rename/RnBinds.hs

    {-# LANGUAGE ScopedTypeVariables, BangPatterns #-}

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

    \section[RnBinds]{Renaming and dependency analysis of bindings}

    This module does renaming and dependency analysis on value bindings in
    the abstract syntax.  It does {\em not} do cycle-checks on class or
    type-synonym declarations; those cannot be done at this stage because
    they may be affected by renaming (which isn't fully worked out yet).
    -}

    module RnBinds (
       -- Renaming top-level bindings
       rnTopBindsLHS, rnTopBindsBoot, rnValBindsRHS,

       -- Renaming local bindings
       rnLocalBindsAndThen, rnLocalValBindsLHS, rnLocalValBindsRHS,

       -- Other bindings
       rnMethodBinds, renameSigs,
       rnMatchGroup, rnGRHSs, rnGRHS,
       makeMiniFixityEnv, MiniFixityEnv,
       HsSigCtxt(..)
       ) where

    import {-# SOURCE #-} RnExpr( rnLExpr, rnStmts )

    import HsSyn
    import TcRnMonad
    import TcEvidence     ( emptyTcEvBinds )
    import RnTypes
    import RnPat
    import RnNames
    import RnEnv
    import DynFlags
    import Module
    import Name
    import NameEnv
    import NameSet
    import RdrName          ( RdrName, rdrNameOcc )
    import SrcLoc
    import ListSetOps       ( findDupsEq )
    import BasicTypes       ( RecFlag(..), LexicalFixity(..) )
    import Digraph          ( SCC(..) )
    import Bag
    import Util
    import Outputable
    import FastString
    import UniqSet
    import Maybes           ( orElse )
    import qualified GHC.LanguageExtensions as LangExt

    import Control.Monad
    import Data.List        ( partition, sort )

    {-
    -- ToDo: Put the annotations into the monad, so that they arrive in the proper
    -- place and can be used when complaining.

    The code tree received by the function @rnBinds@ contains definitions
    in where-clauses which are all apparently mutually recursive, but which may
    not really depend upon each other. For example, in the top level program
    \begin{verbatim}
    f x = y where a = x
                  y = x
    \end{verbatim}
    the definitions of @a@ and @y@ do not depend on each other at all.
    Unfortunately, the typechecker cannot always check such definitions.
    \footnote{Mycroft, A. 1984. Polymorphic type schemes and recursive
    definitions. In Proceedings of the International Symposium on Programming,
    Toulouse, pp. 217-39. LNCS 167. Springer Verlag.}
    However, the typechecker usually can check definitions in which only the
    strongly connected components have been collected into recursive bindings.
    This is precisely what the function @rnBinds@ does.

    ToDo: deal with case where a single monobinds binds the same variable
    twice.

    The vertag tag is a unique @Int@; the tags only need to be unique
    within one @MonoBinds@, so that unique-Int plumbing is done explicitly
    (heavy monad machinery not needed).

    ************************************************************************
    *                                                                      *
    * naming conventions                                                   *
    *                                                                      *
    ************************************************************************

    \subsection[name-conventions]{Name conventions}

    The basic algorithm involves walking over the tree and returning a tuple
    containing the new tree plus its free variables. Some functions, such
    as those walking polymorphic bindings (HsBinds) and qualifier lists in
    list comprehensions (@Quals@), return the variables bound in local
    environments. These are then used to calculate the free variables of the
    expression evaluated in these environments.

    Conventions for variable names are as follows:
    \begin{itemize}
    \item
    new code is given a prime to distinguish it from the old.

    \item
    a set of variables defined in @Exp@ is written @dvExp@

    \item
    a set of variables free in @Exp@ is written @fvExp@
    \end{itemize}

    ************************************************************************
    *                                                                      *
    * analysing polymorphic bindings (HsBindGroup, HsBind)
    *                                                                      *
    ************************************************************************

    \subsubsection[dep-HsBinds]{Polymorphic bindings}

    Non-recursive expressions are reconstructed without any changes at top
    level, although their component expressions may have to be altered.
    However, non-recursive expressions are currently not expected as
    \Haskell{} programs, and this code should not be executed.

    Monomorphic bindings contain information that is returned in a tuple
    (a @FlatMonoBinds@) containing:

    \begin{enumerate}
    \item
    a unique @Int@ that serves as the ``vertex tag'' for this binding.

    \item
    the name of a function or the names in a pattern. These are a set
    referred to as @dvLhs@, the defined variables of the left hand side.

    \item
    the free variables of the body. These are referred to as @fvBody@.

    \item
    the definition's actual code. This is referred to as just @code@.
    \end{enumerate}

    The function @nonRecDvFv@ returns two sets of variables. The first is
    the set of variables defined in the set of monomorphic bindings, while the
    second is the set of free variables in those bindings.

    The set of variables defined in a non-recursive binding is just the
    union of all of them, as @union@ removes duplicates. However, the
    free variables in each successive set of cumulative bindings is the
    union of those in the previous set plus those of the newest binding after
    the defined variables of the previous set have been removed.

    @rnMethodBinds@ deals only with the declarations in class and
    instance declarations.  It expects only to see @FunMonoBind@s, and
    it expects the global environment to contain bindings for the binders
    (which are all class operations).

    ************************************************************************
    *                                                                      *
    \subsubsection{ Top-level bindings}
    *                                                                      *
    ************************************************************************
    -}

    -- for top-level bindings, we need to make top-level names,
    -- so we have a different entry point than for local bindings
    rnTopBindsLHS :: MiniFixityEnv
                  -> HsValBinds RdrName
                  -> RnM (HsValBindsLR Name RdrName)
    rnTopBindsLHS fix_env binds
      = rnValBindsLHS (topRecNameMaker fix_env) binds

    rnTopBindsBoot :: NameSet -> HsValBindsLR Name RdrName -> RnM (HsValBinds Name, DefUses)
    -- A hs-boot file has no bindings.
    -- Return a single HsBindGroup with empty binds and renamed signatures
    rnTopBindsBoot bound_names (ValBindsIn mbinds sigs)
      = do  { checkErr (isEmptyLHsBinds mbinds) (bindsInHsBootFile mbinds)
            ; (sigs', fvs) <- renameSigs (HsBootCtxt bound_names) sigs
            ; return (ValBindsOut [] sigs', usesOnly fvs) }
    rnTopBindsBoot _ b = pprPanic "rnTopBindsBoot" (ppr b)

    {-
    *********************************************************
    *                                                      *
                    HsLocalBinds
    *                                                      *
    *********************************************************
    -}

    rnLocalBindsAndThen :: HsLocalBinds RdrName
                        -> (HsLocalBinds Name -> FreeVars -> RnM (result, FreeVars))
                        -> RnM (result, FreeVars)
    -- This version (a) assumes that the binding vars are *not* already in scope
    --               (b) removes the binders from the free vars of the thing inside
    -- The parser doesn't produce ThenBinds
    rnLocalBindsAndThen EmptyLocalBinds thing_inside =
      thing_inside EmptyLocalBinds emptyNameSet

    rnLocalBindsAndThen (HsValBinds val_binds) thing_inside
      = rnLocalValBindsAndThen val_binds $ \ val_binds' ->
          thing_inside (HsValBinds val_binds')

    rnLocalBindsAndThen (HsIPBinds binds) thing_inside = do
        (binds',fv_binds) <- rnIPBinds binds
        (thing, fvs_thing) <- thing_inside (HsIPBinds binds') fv_binds
        return (thing, fvs_thing `plusFV` fv_binds)

    rnIPBinds :: HsIPBinds RdrName -> RnM (HsIPBinds Name, FreeVars)
    rnIPBinds (IPBinds ip_binds _no_dict_binds) = do
        (ip_binds', fvs_s) <- mapAndUnzipM (wrapLocFstM rnIPBind) ip_binds
        return (IPBinds ip_binds' emptyTcEvBinds, plusFVs fvs_s)

    rnIPBind :: IPBind RdrName -> RnM (IPBind Name, FreeVars)
    rnIPBind (IPBind ~(Left n) expr) = do
        (expr',fvExpr) <- rnLExpr expr
        return (IPBind (Left n) expr', fvExpr)

    {-
    ************************************************************************
    *                                                                      *
                    ValBinds
    *                                                                      *
    ************************************************************************
    -}

    -- Renaming local binding groups
    -- Does duplicate/shadow check
    rnLocalValBindsLHS :: MiniFixityEnv
                       -> HsValBinds RdrName
                       -> RnM ([Name], HsValBindsLR Name RdrName)
    rnLocalValBindsLHS fix_env binds
      = do { binds' <- rnValBindsLHS (localRecNameMaker fix_env) binds

             -- Check for duplicates and shadowing
             -- Must do this *after* renaming the patterns
             -- See Note [Collect binders only after renaming] in HsUtils

             -- We need to check for dups here because we
             -- don't don't bind all of the variables from the ValBinds at once
             -- with bindLocatedLocals any more.
             --
             -- Note that we don't want to do this at the top level, since
             -- sorting out duplicates and shadowing there happens elsewhere.
             -- The behavior is even different. For example,
             --   import A(f)
             --   f = ...
             -- should not produce a shadowing warning (but it will produce
             -- an ambiguity warning if you use f), but
             --   import A(f)
             --   g = let f = ... in f
             -- should.
           ; let bound_names = collectHsValBinders binds'
                 -- There should be only Ids, but if there are any bogus
                 -- pattern synonyms, we'll collect them anyway, so that
                 -- we don't generate subsequent out-of-scope messages
           ; envs <- getRdrEnvs
           ; checkDupAndShadowedNames envs bound_names

           ; return (bound_names, binds') }

    -- renames the left-hand sides
    -- generic version used both at the top level and for local binds
    -- does some error checking, but not what gets done elsewhere at the top level
    rnValBindsLHS :: NameMaker
                  -> HsValBinds RdrName
                  -> RnM (HsValBindsLR Name RdrName)
    rnValBindsLHS topP (ValBindsIn mbinds sigs)
      = do { mbinds' <- mapBagM (wrapLocM (rnBindLHS topP doc)) mbinds
           ; return $ ValBindsIn mbinds' sigs }
      where
        bndrs = collectHsBindsBinders mbinds
        doc   = text "In the binding group for:" <+> pprWithCommas ppr bndrs

    rnValBindsLHS _ b = pprPanic "rnValBindsLHSFromDoc" (ppr b)

    -- General version used both from the top-level and for local things
    -- Assumes the LHS vars are in scope
    --
    -- Does not bind the local fixity declarations
    rnValBindsRHS :: HsSigCtxt
                  -> HsValBindsLR Name RdrName
                  -> RnM (HsValBinds Name, DefUses)

    rnValBindsRHS ctxt (ValBindsIn mbinds sigs)
      = do { (sigs', sig_fvs) <- renameSigs ctxt sigs
           ; binds_w_dus <- mapBagM (rnLBind (mkSigTvFn sigs')) mbinds
           ; let !(anal_binds, anal_dus) = depAnalBinds binds_w_dus

           ; let patsyn_fvs = foldr (unionNameSet . psb_fvs) emptyNameSet $
                              getPatSynBinds anal_binds
                    -- The uses in binds_w_dus for PatSynBinds do not include
                    -- variables used in the patsyn builders; see
                    -- Note [Pattern synonym builders don't yield dependencies]
                    -- But psb_fvs /does/ include those builder fvs.  So we
                    -- add them back in here to avoid bogus warnings about
                    -- unused variables (Trac #12548)

                 valbind'_dus = anal_dus `plusDU` usesOnly sig_fvs
                                         `plusDU` usesOnly patsyn_fvs
                                -- Put the sig uses *after* the bindings
                                -- so that the binders are removed from
                                -- the uses in the sigs

            ; return (ValBindsOut anal_binds sigs', valbind'_dus) }

    rnValBindsRHS _ b = pprPanic "rnValBindsRHS" (ppr b)

    -- Wrapper for local binds
    --
    -- The *client* of this function is responsible for checking for unused binders;
    -- it doesn't (and can't: we don't have the thing inside the binds) happen here
    --
    -- The client is also responsible for bringing the fixities into scope
    rnLocalValBindsRHS :: NameSet  -- names bound by the LHSes
                       -> HsValBindsLR Name RdrName
                       -> RnM (HsValBinds Name, DefUses)
    rnLocalValBindsRHS bound_names binds
      = rnValBindsRHS (LocalBindCtxt bound_names) binds

    -- for local binds
    -- wrapper that does both the left- and right-hand sides
    --
    -- here there are no local fixity decls passed in;
    -- the local fixity decls come from the ValBinds sigs
    rnLocalValBindsAndThen
      :: HsValBinds RdrName
      -> (HsValBinds Name -> FreeVars -> RnM (result, FreeVars))
      -> RnM (result, FreeVars)
    rnLocalValBindsAndThen binds@(ValBindsIn _ sigs) thing_inside
     = do   {     -- (A) Create the local fixity environment
              new_fixities <- makeMiniFixityEnv [L loc sig
                                                      | L loc (FixSig sig) <- sigs]

                  -- (B) Rename the LHSes
            ; (bound_names, new_lhs) <- rnLocalValBindsLHS new_fixities binds

                  --     ...and bring them (and their fixities) into scope
            ; bindLocalNamesFV bound_names              $
              addLocalFixities new_fixities bound_names $ do

            {      -- (C) Do the RHS and thing inside
              (binds', dus) <- rnLocalValBindsRHS (mkNameSet bound_names) new_lhs
            ; (result, result_fvs) <- thing_inside binds' (allUses dus)

                    -- Report unused bindings based on the (accurate)
                    -- findUses.  E.g.
                    --      let x = x in 3
                    -- should report 'x' unused
            ; let real_uses = findUses dus result_fvs
                  -- Insert fake uses for variables introduced implicitly by
                  -- wildcards (#4404)
                  implicit_uses = hsValBindsImplicits binds'
            ; warnUnusedLocalBinds bound_names
                                          (real_uses `unionNameSet` implicit_uses)

            ; let
                -- The variables "used" in the val binds are:
                --   (1) the uses of the binds (allUses)
                --   (2) the FVs of the thing-inside
                all_uses = allUses dus `plusFV` result_fvs
                    -- Note [Unused binding hack]
                    -- ~~~~~~~~~~~~~~~~~~~~~~~~~~
                    -- Note that *in contrast* to the above reporting of
                    -- unused bindings, (1) above uses duUses to return *all*
                    -- the uses, even if the binding is unused.  Otherwise consider:
                    --      x = 3
                    --      y = let p = x in 'x'    -- NB: p not used
                    -- If we don't "see" the dependency of 'y' on 'x', we may put the
                    -- bindings in the wrong order, and the type checker will complain
                    -- that x isn't in scope
                    --
                    -- But note that this means we won't report 'x' as unused,
                    -- whereas we would if we had { x = 3; p = x; y = 'x' }

            ; return (result, all_uses) }}
                    -- The bound names are pruned out of all_uses
                    -- by the bindLocalNamesFV call above

    rnLocalValBindsAndThen bs _ = pprPanic "rnLocalValBindsAndThen" (ppr bs)

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

    -- renaming a single bind

    rnBindLHS :: NameMaker
              -> SDoc
              -> HsBind RdrName
              -- returns the renamed left-hand side,
              -- and the FreeVars *of the LHS*
              -- (i.e., any free variables of the pattern)
              -> RnM (HsBindLR Name RdrName)

    rnBindLHS name_maker _ bind@(PatBind { pat_lhs = pat })
      = do
          -- we don't actually use the FV processing of rnPatsAndThen here
          (pat',pat'_fvs) <- rnBindPat name_maker pat
          return (bind { pat_lhs = pat', bind_fvs = pat'_fvs })
                    -- We temporarily store the pat's FVs in bind_fvs;
                    -- gets updated to the FVs of the whole bind
                    -- when doing the RHS below

    rnBindLHS name_maker _ bind@(FunBind { fun_id = rdr_name })
      = do { name <- applyNameMaker name_maker rdr_name
           ; return (bind { fun_id   = name
                          , bind_fvs = placeHolderNamesTc }) }

    rnBindLHS name_maker _ (PatSynBind psb@PSB{ psb_id = rdrname })
      | isTopRecNameMaker name_maker
      = do { addLocM checkConName rdrname
           ; name <- lookupLocatedTopBndrRn rdrname   -- Should be in scope already
           ; return (PatSynBind psb{ psb_id = name }) }

      | otherwise  -- Pattern synonym, not at top level
      = do { addErr localPatternSynonymErr  -- Complain, but make up a fake
                                            -- name so that we can carry on
           ; name <- applyNameMaker name_maker rdrname
           ; return (PatSynBind psb{ psb_id = name }) }
      where
        localPatternSynonymErr :: SDoc
        localPatternSynonymErr
          = hang (text "Illegal pattern synonym declaration for" <+> quotes (ppr rdrname))
               2 (text "Pattern synonym declarations are only valid at top level")

    rnBindLHS _ _ b = pprPanic "rnBindHS" (ppr b)

    rnLBind :: (Name -> [Name])             -- Signature tyvar function
            -> LHsBindLR Name RdrName
            -> RnM (LHsBind Name, [Name], Uses)
    rnLBind sig_fn (L loc bind)
      = setSrcSpan loc $
        do { (bind', bndrs, dus) <- rnBind sig_fn bind
           ; return (L loc bind', bndrs, dus) }

    -- assumes the left-hands-side vars are in scope
    rnBind :: (Name -> [Name])              -- Signature tyvar function
           -> HsBindLR Name RdrName
           -> RnM (HsBind Name, [Name], Uses)
    rnBind _ bind@(PatBind { pat_lhs = pat
                           , pat_rhs = grhss
                                       -- pat fvs were stored in bind_fvs
                                       -- after processing the LHS
                           , bind_fvs = pat_fvs })
      = do  { mod <- getModule
            ; (grhss', rhs_fvs) <- rnGRHSs PatBindRhs rnLExpr grhss

                    -- No scoped type variables for pattern bindings
            ; let all_fvs = pat_fvs `plusFV` rhs_fvs
                  fvs'    = filterNameSet (nameIsLocalOrFrom mod) all_fvs
                    -- Keep locally-defined Names
                    -- As well as dependency analysis, we need these for the
                    -- MonoLocalBinds test in TcBinds.decideGeneralisationPlan
                  bndrs = collectPatBinders pat
                  bind' = bind { pat_rhs  = grhss',
                                 pat_rhs_ty = placeHolderType, bind_fvs = fvs' }
                  is_wild_pat = case pat of
                                  L _ (WildPat {})                 -> True
                                  L _ (BangPat (L _ (WildPat {}))) -> True -- #9127
                                  _                                -> False

            -- Warn if the pattern binds no variables, except for the
            -- entirely-explicit idiom    _ = rhs
            -- which (a) is not that different from  _v = rhs
            --       (b) is sometimes used to give a type sig for,
            --           or an occurrence of, a variable on the RHS
            ; whenWOptM Opt_WarnUnusedPatternBinds $
              when (null bndrs && not is_wild_pat) $
              addWarn (Reason Opt_WarnUnusedPatternBinds) $ unusedPatBindWarn bind'

            ; fvs' `seq` -- See Note [Free-variable space leak]
              return (bind', bndrs, all_fvs) }

    rnBind sig_fn bind@(FunBind { fun_id = name
                                , fun_matches = matches })
           -- invariant: no free vars here when it's a FunBind
      = do  { let plain_name = unLoc name

            ; (matches', rhs_fvs) <- bindSigTyVarsFV (sig_fn plain_name) $
                                    -- bindSigTyVars tests for LangExt.ScopedTyVars
                                     rnMatchGroup (mkPrefixFunRhs name)
                                                  rnLExpr matches
            ; let is_infix = isInfixFunBind bind
            ; when is_infix $ checkPrecMatch plain_name matches'

            ; mod <- getModule
            ; let fvs' = filterNameSet (nameIsLocalOrFrom mod) rhs_fvs
                    -- Keep locally-defined Names
                    -- As well as dependency analysis, we need these for the
                    -- MonoLocalBinds test in TcBinds.decideGeneralisationPlan

            ; fvs' `seq` -- See Note [Free-variable space leak]
              return (bind { fun_matches = matches'
                           , bind_fvs   = fvs' },
                      [plain_name], rhs_fvs)
          }

    rnBind sig_fn (PatSynBind bind)
      = do  { (bind', name, fvs) <- rnPatSynBind sig_fn bind
            ; return (PatSynBind bind', name, fvs) }

    rnBind _ b = pprPanic "rnBind" (ppr b)

    {-
    Note [Free-variable space leak]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We have
        fvs' = trim fvs
    and we seq fvs' before turning it as part of a record.

    The reason is that trim is sometimes something like
        \xs -> intersectNameSet (mkNameSet bound_names) xs
    and we don't want to retain the list bound_names. This showed up in
    trac ticket #1136.
    -}

    {- *********************************************************************
    *                                                                      *
              Dependency analysis and other support functions
    *                                                                      *
    ********************************************************************* -}

    depAnalBinds :: Bag (LHsBind Name, [Name], Uses)
                 -> ([(RecFlag, LHsBinds Name)], DefUses)
    -- Dependency analysis; this is important so that
    -- unused-binding reporting is accurate
    depAnalBinds binds_w_dus
      = (map get_binds sccs, map get_du sccs)
      where
        sccs = depAnal (\(_, defs, _) -> defs)
                       (\(_, _, uses) -> nonDetEltsUniqSet uses)
                       -- It's OK to use nonDetEltsUniqSet here as explained in
                       -- Note [depAnal determinism] in NameEnv.
                       (bagToList binds_w_dus)

        get_binds (AcyclicSCC (bind, _, _)) = (NonRecursive, unitBag bind)
        get_binds (CyclicSCC  binds_w_dus)  = (Recursive, listToBag [b | (b,_,_) <- binds_w_dus])

        get_du (AcyclicSCC (_, bndrs, uses)) = (Just (mkNameSet bndrs), uses)
        get_du (CyclicSCC  binds_w_dus)      = (Just defs, uses)
            where
              defs = mkNameSet [b | (_,bs,_) <- binds_w_dus, b <- bs]
              uses = unionNameSets [u | (_,_,u) <- binds_w_dus]

    ---------------------
    -- Bind the top-level forall'd type variables in the sigs.
    -- E.g  f :: a -> a
    --      f = rhs
    --      The 'a' scopes over the rhs
    --
    -- NB: there'll usually be just one (for a function binding)
    --     but if there are many, one may shadow the rest; too bad!
    --      e.g  x :: [a] -> [a]
    --           y :: [(a,a)] -> a
    --           (x,y) = e
    --      In e, 'a' will be in scope, and it'll be the one from 'y'!

    mkSigTvFn :: [LSig Name] -> (Name -> [Name])
    -- Return a lookup function that maps an Id Name to the names
    -- of the type variables that should scope over its body.
    mkSigTvFn sigs = \n -> lookupNameEnv env n `orElse` []
      where
        env = mkHsSigEnv get_scoped_tvs sigs

        get_scoped_tvs :: LSig Name -> Maybe ([Located Name], [Name])
        -- Returns (binders, scoped tvs for those binders)
        get_scoped_tvs (L _ (ClassOpSig _ names sig_ty))
          = Just (names, hsScopedTvs sig_ty)
        get_scoped_tvs (L _ (TypeSig names sig_ty))
          = Just (names, hsWcScopedTvs sig_ty)
        get_scoped_tvs (L _ (PatSynSig names sig_ty))
          = Just (names, hsScopedTvs sig_ty)
        get_scoped_tvs _ = Nothing

    -- Process the fixity declarations, making a FastString -> (Located Fixity) map
    -- (We keep the location around for reporting duplicate fixity declarations.)
    --
    -- Checks for duplicates, but not that only locally defined things are fixed.
    -- Note: for local fixity declarations, duplicates would also be checked in
    --       check_sigs below.  But we also use this function at the top level.

    makeMiniFixityEnv :: [LFixitySig RdrName] -> RnM MiniFixityEnv

    makeMiniFixityEnv decls = foldlM add_one_sig emptyFsEnv decls
     where
       add_one_sig env (L loc (FixitySig names fixity)) =
         foldlM add_one env [ (loc,name_loc,name,fixity)
                            | L name_loc name <- names ]

       add_one env (loc, name_loc, name,fixity) = do
         { -- this fixity decl is a duplicate iff
           -- the ReaderName's OccName's FastString is already in the env
           -- (we only need to check the local fix_env because
           --  definitions of non-local will be caught elsewhere)
           let { fs = occNameFS (rdrNameOcc name)
               ; fix_item = L loc fixity };

           case lookupFsEnv env fs of
             Nothing -> return $ extendFsEnv env fs fix_item
             Just (L loc' _) -> do
               { setSrcSpan loc $
                 addErrAt name_loc (dupFixityDecl loc' name)
               ; return env}
         }

    dupFixityDecl :: SrcSpan -> RdrName -> SDoc
    dupFixityDecl loc rdr_name
      = vcat [text "Multiple fixity declarations for" <+> quotes (ppr rdr_name),
              text "also at " <+> ppr loc]

    {- *********************************************************************
    *                                                                      *
                    Pattern synonym bindings
    *                                                                      *
    ********************************************************************* -}

    rnPatSynBind :: (Name -> [Name])                -- Signature tyvar function
                 -> PatSynBind Name RdrName
                 -> RnM (PatSynBind Name Name, [Name], Uses)
    rnPatSynBind sig_fn bind@(PSB { psb_id = L l name
                                  , psb_args = details
                                  , psb_def = pat
                                  , psb_dir = dir })
           -- invariant: no free vars here when it's a FunBind
      = do  { pattern_synonym_ok <- xoptM LangExt.PatternSynonyms
            ; unless pattern_synonym_ok (addErr patternSynonymErr)
            ; let sig_tvs = sig_fn name

            ; ((pat', details'), fvs1) <- bindSigTyVarsFV sig_tvs $
                                          rnPat PatSyn pat $ \pat' ->
             -- We check the 'RdrName's instead of the 'Name's
             -- so that the binding locations are reported
             -- from the left-hand side
                case details of
                   PrefixPatSyn vars ->
                       do { checkDupRdrNames vars
                          ; names <- mapM lookupVar vars
                          ; return ( (pat', PrefixPatSyn names)
                                   , mkFVs (map unLoc names)) }
                   InfixPatSyn var1 var2 ->
                       do { checkDupRdrNames [var1, var2]
                          ; name1 <- lookupVar var1
                          ; name2 <- lookupVar var2
                          -- ; checkPrecMatch -- TODO
                          ; return ( (pat', InfixPatSyn name1 name2)
                                   , mkFVs (map unLoc [name1, name2])) }
                   RecordPatSyn vars ->
                       do { checkDupRdrNames (map recordPatSynSelectorId vars)
                          ; let rnRecordPatSynField
                                  (RecordPatSynField { recordPatSynSelectorId = visible
                                                     , recordPatSynPatVar = hidden })
                                  = do { visible' <- lookupLocatedTopBndrRn visible
                                       ; hidden'  <- lookupVar hidden
                                       ; return $ RecordPatSynField { recordPatSynSelectorId = visible'
                                                                    , recordPatSynPatVar = hidden' } }
                          ; names <- mapM rnRecordPatSynField  vars
                          ; return ( (pat', RecordPatSyn names)
                                   , mkFVs (map (unLoc . recordPatSynPatVar) names)) }

            ; (dir', fvs2) <- case dir of
                Unidirectional -> return (Unidirectional, emptyFVs)
                ImplicitBidirectional -> return (ImplicitBidirectional, emptyFVs)
                ExplicitBidirectional mg ->
                    do { (mg', fvs) <- bindSigTyVarsFV sig_tvs $
                                       rnMatchGroup (mkPrefixFunRhs (L l name))
                                                    rnLExpr mg
                       ; return (ExplicitBidirectional mg', fvs) }

            ; mod <- getModule
            ; let fvs = fvs1 `plusFV` fvs2
                  fvs' = filterNameSet (nameIsLocalOrFrom mod) fvs
                    -- Keep locally-defined Names
                    -- As well as dependency analysis, we need these for the
                    -- MonoLocalBinds test in TcBinds.decideGeneralisationPlan

                  bind' = bind{ psb_args = details'
                              , psb_def = pat'
                              , psb_dir = dir'
                              , psb_fvs = fvs' }
                  selector_names = case details' of
                                     RecordPatSyn names ->
                                      map (unLoc . recordPatSynSelectorId) names
                                     _ -> []

            ; fvs' `seq` -- See Note [Free-variable space leak]
              return (bind', name : selector_names , fvs1)
              -- Why fvs1?  See Note [Pattern synonym builders don't yield dependencies]
          }
      where
        lookupVar = wrapLocM lookupOccRn

        patternSynonymErr :: SDoc
        patternSynonymErr
          = hang (text "Illegal pattern synonym declaration")
               2 (text "Use -XPatternSynonyms to enable this extension")

    {-
    Note [Pattern synonym builders don't yield dependencies]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    When renaming a pattern synonym that has an explicit builder,
    references in the builder definition should not be used when
    calculating dependencies. For example, consider the following pattern
    synonym definition:

    pattern P x <- C1 x where
      P x = f (C1 x)

    f (P x) = C2 x

    In this case, 'P' needs to be typechecked in two passes:

    1. Typecheck the pattern definition of 'P', which fully determines the
       type of 'P'. This step doesn't require knowing anything about 'f',
       since the builder definition is not looked at.

    2. Typecheck the builder definition, which needs the typechecked
       definition of 'f' to be in scope; done by calls oo tcPatSynBuilderBind
       in TcBinds.tcValBinds.

    This behaviour is implemented in 'tcValBinds', but it crucially
    depends on 'P' not being put in a recursive group with 'f' (which
    would make it look like a recursive pattern synonym a la 'pattern P =
    P' which is unsound and rejected).

    So:
     * We do not include builder fvs in the Uses returned by rnPatSynBind
       (which is then used for dependency analysis)
     * But we /do/ include them in the psb_fvs for the PatSynBind
     * In rnValBinds we record these builder uses, to avoid bogus
       unused-variable warnings (Trac #12548)
    -}

    {- *********************************************************************
    *                                                                      *
                    Class/instance method bindings
    *                                                                      *
    ********************************************************************* -}

    {- @rnMethodBinds@ is used for the method bindings of a class and an instance
    declaration.   Like @rnBinds@ but without dependency analysis.

    NOTA BENE: we record each {\em binder} of a method-bind group as a free variable.
    That's crucial when dealing with an instance decl:
    \begin{verbatim}
            instance Foo (T a) where
               op x = ...
    \end{verbatim}
    This might be the {\em sole} occurrence of @op@ for an imported class @Foo@,
    and unless @op@ occurs we won't treat the type signature of @op@ in the class
    decl for @Foo@ as a source of instance-decl gates.  But we should!  Indeed,
    in many ways the @op@ in an instance decl is just like an occurrence, not
    a binder.
    -}

    rnMethodBinds :: Bool                   -- True <=> is a class declaration
                  -> Name                   -- Class name
                  -> [Name]                 -- Type variables from the class/instance header
                  -> LHsBinds RdrName       -- Binds
                  -> [LSig RdrName]         -- and signatures/pragmas
                  -> RnM (LHsBinds Name, [LSig Name], FreeVars)
    -- Used for
    --   * the default method bindings in a class decl
    --   * the method bindings in an instance decl
    rnMethodBinds is_cls_decl cls ktv_names binds sigs
      = do { checkDupRdrNames (collectMethodBinders binds)
                 -- Check that the same method is not given twice in the
                 -- same instance decl      instance C T where
                 --                       f x = ...
                 --                       g y = ...
                 --                       f x = ...
                 -- We must use checkDupRdrNames because the Name of the
                 -- method is the Name of the class selector, whose SrcSpan
                 -- points to the class declaration; and we use rnMethodBinds
                 -- for instance decls too

           -- Rename the bindings LHSs
           ; binds' <- foldrBagM (rnMethodBindLHS is_cls_decl cls) emptyBag binds

           -- Rename the pragmas and signatures
           -- Annoyingly the type variables /are/ in scope for signatures, but
           -- /are not/ in scope in the SPECIALISE instance pramas; e.g.
           --    instance Eq a => Eq (T a) where
           --       (==) :: a -> a -> a
           --       {-# SPECIALISE instance Eq a => Eq (T [a]) #-}
           ; let (spec_inst_prags, other_sigs) = partition isSpecInstLSig sigs
                 bound_nms = mkNameSet (collectHsBindsBinders binds')
                 sig_ctxt | is_cls_decl = ClsDeclCtxt cls
                          | otherwise   = InstDeclCtxt bound_nms
           ; (spec_inst_prags', sip_fvs) <- renameSigs sig_ctxt spec_inst_prags
           ; (other_sigs',      sig_fvs) <- extendTyVarEnvFVRn ktv_names $
                                            renameSigs sig_ctxt other_sigs

           -- Rename the bindings RHSs.  Again there's an issue about whether the
           -- type variables from the class/instance head are in scope.
           -- Answer no in Haskell 2010, but yes if you have -XScopedTypeVariables
           ; scoped_tvs  <- xoptM LangExt.ScopedTypeVariables
           ; (binds'', bind_fvs) <- maybe_extend_tyvar_env scoped_tvs $
                  do { binds_w_dus <- mapBagM (rnLBind (mkSigTvFn other_sigs')) binds'
                     ; let bind_fvs = foldrBag (\(_,_,fv1) fv2 -> fv1 `plusFV` fv2)
                                               emptyFVs binds_w_dus
                     ; return (mapBag fstOf3 binds_w_dus, bind_fvs) }

           ; return ( binds'', spec_inst_prags' ++ other_sigs'
                    , sig_fvs `plusFV` sip_fvs `plusFV` bind_fvs) }
      where
        -- For the method bindings in class and instance decls, we extend
        -- the type variable environment iff -XScopedTypeVariables
        maybe_extend_tyvar_env scoped_tvs thing_inside
           | scoped_tvs = extendTyVarEnvFVRn ktv_names thing_inside
           | otherwise  = thing_inside

    rnMethodBindLHS :: Bool -> Name
                    -> LHsBindLR RdrName RdrName
                    -> LHsBindsLR Name RdrName
                    -> RnM (LHsBindsLR Name RdrName)
    rnMethodBindLHS _ cls (L loc bind@(FunBind { fun_id = name })) rest
      = setSrcSpan loc $ do
        do { sel_name <- wrapLocM (lookupInstDeclBndr cls (text "method")) name
                         -- We use the selector name as the binder
           ; let bind' = bind { fun_id = sel_name
                              , bind_fvs = placeHolderNamesTc }

           ; return (L loc bind' `consBag` rest ) }

    -- Report error for all other forms of bindings
    -- This is why we use a fold rather than map
    rnMethodBindLHS is_cls_decl _ (L loc bind) rest
      = do { addErrAt loc $
             vcat [ what <+> text "not allowed in" <+> decl_sort
                  , nest 2 (ppr bind) ]
           ; return rest }
      where
        decl_sort | is_cls_decl = text "class declaration:"
                  | otherwise   = text "instance declaration:"
        what = case bind of
                  PatBind {}    -> text "Pattern bindings (except simple variables)"
                  PatSynBind {} -> text "Pattern synonyms"
                                   -- Associated pattern synonyms are not implemented yet
                  _ -> pprPanic "rnMethodBind" (ppr bind)

    {-
    ************************************************************************
    *                                                                      *
    \subsubsection[dep-Sigs]{Signatures (and user-pragmas for values)}
    *                                                                      *
    ************************************************************************

    @renameSigs@ checks for:
    \begin{enumerate}
    \item more than one sig for one thing;
    \item signatures given for things not bound here;
    \end{enumerate}

    At the moment we don't gather free-var info from the types in
    signatures.  We'd only need this if we wanted to report unused tyvars.
    -}

    renameSigs :: HsSigCtxt
               -> [LSig RdrName]
               -> RnM ([LSig Name], FreeVars)
    -- Renames the signatures and performs error checks
    renameSigs ctxt sigs
      = do  { mapM_ dupSigDeclErr (findDupSigs sigs)

            ; checkDupMinimalSigs sigs

            ; (sigs', sig_fvs) <- mapFvRn (wrapLocFstM (renameSig ctxt)) sigs

            ; let (good_sigs, bad_sigs) = partition (okHsSig ctxt) sigs'
            ; mapM_ misplacedSigErr bad_sigs                 -- Misplaced

            ; return (good_sigs, sig_fvs) }

    ----------------------
    -- We use lookupSigOccRn in the signatures, which is a little bit unsatisfactory
    -- because this won't work for:
    --      instance Foo T where
    --        {-# INLINE op #-}
    --        Baz.op = ...
    -- We'll just rename the INLINE prag to refer to whatever other 'op'
    -- is in scope.  (I'm assuming that Baz.op isn't in scope unqualified.)
    -- Doesn't seem worth much trouble to sort this.

    renameSig :: HsSigCtxt -> Sig RdrName -> RnM (Sig Name, FreeVars)
    -- FixitySig is renamed elsewhere.
    renameSig _ (IdSig x)
      = return (IdSig x, emptyFVs)    -- Actually this never occurs

    renameSig ctxt sig@(TypeSig vs ty)
      = do  { new_vs <- mapM (lookupSigOccRn ctxt sig) vs
            ; let doc = TypeSigCtx (ppr_sig_bndrs vs)
            ; (new_ty, fvs) <- rnHsSigWcType doc ty
            ; return (TypeSig new_vs new_ty, fvs) }

    renameSig ctxt sig@(ClassOpSig is_deflt vs ty)
      = do  { defaultSigs_on <- xoptM LangExt.DefaultSignatures
            ; when (is_deflt && not defaultSigs_on) $
              addErr (defaultSigErr sig)
            ; new_v <- mapM (lookupSigOccRn ctxt sig) vs
            ; (new_ty, fvs) <- rnHsSigType ty_ctxt ty
            ; return (ClassOpSig is_deflt new_v new_ty, fvs) }
      where
        (v1:_) = vs
        ty_ctxt = GenericCtx (text "a class method signature for"
                              <+> quotes (ppr v1))

    renameSig _ (SpecInstSig src ty)
      = do  { (new_ty, fvs) <- rnHsSigType SpecInstSigCtx ty
            ; return (SpecInstSig src new_ty,fvs) }

    -- {-# SPECIALISE #-} pragmas can refer to imported Ids
    -- so, in the top-level case (when mb_names is Nothing)
    -- we use lookupOccRn.  If there's both an imported and a local 'f'
    -- then the SPECIALISE pragma is ambiguous, unlike all other signatures
    renameSig ctxt sig@(SpecSig v tys inl)
      = do  { new_v <- case ctxt of
                         TopSigCtxt {} -> lookupLocatedOccRn v
                         _             -> lookupSigOccRn ctxt sig v
            ; (new_ty, fvs) <- foldM do_one ([],emptyFVs) tys
            ; return (SpecSig new_v new_ty inl, fvs) }
      where
        ty_ctxt = GenericCtx (text "a SPECIALISE signature for"
                              <+> quotes (ppr v))
        do_one (tys,fvs) ty
          = do { (new_ty, fvs_ty) <- rnHsSigType ty_ctxt ty
               ; return ( new_ty:tys, fvs_ty `plusFV` fvs) }

    renameSig ctxt sig@(InlineSig v s)
      = do  { new_v <- lookupSigOccRn ctxt sig v
            ; return (InlineSig new_v s, emptyFVs) }

    renameSig ctxt sig@(FixSig (FixitySig vs f))
      = do  { new_vs <- mapM (lookupSigOccRn ctxt sig) vs
            ; return (FixSig (FixitySig new_vs f), emptyFVs) }

    renameSig ctxt sig@(MinimalSig s (L l bf))
      = do new_bf <- traverse (lookupSigOccRn ctxt sig) bf
           return (MinimalSig s (L l new_bf), emptyFVs)

    renameSig ctxt sig@(PatSynSig vs ty)
      = do  { new_vs <- mapM (lookupSigOccRn ctxt sig) vs
            ; (ty', fvs) <- rnHsSigType ty_ctxt ty
            ; return (PatSynSig new_vs ty', fvs) }
      where
        ty_ctxt = GenericCtx (text "a pattern synonym signature for"
                              <+> ppr_sig_bndrs vs)

    renameSig ctxt sig@(SCCFunSig st v s)
      = do  { new_v <- lookupSigOccRn ctxt sig v
            ; return (SCCFunSig st new_v s, emptyFVs) }

    -- COMPLETE Sigs can refer to imported IDs which is why we use
    -- lookupLocatedOccRn rather than lookupSigOccRn
    renameSig _ctxt sig@(CompleteMatchSig s (L l bf) mty)
      = do new_bf <- traverse lookupLocatedOccRn bf
           new_mty  <- traverse lookupLocatedOccRn mty

           this_mod <- fmap tcg_mod getGblEnv
           unless (any (nameIsLocalOrFrom this_mod . unLoc) new_bf) $ do
             -- Why 'any'? See Note [Orphan COMPLETE pragmas]
             addErrCtxt (text "In" <+> ppr sig) $ failWithTc orphanError

           return (CompleteMatchSig s (L l new_bf) new_mty, emptyFVs)
      where
        orphanError :: SDoc
        orphanError =
          text "Orphan COMPLETE pragmas not supported" $$
          text "A COMPLETE pragma must mention at least one data constructor" $$
          text "or pattern synonym defined in the same module."

    {-
    Note [Orphan COMPLETE pragmas]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We define a COMPLETE pragma to be a non-orphan if it includes at least
    one conlike defined in the current module. Why is this sufficient?
    Well if you have a pattern match

      case expr of
        P1 -> ...
        P2 -> ...
        P3 -> ...

    any COMPLETE pragma which mentions a conlike other than P1, P2 or P3
    will not be of any use in verifying that the pattern match is
    exhaustive. So as we have certainly read the interface files that
    define P1, P2 and P3, we will have loaded all non-orphan COMPLETE
    pragmas that could be relevant to this pattern match.

    For now we simply disallow orphan COMPLETE pragmas, as the added
    complexity of supporting them properly doesn't seem worthwhile.
    -}

    ppr_sig_bndrs :: [Located RdrName] -> SDoc
    ppr_sig_bndrs bs = quotes (pprWithCommas ppr bs)

    okHsSig :: HsSigCtxt -> LSig a -> Bool
    okHsSig ctxt (L _ sig)
      = case (sig, ctxt) of
         (ClassOpSig {}, ClsDeclCtxt {})  -> True
         (ClassOpSig {}, InstDeclCtxt {}) -> True
         (ClassOpSig {}, _)               -> False

         (TypeSig {}, ClsDeclCtxt {})  -> False
         (TypeSig {}, InstDeclCtxt {}) -> False
         (TypeSig {}, _)               -> True

         (PatSynSig {}, TopSigCtxt{}) -> True
         (PatSynSig {}, _)            -> False

         (FixSig {}, InstDeclCtxt {}) -> False
         (FixSig {}, _)               -> True

         (IdSig {}, TopSigCtxt {})   -> True
         (IdSig {}, InstDeclCtxt {}) -> True
         (IdSig {}, _)               -> False

         (InlineSig {}, HsBootCtxt {}) -> False
         (InlineSig {}, _)             -> True

         (SpecSig {}, TopSigCtxt {})    -> True
         (SpecSig {}, LocalBindCtxt {}) -> True
         (SpecSig {}, InstDeclCtxt {})  -> True
         (SpecSig {}, _)                -> False

         (SpecInstSig {}, InstDeclCtxt {}) -> True
         (SpecInstSig {}, _)               -> False

         (MinimalSig {}, ClsDeclCtxt {}) -> True
         (MinimalSig {}, _)              -> False

         (SCCFunSig {}, HsBootCtxt {}) -> False
         (SCCFunSig {}, _)             -> True

         (CompleteMatchSig {}, TopSigCtxt {} ) -> True
         (CompleteMatchSig {}, _)              -> False

    -------------------
    findDupSigs :: [LSig RdrName] -> [[(Located RdrName, Sig RdrName)]]
    -- Check for duplicates on RdrName version,
    -- because renamed version has unboundName for
    -- not-in-scope binders, which gives bogus dup-sig errors
    -- NB: in a class decl, a 'generic' sig is not considered
    --     equal to an ordinary sig, so we allow, say
    --           class C a where
    --             op :: a -> a
    --             default op :: Eq a => a -> a
    findDupSigs sigs
      = findDupsEq matching_sig (concatMap (expand_sig . unLoc) sigs)
      where
        expand_sig sig@(FixSig (FixitySig ns _)) = zip ns (repeat sig)
        expand_sig sig@(InlineSig n _)           = [(n,sig)]
        expand_sig sig@(TypeSig ns _)            = [(n,sig) | n <- ns]
        expand_sig sig@(ClassOpSig _ ns _)       = [(n,sig) | n <- ns]
        expand_sig sig@(PatSynSig ns  _ )        = [(n,sig) | n <- ns]
        expand_sig sig@(SCCFunSig _ n _)         = [(n,sig)]
        expand_sig _ = []

        matching_sig (L _ n1,sig1) (L _ n2,sig2)       = n1 == n2 && mtch sig1 sig2
        mtch (FixSig {})           (FixSig {})         = True
        mtch (InlineSig {})        (InlineSig {})      = True
        mtch (TypeSig {})          (TypeSig {})        = True
        mtch (ClassOpSig d1 _ _)   (ClassOpSig d2 _ _) = d1 == d2
        mtch (PatSynSig _ _)       (PatSynSig _ _)     = True
        mtch (SCCFunSig{})         (SCCFunSig{})       = True
        mtch _ _ = False

    -- Warn about multiple MINIMAL signatures
    checkDupMinimalSigs :: [LSig RdrName] -> RnM ()
    checkDupMinimalSigs sigs
      = case filter isMinimalLSig sigs of
          minSigs@(_:_:_) -> dupMinimalSigErr minSigs
          _ -> return ()

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Match}
    *                                                                      *
    ************************************************************************
    -}

    rnMatchGroup :: Outputable (body RdrName) => HsMatchContext Name
                 -> (Located (body RdrName) -> RnM (Located (body Name), FreeVars))
                 -> MatchGroup RdrName (Located (body RdrName))
                 -> RnM (MatchGroup Name (Located (body Name)), FreeVars)
    rnMatchGroup ctxt rnBody (MG { mg_alts = L _ ms, mg_origin = origin })
      = do { empty_case_ok <- xoptM LangExt.EmptyCase
           ; when (null ms && not empty_case_ok) (addErr (emptyCaseErr ctxt))
           ; (new_ms, ms_fvs) <- mapFvRn (rnMatch ctxt rnBody) ms
           ; return (mkMatchGroup origin new_ms, ms_fvs) }

    rnMatch :: Outputable (body RdrName) => HsMatchContext Name
            -> (Located (body RdrName) -> RnM (Located (body Name), FreeVars))
            -> LMatch RdrName (Located (body RdrName))
            -> RnM (LMatch Name (Located (body Name)), FreeVars)
    rnMatch ctxt rnBody = wrapLocFstM (rnMatch' ctxt rnBody)

    rnMatch' :: Outputable (body RdrName) => HsMatchContext Name
             -> (Located (body RdrName) -> RnM (Located (body Name), FreeVars))
             -> Match RdrName (Located (body RdrName))
             -> RnM (Match Name (Located (body Name)), FreeVars)
    rnMatch' ctxt rnBody match@(Match { m_ctxt = mf, m_pats = pats
                                      , m_type = maybe_rhs_sig, m_grhss = grhss })
      = do  {       -- Result type signatures are no longer supported
              case maybe_rhs_sig of
                    Nothing -> return ()
                    Just (L loc ty) -> addErrAt loc (resSigErr match ty)

            ; let fixity = if isInfixMatch match then Infix else Prefix
                   -- Now the main event
                   -- Note that there are no local fixity decls for matches
            ; rnPats ctxt pats      $ \ pats' -> do
            { (grhss', grhss_fvs) <- rnGRHSs ctxt rnBody grhss
            ; let mf' = case (ctxt,mf) of
                          (FunRhs (L _ funid) _ _,FunRhs (L lf _) _ strict)
                                                -> FunRhs (L lf funid) fixity strict
                          _                     -> ctxt
            ; return (Match { m_ctxt = mf', m_pats = pats'
                            , m_type = Nothing, m_grhss = grhss'}, grhss_fvs ) }}

    emptyCaseErr :: HsMatchContext Name -> SDoc
    emptyCaseErr ctxt = hang (text "Empty list of alternatives in" <+> pp_ctxt)
                           2 (text "Use EmptyCase to allow this")
      where
        pp_ctxt = case ctxt of
                    CaseAlt    -> text "case expression"
                    LambdaExpr -> text "\\case expression"
                    _ -> text "(unexpected)" <+> pprMatchContextNoun ctxt

    resSigErr :: Outputable body
              => Match RdrName body -> HsType RdrName -> SDoc
    resSigErr match ty
       = vcat [ text "Illegal result type signature" <+> quotes (ppr ty)
              , nest 2 $ ptext (sLit
                     "Result signatures are no longer supported in pattern matches")
              , pprMatchInCtxt match ]

    {-
    ************************************************************************
    *                                                                      *
    \subsubsection{Guarded right-hand sides (GRHSs)}
    *                                                                      *
    ************************************************************************
    -}

    rnGRHSs :: HsMatchContext Name
            -> (Located (body RdrName) -> RnM (Located (body Name), FreeVars))
            -> GRHSs RdrName (Located (body RdrName))
            -> RnM (GRHSs Name (Located (body Name)), FreeVars)
    rnGRHSs ctxt rnBody (GRHSs grhss (L l binds))
      = rnLocalBindsAndThen binds   $ \ binds' _ -> do
        (grhss', fvGRHSs) <- mapFvRn (rnGRHS ctxt rnBody) grhss
        return (GRHSs grhss' (L l binds'), fvGRHSs)

    rnGRHS :: HsMatchContext Name
           -> (Located (body RdrName) -> RnM (Located (body Name), FreeVars))
           -> LGRHS RdrName (Located (body RdrName))
           -> RnM (LGRHS Name (Located (body Name)), FreeVars)
    rnGRHS ctxt rnBody = wrapLocFstM (rnGRHS' ctxt rnBody)

    rnGRHS' :: HsMatchContext Name
            -> (Located (body RdrName) -> RnM (Located (body Name), FreeVars))
            -> GRHS RdrName (Located (body RdrName))
            -> RnM (GRHS Name (Located (body Name)), FreeVars)
    rnGRHS' ctxt rnBody (GRHS guards rhs)
      = do  { pattern_guards_allowed <- xoptM LangExt.PatternGuards
            ; ((guards', rhs'), fvs) <- rnStmts (PatGuard ctxt) rnLExpr guards $ \ _ ->
                                        rnBody rhs

            ; unless (pattern_guards_allowed || is_standard_guard guards')
                     (addWarn NoReason (nonStdGuardErr guards'))

            ; return (GRHS guards' rhs', fvs) }
      where
            -- Standard Haskell 1.4 guards are just a single boolean
            -- expression, rather than a list of qualifiers as in the
            -- Glasgow extension
        is_standard_guard []                       = True
        is_standard_guard [L _ (BodyStmt _ _ _ _)] = True
        is_standard_guard _                        = False

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Error messages}
    *                                                                      *
    ************************************************************************
    -}

    dupSigDeclErr :: [(Located RdrName, Sig RdrName)] -> RnM ()
    dupSigDeclErr pairs@((L loc name, sig) : _)
      = addErrAt loc $
        vcat [ text "Duplicate" <+> what_it_is
               <> text "s for" <+> quotes (ppr name)
             , text "at" <+> vcat (map ppr $ sort $ map (getLoc . fst) pairs) ]
      where
        what_it_is = hsSigDoc sig

    dupSigDeclErr [] = panic "dupSigDeclErr"

    misplacedSigErr :: LSig Name -> RnM ()
    misplacedSigErr (L loc sig)
      = addErrAt loc $
        sep [text "Misplaced" <+> hsSigDoc sig <> colon, ppr sig]

    defaultSigErr :: Sig RdrName -> SDoc
    defaultSigErr sig = vcat [ hang (text "Unexpected default signature:")
                                  2 (ppr sig)
                             , text "Use DefaultSignatures to enable default signatures" ]

    bindsInHsBootFile :: LHsBindsLR Name RdrName -> SDoc
    bindsInHsBootFile mbinds
      = hang (text "Bindings in hs-boot files are not allowed")
           2 (ppr mbinds)

    nonStdGuardErr :: Outputable body => [LStmtLR Name Name body] -> SDoc
    nonStdGuardErr guards
      = hang (text "accepting non-standard pattern guards (use PatternGuards to suppress this message)")
           4 (interpp'SP guards)

    unusedPatBindWarn :: HsBind Name -> SDoc
    unusedPatBindWarn bind
      = hang (text "This pattern-binding binds no variables:")
           2 (ppr bind)

    dupMinimalSigErr :: [LSig RdrName] -> RnM ()
    dupMinimalSigErr sigs@(L loc _ : _)
      = addErrAt loc $
        vcat [ text "Multiple minimal complete definitions"
             , text "at" <+> vcat (map ppr $ sort $ map getLoc sigs)
             , text "Combine alternative minimal complete definitions with `|'" ]
    dupMinimalSigErr [] = panic "dupMinimalSigErr"