Implement -Wforall-identifier (#20609)

In accordance with GHC Proposal #281 "Visible forall in types of terms":

	For three releases before this change takes place, include a new
	warning -Wforall-identifier in -Wdefault. This warning will be triggered
	at definition sites (but not use sites) of forall as an identifier.

Updates the haddock submodule.

29 files changed, 254 insertions, 16 deletions

diff --git a/compiler/GHC/Driver/Flags.hs b/compiler/GHC/Driver/Flags.hs
index d7f72fcf2e..192b983887 100644
--- a/compiler/GHC/Driver/Flags.hs
+++ b/compiler/GHC/Driver/Flags.hs
@@ -536,6 +536,7 @@ data WarningFlag =
    | Opt_WarnMissingKindSignatures        -- Since 9.2
    | Opt_WarnMissingExportedPatternSynonymSignatures -- since 9.2
    | Opt_WarnRedundantStrictnessFlags     -- Since 9.4
+   | Opt_WarnForallIdentifier             -- Since 9.4
    | Opt_WarnUnicodeBidirectionalFormatCharacters -- Since 9.0.2
    deriving (Eq, Ord, Show, Enum)
 
@@ -636,6 +637,7 @@ warnFlagNames wflag = case wflag of
   Opt_WarnOperatorWhitespace                      -> "operator-whitespace" :| []
   Opt_WarnImplicitLift                            -> "implicit-lift" :| []
   Opt_WarnMissingExportedPatternSynonymSignatures -> "missing-exported-pattern-synonym-signatures" :| []
+  Opt_WarnForallIdentifier                        -> "forall-identifier" :| []
   Opt_WarnUnicodeBidirectionalFormatCharacters    -> "unicode-bidirectional-format-characters" :| []
 
 -- -----------------------------------------------------------------------------
@@ -728,6 +730,7 @@ standardWarnings -- see Note [Documenting warning flags]
         Opt_WarnNonCanonicalMonadInstances,
         Opt_WarnNonCanonicalMonoidInstances,
         Opt_WarnOperatorWhitespaceExtConflict,
+        Opt_WarnForallIdentifier,
         Opt_WarnUnicodeBidirectionalFormatCharacters
       ]
 
diff --git a/compiler/GHC/Driver/Session.hs b/compiler/GHC/Driver/Session.hs
index b4983d9218..5f4479939a 100644
--- a/compiler/GHC/Driver/Session.hs
+++ b/compiler/GHC/Driver/Session.hs
@@ -3218,6 +3218,7 @@ wWarningFlagsDeps = mconcat [
   warnSpec    Opt_WarnOperatorWhitespace,
   warnSpec    Opt_WarnImplicitLift,
   warnSpec    Opt_WarnMissingExportedPatternSynonymSignatures,
+  warnSpec    Opt_WarnForallIdentifier,
   warnSpec    Opt_WarnUnicodeBidirectionalFormatCharacters
  ]
 
diff --git a/compiler/GHC/Rename/Bind.hs b/compiler/GHC/Rename/Bind.hs
index b85cee2a51..08e8672d00 100644
--- a/compiler/GHC/Rename/Bind.hs
+++ b/compiler/GHC/Rename/Bind.hs
@@ -43,7 +43,9 @@ import GHC.Rename.Names
 import GHC.Rename.Env
 import GHC.Rename.Fixity
 import GHC.Rename.Utils ( HsDocContext(..), mapFvRn
-                        , checkDupRdrNames, checkDupRdrNamesN, warnUnusedLocalBinds
+                        , checkDupRdrNames, checkDupRdrNamesN
+                        , warnUnusedLocalBinds
+                        , warnForallIdentifier
                         , checkUnusedRecordWildcard
                         , checkDupAndShadowedNames, bindLocalNamesFV
                         , addNoNestedForallsContextsErr, checkInferredVars )
@@ -981,6 +983,7 @@ renameSig ctxt sig@(ClassOpSig _ is_deflt vs ty)
   = do  { defaultSigs_on <- xoptM LangExt.DefaultSignatures
         ; when (is_deflt && not defaultSigs_on) $
           addErr (defaultSigErr sig)
+        ; mapM_ warnForallIdentifier vs
         ; new_v <- mapM (lookupSigOccRnN ctxt sig) vs
         ; (new_ty, fvs) <- rnHsSigType ty_ctxt TypeLevel ty
         ; return (ClassOpSig noAnn is_deflt new_v new_ty, fvs) }
diff --git a/compiler/GHC/Rename/HsType.hs b/compiler/GHC/Rename/HsType.hs
index 1340993084..6740e02430 100644
--- a/compiler/GHC/Rename/HsType.hs
+++ b/compiler/GHC/Rename/HsType.hs
@@ -51,7 +51,8 @@ import GHC.Rename.Env
 import GHC.Rename.Utils  ( HsDocContext(..), inHsDocContext, withHsDocContext
                          , mapFvRn, pprHsDocContext, bindLocalNamesFV
                          , typeAppErr, newLocalBndrRn, checkDupRdrNamesN
-                         , checkShadowedRdrNames )
+                         , checkShadowedRdrNames
+                         , warnForallIdentifier )
 import GHC.Rename.Fixity ( lookupFieldFixityRn, lookupFixityRn
                          , lookupTyFixityRn )
 import GHC.Rename.Unbound ( notInScopeErr, WhereLooking(WL_LocalOnly) )
@@ -1281,7 +1282,8 @@ rnConDeclFields ctxt fls fields
 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 fl_env)) names
+  = do { mapM_ (\(L _ (FieldOcc _ rdr_name)) -> warnForallIdentifier rdr_name) names
+       ; let new_names = map (fmap (lookupField fl_env)) names
        ; (new_ty, fvs) <- rnLHsTyKi env ty
        ; return (L l (ConDeclField noAnn new_names new_ty haddock_doc)
                 , fvs) }
diff --git a/compiler/GHC/Rename/Module.hs b/compiler/GHC/Rename/Module.hs
index 2febede5c5..d2f5463d58 100644
--- a/compiler/GHC/Rename/Module.hs
+++ b/compiler/GHC/Rename/Module.hs
@@ -31,6 +31,7 @@ import GHC.Rename.Env
 import GHC.Rename.Utils ( HsDocContext(..), mapFvRn, bindLocalNames
                         , checkDupRdrNamesN, bindLocalNamesFV
                         , checkShadowedRdrNames, warnUnusedTypePatterns
+                        , warnForallIdentifier
                         , newLocalBndrsRn
                         , withHsDocContext, noNestedForallsContextsErr
                         , addNoNestedForallsContextsErr, checkInferredVars )
@@ -351,6 +352,7 @@ rnDefaultDecl (DefaultDecl _ tys)
 rnHsForeignDecl :: ForeignDecl GhcPs -> RnM (ForeignDecl GhcRn, FreeVars)
 rnHsForeignDecl (ForeignImport { fd_name = name, fd_sig_ty = ty, fd_fi = spec })
   = do { topEnv :: HscEnv <- getTopEnv
+       ; warnForallIdentifier name
        ; name' <- lookupLocatedTopBndrRnN name
        ; (ty', fvs) <- rnHsSigType (ForeignDeclCtx name) TypeLevel ty
 
@@ -1220,6 +1222,7 @@ rnHsRuleDecl (HsRule { rd_name = rule_name
                      , rd_lhs  = lhs
                      , rd_rhs  = rhs })
   = do { let rdr_names_w_loc = map (get_var . unLoc) tmvs
+       ; mapM_ warnForallIdentifier rdr_names_w_loc
        ; checkDupRdrNamesN rdr_names_w_loc
        ; checkShadowedRdrNames rdr_names_w_loc
        ; names <- newLocalBndrsRn rdr_names_w_loc
@@ -2299,7 +2302,7 @@ rnConDecls = mapFvRn (wrapLocFstMA rnConDecl)
 rnConDecl :: ConDecl GhcPs -> RnM (ConDecl GhcRn, FreeVars)
 rnConDecl decl@(ConDeclH98 { con_name = name, con_ex_tvs = ex_tvs
                            , con_mb_cxt = mcxt, con_args = args
-                           , con_doc = mb_doc, con_forall = forall })
+                           , con_doc = mb_doc, con_forall = forall_ })
   = do  { _        <- addLocMA checkConName name
         ; new_name <- lookupLocatedTopConstructorRnN name
 
@@ -2326,7 +2329,7 @@ rnConDecl decl@(ConDeclH98 { con_name = name, con_ex_tvs = ex_tvs
                        , con_name = new_name, con_ex_tvs = new_ex_tvs
                        , con_mb_cxt = new_context, con_args = new_args
                        , con_doc = mb_doc
-                       , con_forall = forall }, -- Remove when #18311 is fixed
+                       , con_forall = forall_ }, -- Remove when #18311 is fixed
                   all_fvs) }}
 
 rnConDecl (ConDeclGADT { con_names   = names
diff --git a/compiler/GHC/Rename/Pat.hs b/compiler/GHC/Rename/Pat.hs
index a011b709cf..79eeaa3477 100644
--- a/compiler/GHC/Rename/Pat.hs
+++ b/compiler/GHC/Rename/Pat.hs
@@ -53,7 +53,7 @@ import GHC.Tc.Utils.Zonk   ( hsOverLitName )
 import GHC.Rename.Env
 import GHC.Rename.Fixity
 import GHC.Rename.Utils    ( HsDocContext(..), newLocalBndrRn, bindLocalNames
-                           , warnUnusedMatches, newLocalBndrRn
+                           , warnUnusedMatches, warnForallIdentifier
                            , checkUnusedRecordWildcard
                            , checkDupNames, checkDupAndShadowedNames
                            , wrapGenSpan, genHsApps, genLHsVar, genHsIntegralLit )
@@ -232,14 +232,16 @@ newPatLName name_maker rdr_name@(L loc _)
 newPatName :: NameMaker -> LocatedN RdrName -> CpsRn Name
 newPatName (LamMk report_unused) rdr_name
   = CpsRn (\ thing_inside ->
-        do { name <- newLocalBndrRn rdr_name
+        do { warnForallIdentifier rdr_name
+           ; name <- newLocalBndrRn rdr_name
            ; (res, fvs) <- bindLocalNames [name] (thing_inside name)
            ; when report_unused $ warnUnusedMatches [name] fvs
            ; return (res, name `delFV` fvs) })
 
 newPatName (LetMk is_top fix_env) rdr_name
   = CpsRn (\ thing_inside ->
-        do { name <- case is_top of
+        do { warnForallIdentifier rdr_name
+           ; name <- case is_top of
                        NotTopLevel -> newLocalBndrRn rdr_name
                        TopLevel    -> newTopSrcBinder rdr_name
            ; bindLocalNames [name] $
diff --git a/compiler/GHC/Rename/Utils.hs b/compiler/GHC/Rename/Utils.hs
index fc27dac004..4041b0b6c8 100644
--- a/compiler/GHC/Rename/Utils.hs
+++ b/compiler/GHC/Rename/Utils.hs
@@ -15,6 +15,7 @@ module GHC.Rename.Utils (
         addFvRn, mapFvRn, mapMaybeFvRn,
         warnUnusedMatches, warnUnusedTypePatterns,
         warnUnusedTopBinds, warnUnusedLocalBinds,
+        warnForallIdentifier,
         checkUnusedRecordWildcard,
         mkFieldEnv,
         unknownSubordinateErr, badQualBndrErr, typeAppErr,
@@ -426,6 +427,13 @@ check_unused flag bound_names used_names
   = whenWOptM flag (warnUnused flag (filterOut (`elemNameSet` used_names)
                                                bound_names))
 
+warnForallIdentifier :: LocatedN RdrName -> RnM ()
+warnForallIdentifier (L l rdr_name@(Unqual occ))
+  | isKw (fsLit "forall") || isKw (fsLit "∀")
+  = addDiagnosticAt (locA l) (TcRnForallIdentifier rdr_name)
+  where isKw = (occNameFS occ ==)
+warnForallIdentifier _ = return ()
+
 -------------------------
 --      Helpers
 warnUnusedGREs :: [GlobalRdrElt] -> RnM ()
diff --git a/compiler/GHC/Tc/Errors/Ppr.hs b/compiler/GHC/Tc/Errors/Ppr.hs
index e282d8fe8d..b5a0dbb284 100644
--- a/compiler/GHC/Tc/Errors/Ppr.hs
+++ b/compiler/GHC/Tc/Errors/Ppr.hs
@@ -517,6 +517,11 @@ instance Diagnostic TcRnMessage where
               = text " when Safe Haskell is enabled."
               | otherwise
               = dot
+    TcRnForallIdentifier rdr_name
+      -> mkSimpleDecorated $
+            fsep [ text "The use of" <+> quotes (ppr rdr_name)
+                                     <+> text "as an identifier",
+                   text "will become an error in a future GHC release." ]
 
   diagnosticReason = \case
     TcRnUnknownMessage m
@@ -733,6 +738,8 @@ instance Diagnostic TcRnMessage where
       -> ErrorWithoutFlag
     TcRnSpecialClassInst {}
       -> ErrorWithoutFlag
+    TcRnForallIdentifier {}
+      -> WarningWithFlag Opt_WarnForallIdentifier
 
   diagnosticHints = \case
     TcRnUnknownMessage m
@@ -943,6 +950,8 @@ instance Diagnostic TcRnMessage where
       -> noHints
     TcRnSpecialClassInst {}
       -> noHints
+    TcRnForallIdentifier {}
+      -> [SuggestRenameForall]
 
 deriveInstanceErrReasonHints :: Class
                              -> UsingGeneralizedNewtypeDeriving
diff --git a/compiler/GHC/Tc/Errors/Types.hs b/compiler/GHC/Tc/Errors/Types.hs
index 10cc3524df..98e2479e52 100644
--- a/compiler/GHC/Tc/Errors/Types.hs
+++ b/compiler/GHC/Tc/Errors/Types.hs
@@ -1412,6 +1412,17 @@ data TcRnMessage where
   -}
   TcRnArrowProcGADTPattern :: TcRnMessage
 
+  {- TcRnForallIdentifier is a warning (controlled with -Wforall-identifier) that occurs
+     when a definition uses 'forall' as an identifier.
+
+     Example:
+       forall x = ()
+       g forall = ()
+
+     Test cases: T20609 T20609a T20609b T20609c T20609d
+  -}
+  TcRnForallIdentifier :: RdrName -> TcRnMessage
+
 -- | Which parts of a record field are affected by a particular error or warning.
 data RecordFieldPart
   = RecordFieldConstructor !Name
diff --git a/compiler/GHC/Types/Hint.hs b/compiler/GHC/Types/Hint.hs
index d0980bce95..1f76f028d7 100644
--- a/compiler/GHC/Types/Hint.hs
+++ b/compiler/GHC/Types/Hint.hs
@@ -309,6 +309,11 @@ data GhcHint
     -}
   | SuggestFillInWildcardConstraint
 
+    {-| Suggests to use an identifier other than 'forall'
+        Triggered by: 'GHC.Tc.Errors.Types.TcRnForallIdentifier'
+    -}
+  | SuggestRenameForall
+
 -- | An 'InstantiationSuggestion' for a '.hsig' file. This is generated
 -- by GHC in case of a 'DriverUnexpectedSignature' and suggests a way
 -- to instantiate a particular signature, where the first argument is
diff --git a/compiler/GHC/Types/Hint/Ppr.hs b/compiler/GHC/Types/Hint/Ppr.hs
index 00ffb9173a..1419078df6 100644
--- a/compiler/GHC/Types/Hint/Ppr.hs
+++ b/compiler/GHC/Types/Hint/Ppr.hs
@@ -126,6 +126,11 @@ instance Outputable GhcHint where
       -> text "Use a standalone deriving declaration instead"
     SuggestFillInWildcardConstraint
       -> text "Fill in the wildcard constraint yourself"
+    SuggestRenameForall
+      -> vcat [ text "Consider using another name, such as"
+              , quotes (text "forAll") <> comma <+>
+                quotes (text "for_all") <> comma <+> text "or" <+>
+                quotes (text "forall_") <> dot ]
 
 perhapsAsPat :: SDoc
 perhapsAsPat = text "Perhaps you meant an as-pattern, which must not be surrounded by whitespace"
diff --git a/docs/users_guide/using-warnings.rst b/docs/users_guide/using-warnings.rst
index 8cdd5677fe..baabbbdc37 100644
--- a/docs/users_guide/using-warnings.rst
+++ b/docs/users_guide/using-warnings.rst
@@ -56,6 +56,7 @@ To reverse ``-Werror``, which makes all warnings into errors, use ``-Wwarn``.
         * :ghc-flag:`-Woperator-whitespace-ext-conflict`
         * :ghc-flag:`-Wambiguous-fields`
         * :ghc-flag:`-Wunicode-bidirectional-format-characters`
+        * :ghc-flag:`-Wforall-identifier`
 
 The following flags are simple ways to select standard "packages" of warnings:
 
@@ -2170,6 +2171,33 @@ of ``-W(no-)*``.
     This warning has no effect when :extension:`DuplicateRecordFields` is
     disabled.
 
+.. ghc-flag:: -Wforall-identifier
+    :shortdesc: warn when ``forall`` is used as an identifier (at definition sites)
+    :type: dynamic
+    :reverse: -Wno-forall-identifier
+
+    :since: 9.4
+
+    In a future GHC release, ``forall`` will become a keyword regardless of
+    enabled extensions. This will make definitions such as the following
+    illegal::
+
+      -- from constraints-0.13
+      forall :: forall p. (forall a. Dict (p a)) -> Dict (Forall p)
+      forall d = ...
+
+    Library authors are advised to use a different identifier, such as
+    ``forAll``, ``forall_``, or ``for_all``::
+
+      forall_ :: forall p. (forall a. Dict (p a)) -> Dict (Forall p)
+      forall_ d = ...
+
+    The warning is only triggered at definition sites where it can be
+    addressed by using a different name.
+
+    Users of a library that exports ``forall`` as an identifier cannot address
+    the issue themselves, so the warning is not reported at use sites.
+
 .. ghc-flag:: -Wunicode-bidirectional-format-characters
     :shortdesc: warn about the usage of unicode bidirectional layout override characters
     :type: dynamic
diff --git a/libraries/template-haskell/Language/Haskell/TH/Ppr.hs b/libraries/template-haskell/Language/Haskell/TH/Ppr.hs
index 7ed842ca94..9f19d75dcd 100644
--- a/libraries/template-haskell/Language/Haskell/TH/Ppr.hs
+++ b/libraries/template-haskell/Language/Haskell/TH/Ppr.hs
@@ -93,10 +93,10 @@ pprPatSynType :: PatSynType -> Doc
 pprPatSynType ty@(ForallT uniTys reqs ty'@(ForallT exTys provs ty''))
   | null exTys,  null provs = ppr (ForallT uniTys reqs ty'')
   | null uniTys, null reqs  = noreqs <+> ppr ty'
-  | null reqs               = forall uniTys <+> noreqs <+> ppr ty'
+  | null reqs               = pprForallBndrs uniTys <+> noreqs <+> ppr ty'
   | otherwise               = ppr ty
-  where noreqs     = text "() =>"
-        forall tvs = text "forall" <+> (hsep (map ppr tvs)) <+> text "."
+  where noreqs = text "() =>"
+        pprForallBndrs tvs = text "forall" <+> hsep (map ppr tvs) <+> text "."
 pprPatSynType ty            = ppr ty
 
 ------------------------------
diff --git a/testsuite/tests/ghci/prog017/TopLevel.hs b/testsuite/tests/ghci/prog017/TopLevel.hs
index c2e562b1f9..6e9d38812a 100644
--- a/testsuite/tests/ghci/prog017/TopLevel.hs
+++ b/testsuite/tests/ghci/prog017/TopLevel.hs
@@ -72,8 +72,8 @@ import Level2.Level2
 (➾➔) = undefined :: () -- ... U+2794
 
 -- mathematicalOperators
-(∀) = undefined :: () -- U+2200
-(∀⋙) = undefined :: () -- ... U+22D9
+(∁) = undefined :: () -- U+2201 (skip U+2200 because it's the keyword ∀)
+(∁⋙) = undefined :: () -- ... U+22D9
 
 -- miscellaneousMathematicalSymbolsAR
 (⟑) = undefined :: () -- U+27D1
diff --git a/testsuite/tests/ghci/prog017/prog017.script b/testsuite/tests/ghci/prog017/prog017.script
index 302233869b..7144d087fd 100644
--- a/testsuite/tests/ghci/prog017/prog017.script
+++ b/testsuite/tests/ghci/prog017/prog017.script
@@ -61,7 +61,7 @@
 :complete repl "➾"
 
 -- mathematicalOperators
-:complete repl "∀"
+:complete repl "∁"
 
 -- miscellaneousMathematicalSymbolsAR
 :complete repl "⟑"
diff --git a/testsuite/tests/ghci/prog017/prog017.stdout b/testsuite/tests/ghci/prog017/prog017.stdout
index 903660d369..57b1ba0d6c 100644
--- a/testsuite/tests/ghci/prog017/prog017.stdout
+++ b/testsuite/tests/ghci/prog017/prog017.stdout
@@ -68,8 +68,8 @@
 "\10174"
 "\10174\10132"
 2 2 ""
-"\8704"
-"\8704\8921"
+"\8705"
+"\8705\8921"
 2 2 ""
 "\10193"
 "\10193\10193"
diff --git a/testsuite/tests/rename/should_compile/ExplicitForAllRules1.stderr b/testsuite/tests/rename/should_compile/ExplicitForAllRules1.stderr
index 5852074450..88c7e7c771 100644
--- a/testsuite/tests/rename/should_compile/ExplicitForAllRules1.stderr
+++ b/testsuite/tests/rename/should_compile/ExplicitForAllRules1.stderr
@@ -1,4 +1,11 @@
 
+ExplicitForAllRules1.hs:46:31: warning: [-Wforall-identifier (in -Wdefault)]
+    The use of ‘forall’ as an identifier
+    will become an error in a future GHC release.
+    Suggested fix:
+      Consider using another name, such as
+      ‘forAll’, ‘for_all’, or ‘forall_’.
+
 ExplicitForAllRules1.hs:49:31: warning: [-Wunused-foralls (in -Wextra)]
     Unused quantified type variable ‘b’
     In the rewrite rule "example7"
diff --git a/testsuite/tests/rename/should_compile/T20609.hs b/testsuite/tests/rename/should_compile/T20609.hs
new file mode 100644
index 0000000000..7e8955f29f
--- /dev/null
+++ b/testsuite/tests/rename/should_compile/T20609.hs
@@ -0,0 +1,23 @@
+module T20609 where
+
+-- Triggers the warning (definition/binding sites):
+-- ------------------------------------------------
+
+forall x = ()
+
+(∀) x = ()
+
+fparam forall = ()
+
+asPattern forall@Nothing = ()
+
+localLet = let forall = () in forall
+
+{-# RULES "rule" forall forall. id forall = forall #-}
+
+{-# RULES "rule_sig" forall a. forall (forall :: a). id forall = forall #-}
+
+-- Does not trigger the warning (use sites):
+-- -----------------------------------------
+
+other = forall
diff --git a/testsuite/tests/rename/should_compile/T20609.stderr b/testsuite/tests/rename/should_compile/T20609.stderr
new file mode 100644
index 0000000000..a9958e2b85
--- /dev/null
+++ b/testsuite/tests/rename/should_compile/T20609.stderr
@@ -0,0 +1,49 @@
+
+T20609.hs:6:1: warning: [-Wforall-identifier (in -Wdefault)]
+    The use of ‘forall’ as an identifier
+    will become an error in a future GHC release.
+    Suggested fix:
+      Consider using another name, such as
+      ‘forAll’, ‘for_all’, or ‘forall_’.
+
+T20609.hs:8:1: warning: [-Wforall-identifier (in -Wdefault)]
+    The use of ‘∀’ as an identifier
+    will become an error in a future GHC release.
+    Suggested fix:
+      Consider using another name, such as
+      ‘forAll’, ‘for_all’, or ‘forall_’.
+
+T20609.hs:10:8: warning: [-Wforall-identifier (in -Wdefault)]
+    The use of ‘forall’ as an identifier
+    will become an error in a future GHC release.
+    Suggested fix:
+      Consider using another name, such as
+      ‘forAll’, ‘for_all’, or ‘forall_’.
+
+T20609.hs:12:11: warning: [-Wforall-identifier (in -Wdefault)]
+    The use of ‘forall’ as an identifier
+    will become an error in a future GHC release.
+    Suggested fix:
+      Consider using another name, such as
+      ‘forAll’, ‘for_all’, or ‘forall_’.
+
+T20609.hs:14:16: warning: [-Wforall-identifier (in -Wdefault)]
+    The use of ‘forall’ as an identifier
+    will become an error in a future GHC release.
+    Suggested fix:
+      Consider using another name, such as
+      ‘forAll’, ‘for_all’, or ‘forall_’.
+
+T20609.hs:16:25: warning: [-Wforall-identifier (in -Wdefault)]
+    The use of ‘forall’ as an identifier
+    will become an error in a future GHC release.
+    Suggested fix:
+      Consider using another name, such as
+      ‘forAll’, ‘for_all’, or ‘forall_’.
+
+T20609.hs:18:40: warning: [-Wforall-identifier (in -Wdefault)]
+    The use of ‘forall’ as an identifier
+    will become an error in a future GHC release.
+    Suggested fix:
+      Consider using another name, such as
+      ‘forAll’, ‘for_all’, or ‘forall_’.
diff --git a/testsuite/tests/rename/should_compile/T20609a.hs b/testsuite/tests/rename/should_compile/T20609a.hs
new file mode 100644
index 0000000000..0cf753760f
--- /dev/null
+++ b/testsuite/tests/rename/should_compile/T20609a.hs
@@ -0,0 +1,12 @@
+module T20609a where
+
+-- Triggers the warning (definition/binding sites):
+-- ------------------------------------------------
+
+data MyRecord a = R { forall :: a }
+
+-- Does not trigger the warning (use sites):
+-- -----------------------------------------
+
+x = forall (R { forall = () })
+f (R { forall = r }) = r
diff --git a/testsuite/tests/rename/should_compile/T20609a.stderr b/testsuite/tests/rename/should_compile/T20609a.stderr
new file mode 100644
index 0000000000..d828a1b269
--- /dev/null
+++ b/testsuite/tests/rename/should_compile/T20609a.stderr
@@ -0,0 +1,7 @@
+
+T20609a.hs:6:23: warning: [-Wforall-identifier (in -Wdefault)]
+    The use of ‘forall’ as an identifier
+    will become an error in a future GHC release.
+    Suggested fix:
+      Consider using another name, such as
+      ‘forAll’, ‘for_all’, or ‘forall_’.
diff --git a/testsuite/tests/rename/should_compile/T20609b.hs b/testsuite/tests/rename/should_compile/T20609b.hs
new file mode 100644
index 0000000000..2e08bdcee2
--- /dev/null
+++ b/testsuite/tests/rename/should_compile/T20609b.hs
@@ -0,0 +1,13 @@
+module T20609b where
+
+-- Triggers the warning (definition/binding sites):
+-- ------------------------------------------------
+
+class MyClass c where
+  forall :: c -> ()
+
+-- Does not trigger the warning (use sites):
+-- -----------------------------------------
+
+instance MyClass () where
+  forall = id
diff --git a/testsuite/tests/rename/should_compile/T20609b.stderr b/testsuite/tests/rename/should_compile/T20609b.stderr
new file mode 100644
index 0000000000..c356dde8b0
--- /dev/null
+++ b/testsuite/tests/rename/should_compile/T20609b.stderr
@@ -0,0 +1,7 @@
+
+T20609b.hs:7:3: warning: [-Wforall-identifier (in -Wdefault)]
+    The use of ‘forall’ as an identifier
+    will become an error in a future GHC release.
+    Suggested fix:
+      Consider using another name, such as
+      ‘forAll’, ‘for_all’, or ‘forall_’.
diff --git a/testsuite/tests/rename/should_compile/T20609c.hs b/testsuite/tests/rename/should_compile/T20609c.hs
new file mode 100644
index 0000000000..2e36f8c7e1
--- /dev/null
+++ b/testsuite/tests/rename/should_compile/T20609c.hs
@@ -0,0 +1,8 @@
+{-# LANGUAGE PatternSynonyms #-}
+
+module T20609c where
+
+pattern Pat forall = forall
+
+pattern RPat { forall } = forall
+
diff --git a/testsuite/tests/rename/should_compile/T20609c.stderr b/testsuite/tests/rename/should_compile/T20609c.stderr
new file mode 100644
index 0000000000..c22ead3d0b
--- /dev/null
+++ b/testsuite/tests/rename/should_compile/T20609c.stderr
@@ -0,0 +1,14 @@
+
+T20609c.hs:5:22: warning: [-Wforall-identifier (in -Wdefault)]
+    The use of ‘forall’ as an identifier
+    will become an error in a future GHC release.
+    Suggested fix:
+      Consider using another name, such as
+      ‘forAll’, ‘for_all’, or ‘forall_’.
+
+T20609c.hs:7:27: warning: [-Wforall-identifier (in -Wdefault)]
+    The use of ‘forall’ as an identifier
+    will become an error in a future GHC release.
+    Suggested fix:
+      Consider using another name, such as
+      ‘forAll’, ‘for_all’, or ‘forall_’.
diff --git a/testsuite/tests/rename/should_compile/T20609d.hs b/testsuite/tests/rename/should_compile/T20609d.hs
new file mode 100644
index 0000000000..bed945bcfa
--- /dev/null
+++ b/testsuite/tests/rename/should_compile/T20609d.hs
@@ -0,0 +1,6 @@
+module T20609d where
+
+import Data.Word (Word8)
+
+foreign import ccall unsafe "forall"
+  forall :: IO Word8
diff --git a/testsuite/tests/rename/should_compile/T20609d.stderr b/testsuite/tests/rename/should_compile/T20609d.stderr
new file mode 100644
index 0000000000..8060b3298f
--- /dev/null
+++ b/testsuite/tests/rename/should_compile/T20609d.stderr
@@ -0,0 +1,7 @@
+
+T20609d.hs:6:3: warning: [-Wforall-identifier (in -Wdefault)]
+    The use of ‘forall’ as an identifier
+    will become an error in a future GHC release.
+    Suggested fix:
+      Consider using another name, such as
+      ‘forAll’, ‘for_all’, or ‘forall_’.
diff --git a/testsuite/tests/rename/should_compile/all.T b/testsuite/tests/rename/should_compile/all.T
index 2148470c66..8e55b3705a 100644
--- a/testsuite/tests/rename/should_compile/all.T
+++ b/testsuite/tests/rename/should_compile/all.T
@@ -177,3 +177,8 @@ test('T18302', expect_broken(18302), compile, [''])
 test('T17853', [], multimod_compile, ['T17853', '-v0'])
 test('T19966', expect_broken(19966), compile, ['-fdefer-out-of-scope-variables'])
 test('T20472', normal, compile, [''])
+test('T20609', normal, compile, [''])
+test('T20609a', normal, compile, [''])
+test('T20609b', normal, compile, [''])
+test('T20609c', normal, compile, [''])
+test('T20609d', normal, compile, [''])
diff --git a/utils/haddock b/utils/haddock
-Subproject 1ef24e617651955f07c4fb6f2d488806cc6785e
+Subproject f7bfa0013f2bc3934a63ea7af21fe41a4e91058