsTyVarBndrs in turn. As such, it
has a choice to make about whether to emit -Wunused-foralls warnings or not.
If it /did/ emit warnings, then the `a` would be flagged as unused. However,
this is not what we want! Removing the `a` in `Proxy a` would change its kind
entirely, which is a huge price to pay for fixing a warning.

Unlike other forms of type variable binders, dropping "unused" variables in
an LHsQTyVars can be semantically significant. As a result, we suppress
-Wunused-foralls warnings in exactly one place: in bindHsQTyVars.
-}

bindHsForAllTelescope :: HsDocContext
                      -> HsForAllTelescope GhcPs
                      -> (HsForAllTelescope GhcRn -> RnM (a, FreeVars))
                      -> RnM (a, FreeVars)
bindHsForAllTelescope doc tele thing_inside =
  case tele of
    HsForAllVis { hsf_vis_bndrs = bndrs } ->
      bindLHsTyVarBndrs doc WarnUnusedForalls Nothing bndrs $ \bndrs' ->
        thing_inside $ mkHsForAllVisTele bndrs'
    HsForAllInvis { hsf_invis_bndrs = bndrs } ->
      bindLHsTyVarBndrs doc WarnUnusedForalls Nothing bndrs $ \bndrs' ->
        thing_inside $ mkHsForAllInvisTele bndrs'

-- | Should GHC warn if a quantified type variable goes unused? Usually, the
-- answer is \"yes\", but in the particular case of binding 'LHsQTyVars', we
-- avoid emitting warnings.
-- See @Note [Suppress -Wunused-foralls when binding LHsQTyVars]@.
data WarnUnusedForalls
  = WarnUnusedForalls
  | NoWarnUnusedForalls

instance Outputable WarnUnusedForalls where
  ppr wuf = text $ case wuf of
    WarnUnusedForalls   -> "WarnUnusedForalls"
    NoWarnUnusedForalls -> "NoWarnUnusedForalls"

bindLHsTyVarBndrs :: (OutputableBndrFlag flag)
                  => HsDocContext
                  -> WarnUnusedForalls
                  -> Maybe a               -- Just _  => an associated type decl
                  -> [LHsTyVarBndr flag GhcPs]  -- User-written tyvars
                  -> ([LHsTyVarBndr flag GhcRn] -> RnM (b, FreeVars))
                  -> RnM (b, FreeVars)
bindLHsTyVarBndrs doc wuf mb_assoc tv_bndrs thing_inside
  = do { when (isNothing mb_assoc) (checkShadowedRdrNames tv_names_w_loc)
       ; checkDupRdrNames tv_names_w_loc
       ; go tv_bndrs thing_inside }
  where
    tv_names_w_loc = map hsLTyVarLocName tv_bndrs

    go []     thing_inside = thing_inside []
    go (b:bs) thing_inside = bindLHsTyVarBndr doc mb_assoc b $ \ b' ->
                             do { (res, fvs) <- go bs $ \ bs' ->
                                                thing_inside (b' : bs')
                                ; warn_unused b' fvs
                                ; return (res, fvs) }

    warn_unused tv_bndr fvs = case wuf of
      WarnUnusedForalls   -> warnUnusedForAll doc tv_bndr fvs
      NoWarnUnusedForalls -> return ()

bindLHsTyVarBndr :: HsDocContext
                 -> Maybe a   -- associated class
                 -> LHsTyVarBndr flag GhcPs
                 -> (LHsTyVarBndr flag GhcRn -> RnM (b, FreeVars))
                 -> RnM (b, FreeVars)
bindLHsTyVarBndr _doc mb_assoc (L loc
                                 (UserTyVar x fl
                                    lrdr@(L lv _))) thing_inside
  = do { nm <- newTyVarNameRn mb_assoc lrdr
       ; bindLocalNamesFV [nm] $
         thing_inside (L loc (UserTyVar x fl (L lv nm))) }

bindLHsTyVarBndr doc mb_assoc (L loc (KindedTyVar x fl lrdr@(L lv _) kind))
                 thing_inside
  = do { sig_ok <- xoptM LangExt.KindSignatures
           ; unless sig_ok (badKindSigErr doc kind)
           ; (kind', fvs1) <- rnLHsKind doc kind
           ; tv_nm  <- newTyVarNameRn mb_assoc lrdr
           ; (b, fvs2) <- bindLocalNamesFV [tv_nm]
               $ thing_inside (L loc (KindedTyVar x fl (L lv tv_nm) kind'))
           ; return (b, fvs1 `plusFV` fvs2) }

newTyVarNameRn :: Maybe a -- associated class
               -> Located RdrName -> RnM Name
newTyVarNameRn mb_assoc lrdr@(L _ rdr)
  = do { rdr_env <- getLocalRdrEnv
       ; case (mb_assoc, lookupLocalRdrEnv rdr_env rdr) of
           (Just _, Just n) -> return n
              -- Use the same Name as the parent class decl

           _                -> newLocalBndrRn lrdr }
{-
*********************************************************
*                                                       *
        ConDeclField
*                                                       *
*********************************************************

When renaming a ConDeclField, we have to find the FieldLabel
associated with each field.  But we already have all the FieldLabels
available (since they were brought into scope by
GHC.Rename.Names.getLocalNonValBinders), so we just take the list as an
argument, build a map and look them up.
-}

rnConDeclFields :: HsDocContext -> [FieldLabel] -> [LConDeclField GhcPs]
                -> RnM ([LConDeclField GhcRn], FreeVars)
-- Also called from GHC.Rename.Module
-- No wildcards can appear in record fields
rnConDeclFields ctxt fls fields
   = mapFvRn (rnField fl_env env) fields
  where
    env    = mkTyKiEnv ctxt TypeLevel RnTypeBody
    fl_env = mkFsEnv [ (flLabel fl, fl) | fl <- fls ]

rnField :: FastStringEnv FieldLabel -> RnTyKiEnv -> LConDeclField GhcPs
        -> RnM (LConDeclField GhcRn, FreeVars)
rnField fl_env env (L l (ConDeclField _ names ty haddock_doc))
  = do { let new_names = map (fmap lookupField) names
       ; (new_ty, fvs) <- rnLHsTyKi env ty
       ; new_haddock_doc <- rnMbLHsDoc haddock_doc
       ; return (L l (ConDeclField noExtField new_names new_ty new_haddock_doc)
                , fvs) }
  where
    lookupField :: FieldOcc GhcPs -> FieldOcc GhcRn
    lookupField (FieldOcc _ (L lr rdr)) =
        FieldOcc (flSelector fl) (L lr rdr)
      where
        lbl = occNameFS $ rdrNameOcc rdr
        fl  = expectJust "rnField" $ lookupFsEnv fl_env lbl

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

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

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

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

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

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

mkHsOpTyRn mk1 pp_op1 fix1 ty1 (L loc2 (HsFunTy _ mult ty21 ty22))
  = mk_hs_op_ty mk1 pp_op1 fix1 ty1
                hs_fun_ty funTyConName funTyFixity ty21 ty22 loc2
  where
    hs_fun_ty a b = HsFunTy noExtField mult a b

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

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


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

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

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

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

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

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

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

data NegationHandling = ReassociateNegation | KeepNegationIntact

----------------------------

-- | Name of an operator in an operator application or section
data OpName = NormalOp Name         -- ^ A normal identifier
            | NegateOp              -- ^ Prefix negation
            | UnboundOp OccName     -- ^ An unbound indentifier
            | RecFldOp (AmbiguousFieldOcc GhcRn)
              -- ^ A (possibly ambiguous) record field occurrence

instance Outputable OpName where
  ppr (NormalOp n)   = ppr n
  ppr NegateOp       = ppr negateName
  ppr (UnboundOp uv) = ppr uv
  ppr (RecFldOp fld) = ppr fld

get_op :: LHsExpr GhcRn -> OpName
-- An unbound name could be either HsVar or HsUnboundVar
-- See GHC.Rename.Expr.rnUnboundVar
get_op (L _ (HsVar _ n))         = NormalOp (unLoc n)
get_op (L _ (HsUnboundVar _ uv)) = UnboundOp uv
get_op (L _ (HsRecFld _ fld))    = RecFldOp fld
get_op other                     = pprPanic "get_op" (ppr other)

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

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

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

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

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

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

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


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

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

        ; if nofix_error then do
                { precParseErr (NormalOp (unLoc op1),fix1)
                               (NormalOp (unLoc op2),fix2)
                ; return $ ConPat
                    { pat_con_ext = noExtField
                    , pat_con = op2
                    , pat_args = InfixCon p1 p2
                    }
                }

          else if associate_right then do
                { new_p <- mkConOpPatRn op2 fix2 p12 p2
                ; return $ ConPat
                    { pat_con_ext = noExtField
                    , pat_con = op1
                    , pat_args = InfixCon p11 (L loc new_p)
                    }
                }
                -- XXX loc right?
          else return $ ConPat
                 { pat_con_ext = noExtField
                 , pat_con = op2
                 , pat_args = InfixCon p1 p2
                 }
        }

mkConOpPatRn op _ p1 p2                         -- Default case, no rearrangment
  = ASSERT( not_op_pat (unLoc p2) )
    return $ ConPat
      { pat_con_ext = noExtField
      , pat_con = op
      , pat_args = InfixCon p1 p2
      }

not_op_pat :: Pat GhcRn -> Bool
not_op_pat (ConPat NoExtField _ (InfixCon _ _)) = False
not_op_pat _                                    = True

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

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

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

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

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

checkPrec _ _ _
  = return ()

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

-- | Look up the fixity for an operator name.  Be careful to use
-- 'lookupFieldFixityRn' for (possibly ambiguous) record fields
-- (see #13132).
lookupFixityOp :: OpName -> RnM Fixity
lookupFixityOp (NormalOp n)  = lookupFixityRn n
lookupFixityOp NegateOp      = lookupFixityRn negateName
lookupFixityOp (UnboundOp u) = lookupFixityRn (mkUnboundName u)
lookupFixityOp (RecFldOp f)  = lookupFieldFixityRn f


-- Precedence-related error messages

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

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

is_unbound :: OpName -> Bool
is_unbound (NormalOp n) = isUnboundName n
is_unbound UnboundOp{}  = True
is_unbound _            = False

ppr_opfix :: (OpName, Fixity) -> SDoc
ppr_opfix (op, fixity) = pp_op <+> brackets (ppr fixity)
   where
     pp_op | NegateOp <- op = text "prefix `-'"
           | otherwise      = quotes (ppr op)


{- *****************************************************
*                                                      *
                 Errors
*                                                      *
***************************************************** -}

unexpectedPatSigTypeErr :: HsPatSigType GhcPs -> SDoc
unexpectedPatSigTypeErr ty
  = hang (text "Illegal type signature:" <+> quotes (ppr ty))
       2 (text "Type signatures are only allowed in patterns with ScopedTypeVariables")

badKindSigErr :: HsDocContext -> LHsType GhcPs -> TcM ()
badKindSigErr doc (L loc ty)
  = setSrcSpan loc $ addErr $
    withHsDocContext doc $
    hang (text "Illegal kind signature:" <+> quotes (ppr ty))
       2 (text "Perhaps you intended to use KindSignatures")

dataKindsErr :: RnTyKiEnv -> HsType GhcPs -> SDoc
dataKindsErr env thing
  = hang (text "Illegal" <+> pp_what <> colon <+> quotes (ppr thing))
       2 (text "Perhaps you intended to use DataKinds")
  where
    pp_what | isRnKindLevel env = text "kind"
            | otherwise          = text "type"

warnUnusedForAll :: OutputableBndrFlag flag
                 => HsDocContext -> LHsTyVarBndr flag GhcRn -> FreeVars -> TcM ()
warnUnusedForAll doc (L loc tv) used_names
  = whenWOptM Opt_WarnUnusedForalls $
    unless (hsTyVarName tv `elemNameSet` used_names) $
    addWarnAt (Reason Opt_WarnUnusedForalls) loc $
    vcat [ text "Unused quantified type variable" <+> quotes (ppr tv)
         , inHsDocContext doc ]

opTyErr :: Outputable a => RdrName -> a -> SDoc
opTyErr op overall_ty
  = hang (text "Illegal operator" <+> quotes (ppr op) <+> ptext (sLit "in type") <+> quotes (ppr overall_ty))
         2 (text "Use TypeOperators to allow operators in types")

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


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

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

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

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

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

To do that, we need to walk over a type and find its free type/kind variables.
We preserve the left-to-right order of each variable occurrence.
See Note [Ordering of implicit variables].

It is common for lists of free type variables to contain duplicates. For
example, in `f :: a -> a`, the free type variable list is [a, a]. When these
implicitly bound variables are brought into scope (with rnImplicitBndrs),
duplicates are removed with nubL.

Note [Ordering of implicit variables]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Since the advent of -XTypeApplications, GHC makes promises about the ordering
of implicit variable quantification. Specifically, we offer that implicitly
quantified variables (such as those in const :: a -> b -> a, without a `forall`)
will occur in left-to-right order of first occurrence. Here are a few examples:

  const :: a -> b -> a       -- forall a b. ...
  f :: Eq a => b -> a -> a   -- forall a b. ...  contexts are included

  type a <-< b = b -> a
  g :: a <-< b               -- forall a b. ...  type synonyms matter

  class Functor f where
    fmap :: (a -> b) -> f a -> f b   -- forall f a b. ...
    -- The f is quantified by the class, so only a and b are considered in fmap

This simple story is complicated by the possibility of dependency: all variables
must come after any variables mentioned in their kinds.

  typeRep :: Typeable a => TypeRep (a :: k)   -- forall k a. ...

The k comes first because a depends on k, even though the k appears later than
the a in the code. Thus, GHC does ScopedSort on the variables.
See Note [ScopedSort] in GHC.Core.Type.

Implicitly bound variables are collected by any function which returns a
FreeKiTyVars, which notably includes the `extract-` family of functions
(extractHsTysRdrTyVars, extractHsTyVarBndrsKVs, etc.).
These functions thus promise to keep left-to-right ordering.

Note [Implicit quantification in type synonyms]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We typically bind type/kind variables implicitly when they are in a kind
annotation on the LHS, for example:

  data Proxy (a :: k) = Proxy
  type KindOf (a :: k) = k

Here 'k' is in the kind annotation of a type variable binding, KindedTyVar, and
we want to implicitly quantify over it.  This is easy: just extract all free
variables from the kind signature. That's what we do in extract_hs_tv_bndrs_kvs

By contrast, on the RHS we can't simply collect *all* free variables. Which of
the following are allowed?

  type TySyn1 = a :: Type
  type TySyn2 = 'Nothing :: Maybe a
  type TySyn3 = 'Just ('Nothing :: Maybe a)
  type TySyn4 = 'Left a :: Either Type a

After some design deliberations (see non-taken alternatives below), the answer
is to reject TySyn1 and TySyn3, but allow TySyn2 and TySyn4, at least for now.
We implicitly quantify over free variables of the outermost kind signature, if
one exists:

  * In TySyn1, the outermost kind signature is (:: Type), and it does not have
    any free variables.
  * In TySyn2, the outermost kind signature is (:: Maybe a), it contains a
    free variable 'a', which we implicitly quantify over.
  * In TySyn3, there is no outermost kind signature. The (:: Maybe a) signature
    is hidden inside 'Just.
  * In TySyn4, the outermost kind signature is (:: Either Type a), it contains
    a free variable 'a', which we implicitly quantify over. That is why we can
    also use it to the left of the double colon: 'Left a

The logic resides in extractHsTyRdrTyVarsKindVars. We use it both for type
synonyms and type family instances.

This is something of a stopgap solution until we can explicitly bind invisible
type/kind variables:

  type TySyn3 :: forall a. Maybe a
  type TySyn3 @a = 'Just ('Nothing :: Maybe a)

Note [Implicit quantification in type synonyms: non-taken alternatives]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Alternative I: No quantification
--------------------------------
We could offer no implicit quantification on the RHS, accepting none of the
TySyn<N> examples. The user would have to bind the variables explicitly:

  type TySyn1 a = a :: Type
  type TySyn2 a = 'Nothing :: Maybe a
  type TySyn3 a = 'Just ('Nothing :: Maybe a)
  type TySyn4 a = 'Left a :: Either Type a

However, this would mean that one would have to specify 'a' at call sites every
time, which could be undesired.

Alternative II: Indiscriminate quantification
---------------------------------------------
We could implicitly quantify over all free variables on the RHS just like we do
on the LHS. Then we would infer the following kinds:

  TySyn1 :: forall {a}. Type
  TySyn2 :: forall {a}. Maybe a
  TySyn3 :: forall {a}. Maybe (Maybe a)
  TySyn4 :: forall {a}. Either Type a

This would work fine for TySyn<2,3,4>, but TySyn1 is clearly bogus: the variable
is free-floating, not fixed by anything.

Alternative III: reportFloatingKvs
----------------------------------
We could augment Alternative II by hunting down free-floating variables during
type checking. While viable, this would mean we'd end up accepting this:

  data Prox k (a :: k)
  type T = Prox k

-}

-- A list of free type/kind variables, which can contain duplicates.
-- See Note [Kind and type-variable binders]
-- These lists are guaranteed to preserve left-to-right ordering of
-- the types the variables were extracted from. See also
-- Note [Ordering of implicit variables].
type FreeKiTyVars = [Located RdrName]

-- | Filter out any type and kind variables that are already in scope in the
-- the supplied LocalRdrEnv. Note that this includes named wildcards, which
-- look like perfectly ordinary type variables at this point.
filterInScope :: LocalRdrEnv -> FreeKiTyVars -> FreeKiTyVars
filterInScope rdr_env = filterOut (inScope rdr_env . unLoc)

-- | Filter out any type and kind variables that are already in scope in the
-- the environment's LocalRdrEnv. Note that this includes named wildcards,
-- which look like perfectly ordinary type variables at this point.
filterInScopeM :: FreeKiTyVars -> RnM FreeKiTyVars
filterInScopeM vars
  = do { rdr_env <- getLocalRdrEnv
       ; return (filterInScope rdr_env vars) }

inScope :: LocalRdrEnv -> RdrName -> Bool
inScope rdr_env rdr = rdr `elemLocalRdrEnv` rdr_env

extract_tyarg :: LHsTypeArg GhcPs -> FreeKiTyVars -> FreeKiTyVars
extract_tyarg (HsValArg ty) acc = extract_lty ty acc
extract_tyarg (HsTypeArg _ ki) acc = extract_lty ki acc
extract_tyarg (HsArgPar _) acc = acc

extract_tyargs :: [LHsTypeArg GhcPs] -> FreeKiTyVars -> FreeKiTyVars
extract_tyargs args acc = foldr extract_tyarg acc args

extractHsTyArgRdrKiTyVars :: [LHsTypeArg GhcPs] -> FreeKiTyVars
extractHsTyArgRdrKiTyVars args
  = extract_tyargs args []

-- | 'extractHsTyRdrTyVars' finds the type/kind variables
--                          of a HsType/HsKind.
-- It's used when making the @forall@s explicit.
-- See Note [Kind and type-variable binders]
extractHsTyRdrTyVars :: LHsType GhcPs -> FreeKiTyVars
extractHsTyRdrTyVars ty = extract_lty ty []

extractHsScaledTysRdrTyVars :: [HsScaled GhcPs (LHsType GhcPs)] -> FreeKiTyVars -> FreeKiTyVars
extractHsScaledTysRdrTyVars args acc = foldr (\(HsScaled m ty) -> extract_lty ty . extract_hs_arrow m) acc args

-- | Extracts the free type/kind variables from the kind signature of a HsType.
--   This is used to implicitly quantify over @k@ in @type T = Nothing :: Maybe k@.
-- The left-to-right order of variables is preserved.
-- See Note [Kind and type-variable binders] and
--     Note [Ordering of implicit variables] and
--     Note [Implicit quantification in type synonyms].
extractHsTyRdrTyVarsKindVars :: LHsType GhcPs -> FreeKiTyVars
extractHsTyRdrTyVarsKindVars (L _ ty) =
  case ty of
    HsParTy _ ty -> extractHsTyRdrTyVarsKindVars ty
    HsKindSig _ _ ki -> extractHsTyRdrTyVars ki
    _ -> []

-- | Extracts free type and kind variables from types in a list.
-- When the same name occurs multiple times in the types, all occurrences
-- are returned.
extractHsTysRdrTyVars :: [LHsType GhcPs] -> FreeKiTyVars -> FreeKiTyVars
extractHsTysRdrTyVars tys = extract_ltys tys

-- Returns the free kind variables of any explicitly-kinded binders, returning
-- variable occurrences in left-to-right order.
-- See Note [Ordering of implicit variables].
-- NB: Does /not/ delete the binders themselves.
--     E.g. given  [k1, a:k1, b:k2]
--          the function returns [k1,k2], even though k1 is bound here
extractHsTyVarBndrsKVs :: [LHsTyVarBndr flag GhcPs] -> FreeKiTyVars
extractHsTyVarBndrsKVs tv_bndrs = extract_hs_tv_bndrs_kvs tv_bndrs

-- Returns the free kind variables in a type family result signature, returning
-- variable occurrences in left-to-right order.
-- See Note [Ordering of implicit variables].
extractRdrKindSigVars :: LFamilyResultSig GhcPs -> FreeKiTyVars
extractRdrKindSigVars (L _ resultSig) = case resultSig of
  KindSig _ k                            -> extractHsTyRdrTyVars k
  TyVarSig _ (L _ (KindedTyVar _ _ _ k)) -> extractHsTyRdrTyVars k
  _ -> []

-- | Get type/kind variables mentioned in the kind signature, preserving
-- left-to-right order:
--
--  * data T a (b :: k1) :: k2 -> k1 -> k2 -> Type   -- result: [k2,k1]
--  * data T a (b :: k1)                             -- result: []
--
-- See Note [Ordering of implicit variables].
extractDataDefnKindVars :: HsDataDefn GhcPs ->  FreeKiTyVars
extractDataDefnKindVars (HsDataDefn { dd_kindSig = ksig })
  = maybe [] extractHsTyRdrTyVars ksig

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

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

extract_lty :: LHsType GhcPs -> FreeKiTyVars -> FreeKiTyVars
extract_lty (L _ ty) acc
  = case ty of
      HsTyVar _ _  ltv            -> extract_tv ltv acc
      HsBangTy _ _ ty             -> extract_lty ty acc
      HsRecTy _ flds              -> foldr (extract_lty
                                            . cd_fld_type . unLoc) acc
                                           flds
      HsAppTy _ ty1 ty2           -> extract_lty ty1 $
                                     extract_lty ty2 acc
      HsAppKindTy _ ty k          -> extract_lty ty $
                                     extract_lty k acc
      HsListTy _ ty               -> extract_lty ty acc
      HsTupleTy _ _ tys           -> extract_ltys tys acc
      HsSumTy _ tys               -> extract_ltys tys acc
      HsFunTy _ w ty1 ty2         -> extract_lty ty1 $
                                     extract_lty ty2 $
                                     extract_hs_arrow w acc
      HsIParamTy _ _ ty           -> extract_lty ty acc
      HsOpTy _ ty1 tv ty2         -> extract_tv tv $
                                     extract_lty ty1 $
                                     extract_lty ty2 acc
      HsParTy _ ty                -> extract_lty ty acc
      HsSpliceTy {}               -> acc  -- Type splices mention no tvs
      HsDocTy _ ty _              -> extract_lty ty acc
      HsExplicitListTy _ _ tys    -> extract_ltys tys acc
      HsExplicitTupleTy _ tys     -> extract_ltys tys acc
      HsTyLit _ _                 -> acc
      HsStarTy _ _                -> acc
      HsKindSig _ ty ki           -> extract_lty ty $
                                     extract_lty ki acc
      HsForAllTy { hst_tele = tele, hst_body = ty }
                                  -> extract_hs_for_all_telescope tele acc $
                                     extract_lty ty []
      HsQualTy { hst_ctxt = ctxt, hst_body = ty }
                                  -> extract_lctxt ctxt $
                                     extract_lty ty acc
      XHsType {}                  -> acc
      -- We deal with these separately in rnLHsTypeWithWildCards
      HsWildCardTy {}             -> acc

extract_hs_arrow :: HsArrow GhcPs -> FreeKiTyVars ->
                   FreeKiTyVars
extract_hs_arrow (HsExplicitMult _ p) acc = extract_lty p acc
extract_hs_arrow _ acc = acc

extract_hs_for_all_telescope :: HsForAllTelescope GhcPs
                             -> FreeKiTyVars -- Accumulator
                             -> FreeKiTyVars -- Free in body
                             -> FreeKiTyVars
extract_hs_for_all_telescope tele acc_vars body_fvs =
  case tele of
    HsForAllVis { hsf_vis_bndrs = bndrs } ->
      extract_hs_tv_bndrs bndrs acc_vars body_fvs
    HsForAllInvis { hsf_invis_bndrs = bndrs } ->
      extract_hs_tv_bndrs bndrs acc_vars body_fvs

extractHsTvBndrs :: [LHsTyVarBndr flag GhcPs]
                 -> FreeKiTyVars       -- Free in body
                 -> FreeKiTyVars       -- Free in result
extractHsTvBndrs tv_bndrs body_fvs
  = extract_hs_tv_bndrs tv_bndrs [] body_fvs

extract_hs_tv_bndrs :: [LHsTyVarBndr flag GhcPs]
                    -> FreeKiTyVars  -- Accumulator
                    -> FreeKiTyVars  -- Free in body
                    -> FreeKiTyVars
-- In (forall (a :: Maybe e). a -> b) we have
--     'a' is bound by the forall
--     'b' is a free type variable
--     'e' is a free kind variable
extract_hs_tv_bndrs tv_bndrs acc_vars body_vars = new_vars ++ acc_vars
  where
    new_vars
      | null tv_bndrs = body_vars
      | otherwise = filterFreeVarsToBind tv_bndr_rdrs $ bndr_vars ++ body_vars
    -- NB: delete all tv_bndr_rdrs from bndr_vars as well as body_vars.
    -- See Note [Kind variable scoping]
    bndr_vars = extract_hs_tv_bndrs_kvs tv_bndrs
    tv_bndr_rdrs = map hsLTyVarLocName tv_bndrs

extract_hs_tv_bndrs_kvs :: [LHsTyVarBndr flag GhcPs] -> FreeKiTyVars
-- Returns the free kind variables of any explicitly-kinded binders, returning
-- variable occurrences in left-to-right order.
-- See Note [Ordering of implicit variables].
-- NB: Does /not/ delete the binders themselves.
--     E.g. given  [k1, a:k1, b:k2]
--          the function returns [k1,k2], even though k1 is bound here
extract_hs_tv_bndrs_kvs tv_bndrs =
    foldr extract_lty []
          [k | L _ (KindedTyVar _ _ _ k) <- tv_bndrs]

extract_tv :: Located RdrName -> FreeKiTyVars -> FreeKiTyVars
extract_tv tv acc =
  if isRdrTyVar (unLoc tv) then tv:acc else acc

-- Deletes duplicates in a list of Located things. This is used to:
--
-- * Delete duplicate occurrences of implicitly bound type/kind variables when
--   bringing them into scope (in rnImplicitBndrs).
--
-- * Delete duplicate occurrences of named wildcards (in rn_hs_sig_wc_type and
--   rnHsWcType).
--
-- Importantly, this function is stable with respect to the original ordering
-- of things in the list. This is important, as it is a property that GHC
-- relies on to maintain the left-to-right ordering of implicitly quantified
-- type variables.
-- See Note [Ordering of implicit variables].
nubL :: Eq a => [Located a] -> [Located a]
nubL = nubBy eqLocated

-- | Filter out any potential implicit binders that are either
-- already in scope, or are explicitly bound in the binder.
filterFreeVarsToBind :: FreeKiTyVars
                     -- ^ Explicitly bound here
                     -> FreeKiTyVars
                     -- ^ Potential implicit binders
                     -> FreeKiTyVars
                     -- ^ Final implicit binders
filterFreeVarsToBind bndrs = filterOut is_in_scope
    -- Make sure to list the binder kvs before the body kvs, as mandated by
    -- Note [Ordering of implicit variables]
  where
    is_in_scope locc = any (eqLocated locc) bndrs
