diff --git a/compiler/GHC/Driver/Flags.hs b/compiler/GHC/Driver/Flags.hs
index bf4ac2bc788cf241c6dfe4eb745b52536497fdf8..17be75a7cc3a07eab6fc400c02730a9c9c7143fe 100644
--- a/compiler/GHC/Driver/Flags.hs
+++ b/compiler/GHC/Driver/Flags.hs
@@ -697,6 +697,7 @@ data WarningFlag =
| Opt_WarnBadlyStagedTypes -- Since 9.10
| Opt_WarnInconsistentFlags -- Since 9.8
| Opt_WarnDataKindsTC -- Since 9.10
+ | Opt_WarnDeprecatedTypeAbstractions -- Since 9.10
deriving (Eq, Ord, Show, Enum, Bounded)
-- | Return the names of a WarningFlag
@@ -811,6 +812,7 @@ warnFlagNames wflag = case wflag of
Opt_WarnBadlyStagedTypes -> "badly-staged-types" :| []
Opt_WarnInconsistentFlags -> "inconsistent-flags" :| []
Opt_WarnDataKindsTC -> "data-kinds-tc" :| []
+ Opt_WarnDeprecatedTypeAbstractions -> "deprecated-type-abstractions" :| []
-- -----------------------------------------------------------------------------
-- Standard sets of warning options
@@ -1006,6 +1008,7 @@ minusWcompatOpts
, Opt_WarnCompatUnqualifiedImports
, Opt_WarnTypeEqualityOutOfScope
, Opt_WarnImplicitRhsQuantification
+ , Opt_WarnDeprecatedTypeAbstractions
]
-- | Things you get with -Wunused-binds
diff --git a/compiler/GHC/Driver/Session.hs b/compiler/GHC/Driver/Session.hs
index 4b5ec2216cdc4c31399401006f98227de9ab632e..161eadb9cae0c015afc8f5e374675e40c5178411 100644
--- a/compiler/GHC/Driver/Session.hs
+++ b/compiler/GHC/Driver/Session.hs
@@ -2285,6 +2285,7 @@ wWarningFlagsDeps = [minBound..maxBound] >>= \x -> case x of
Opt_WarnIncompleteExportWarnings -> warnSpec x
Opt_WarnIncompleteRecordSelectors -> warnSpec x
Opt_WarnDataKindsTC -> warnSpec x
+ Opt_WarnDeprecatedTypeAbstractions -> warnSpec x
warningGroupsDeps :: [(Deprecation, FlagSpec WarningGroup)]
warningGroupsDeps = map mk warningGroups
diff --git a/compiler/GHC/Rename/Pat.hs b/compiler/GHC/Rename/Pat.hs
index b563a3ebf4fd0fbd39e8c32ddc64582c4dda778b..5951da22b74a01306d5e4ef8aaa4b4d4b1198c63 100644
--- a/compiler/GHC/Rename/Pat.hs
+++ b/compiler/GHC/Rename/Pat.hs
@@ -663,21 +663,10 @@ rnConPatAndThen mk con (PrefixCon tyargs pats)
do { type_abs <- xoptM LangExt.TypeAbstractions
; type_app <- xoptM LangExt.TypeApplications
; scoped_tvs <- xoptM LangExt.ScopedTypeVariables
- ; if | type_abs
- -> return ()
-
- -- As per [GHC Proposal 604](https://github.com/ghc-proposals/ghc-proposals/pull/604/),
- -- we allow type applications in constructor patterns when -XTypeApplications and
- -- -XScopedTypeVariables are both enabled, but we emit a warning when doing so.
- --
- -- This warning is scheduled to become an error in GHC 9.12, in
- -- which case we will get the usual error (below),
- -- which suggests enabling -XTypeAbstractions.
- | type_app && scoped_tvs
- -> addDiagnostic TcRnDeprecatedInvisTyArgInConPat
-
- | otherwise
- -> addErrTc $ TcRnTypeApplicationsDisabled (TypeApplicationInPattern arg)
+ -- See Note [Deprecated type abstractions in constructor patterns]
+ ; if | type_abs -> return ()
+ | type_app && scoped_tvs -> addDiagnostic TcRnDeprecatedInvisTyArgInConPat
+ | otherwise -> addErrTc $ TcRnTypeApplicationsDisabled (TypeApplicationInPattern arg)
}
rnConPatTyArg (HsConPatTyArg at t) = do
@@ -701,6 +690,29 @@ rnConPatAndThen mk con (RecCon rpats)
}
}
+{- Note [Deprecated type abstractions in constructor patterns]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Type abstractions in constructor patterns allow the user to bind
+existential type variables:
+
+ import Type.Reflection (Typeable, typeRep)
+ data Ex = forall e. (Typeable e, Show e) => MkEx e
+ showEx (MkEx @e a) = show a ++ " :: " ++ show (typeRep @e)
+
+Note the pattern `MkEx @e a`, and specifically the `@e` binder.
+
+For historical reasons, using this feature only required TypeApplications
+and ScopedTypeVariables to be enabled. As per GHC Proposal #448 (and especially
+its amendment #604) we are now transitioning towards guarding this feature
+behind TypeAbstractions instead.
+
+As a compatibility measure, we continue to support old programs that use
+TypeApplications with ScopedTypeVariables instead of TypeAbstractions,
+but emit the appropriate compatibility warning, -Wdeprecated-type-abstractions.
+This warning is scheduled to become an error in GHC 9.14, at which point
+we can simply require TypeAbstractions.
+-}
+
checkUnusedRecordWildcardCps :: SrcSpan
-> Maybe [ImplicitFieldBinders]
-> CpsRn ()
diff --git a/compiler/GHC/Tc/Errors/Ppr.hs b/compiler/GHC/Tc/Errors/Ppr.hs
index 87bed258dfe2c0568efec84dfc8bfae1de020e71..196a1909a422eefa4d2486dbe0ea049830076ce3 100644
--- a/compiler/GHC/Tc/Errors/Ppr.hs
+++ b/compiler/GHC/Tc/Errors/Ppr.hs
@@ -1865,7 +1865,7 @@ instance Diagnostic TcRnMessage where
TcRnDeprecatedInvisTyArgInConPat ->
mkSimpleDecorated $
cat [ text "Type applications in constructor patterns will require"
- , text "the TypeAbstractions extension starting from GHC 9.12." ]
+ , text "the TypeAbstractions extension starting from GHC 9.14." ]
TcRnInvisBndrWithoutSig _ hs_bndr ->
mkSimpleDecorated $
@@ -2502,7 +2502,7 @@ instance Diagnostic TcRnMessage where
TcRnIllegalInvisTyVarBndr{}
-> ErrorWithoutFlag
TcRnDeprecatedInvisTyArgInConPat {}
- -> WarningWithoutFlag
+ -> WarningWithFlag Opt_WarnDeprecatedTypeAbstractions
TcRnInvalidInvisTyVarBndr{}
-> ErrorWithoutFlag
TcRnInvisBndrWithoutSig{}
diff --git a/docs/users_guide/9.10.1-notes.rst b/docs/users_guide/9.10.1-notes.rst
index 08b9d740b1b2165a9c677e49cd64c4563455bec8..8fd0c11c2227e272d651a3508139cd4abbcc2353 100644
--- a/docs/users_guide/9.10.1-notes.rst
+++ b/docs/users_guide/9.10.1-notes.rst
@@ -83,6 +83,10 @@ Compiler
This makes core optimizations more consistent between 32bit and 64bit platforms
at the cost of slightly worse 32bit performance in edge cases.
+- Type abstractions in constructor patterns that were previously admitted without enabling the :extension:`TypeAbstractions`
+ extension now trigger a warning, :ghc-flag:`-Wdeprecated-type-abstractions`.
+ This new warning is part of the :ghc-flag:`-Wcompat` warning group and will become an error in a future GHC release.
+
GHCi
~~~~
diff --git a/docs/users_guide/using-warnings.rst b/docs/users_guide/using-warnings.rst
index 1cd7db47018088e3465ec8d72c740964290068fa..cb2fbc8812fdb26ed69d2cfc528a0643eef0ca6d 100644
--- a/docs/users_guide/using-warnings.rst
+++ b/docs/users_guide/using-warnings.rst
@@ -171,6 +171,7 @@ as ``-Wno-...`` for every individual warning in the group.
* :ghc-flag:`-Wcompat-unqualified-imports`
* :ghc-flag:`-Wtype-equality-out-of-scope`
* :ghc-flag:`-Wimplicit-rhs-quantification`
+ * :ghc-flag:`-Wdeprecated-type-abstractions`
.. ghc-flag:: -w
:shortdesc: disable all warnings
@@ -2501,6 +2502,36 @@ of ``-W(no-)*``.
This warning detects code that will be affected by this breaking change.
+.. ghc-flag:: -Wdeprecated-type-abstractions
+ :shortdesc: warn when type abstractions in constructor patterns are used without enabling :extension:`TypeApplications`
+ :type: dynamic
+ :reverse: -Wno-deprecated-type-abstractions
+ :category:
+
+ :since: 9.10.1
+ :default: off
+
+ Type abstractions in constructor patterns allow binding existential type variables: ::
+
+ import Type.Reflection (Typeable, typeRep)
+ data Ex = forall e. (Typeable e, Show e) => MkEx e
+ showEx (MkEx @e a) = show a ++ " :: " ++ show (typeRep @e)
+
+ Note the pattern ``MkEx @e a``, and specifically the ``@e`` binder.
+
+ Support for this feature was added to GHC in version 9.2, but instead of getting
+ its own language extension the feature was enabled by a combination of
+ :extension:`TypeApplications` and :extension:`ScopedTypeVariables`.
+ As per `GHC Proposal #448
+ <https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0448-type-variable-scoping.rst>`__
+ and its amendment `#604 <https://github.com/ghc-proposals/ghc-proposals/pull/604>`__
+ we are now transitioning towards guarding this feature behind :extension:`TypeAbstractions` instead.
+
+ As a compatibility measure, GHC continues to support old programs that use type abstractions
+ in constructor patterns without enabling the appropriate extension :extension:`TypeAbstractions`,
+ but it will stop doing so in a future release.
+
+ This warning detects code that will be affected by this breaking change.
.. ghc-flag:: -Wincomplete-export-warnings
:shortdesc: warn when some but not all of exports for a name are warned about
diff --git a/testsuite/tests/ghci/scripts/ghci024.stdout b/testsuite/tests/ghci/scripts/ghci024.stdout
index a3ceebd9d85e089ffdd7c7bc88f3b394bb5a2e89..e82226aff141c55eb9bd76bd635db0d62b72d7e4 100644
--- a/testsuite/tests/ghci/scripts/ghci024.stdout
+++ b/testsuite/tests/ghci/scripts/ghci024.stdout
@@ -20,6 +20,7 @@ warning settings:
-Wcompat-unqualified-imports
-Wtype-equality-out-of-scope
-Wimplicit-rhs-quantification
+ -Wdeprecated-type-abstractions
~~~~~~~~~~ Testing :set -a
options currently set: none.
base language is: GHC2021
diff --git a/testsuite/tests/ghci/scripts/ghci024.stdout-mingw32 b/testsuite/tests/ghci/scripts/ghci024.stdout-mingw32
index 2563472740ff6df9f8b2377b52fefdb3ecffa724..c7658f986c2f80fbeb3831f63e99838512b7bcc3 100644
--- a/testsuite/tests/ghci/scripts/ghci024.stdout-mingw32
+++ b/testsuite/tests/ghci/scripts/ghci024.stdout-mingw32
@@ -19,6 +19,7 @@ warning settings:
-Wcompat-unqualified-imports
-Wtype-equality-out-of-scope
-Wimplicit-rhs-quantification
+ -Wdeprecated-type-abstractions
~~~~~~~~~~ Testing :set -a
options currently set: none.
base language is: GHC2021
diff --git a/testsuite/tests/typecheck/should_fail/T23776.stderr b/testsuite/tests/typecheck/should_fail/T23776.stderr
index 1379bfaff8495c91d653e9ce970a750ade081bd8..cd734bc880a05b8989befb0ac9bf7b58dde31e5f 100644
--- a/testsuite/tests/typecheck/should_fail/T23776.stderr
+++ b/testsuite/tests/typecheck/should_fail/T23776.stderr
@@ -1,5 +1,5 @@
-T23776.hs:8:6: warning: [GHC-69797]
+T23776.hs:8:6: error: [GHC-69797] [-Wdeprecated-type-abstractions (in -Wcompat), Werror=deprecated-type-abstractions]
Type applications in constructor patterns will require
- the TypeAbstractions extension starting from GHC 9.12.
+ the TypeAbstractions extension starting from GHC 9.14.
Suggested fix: Perhaps you intended to use TypeAbstractions
diff --git a/testsuite/tests/typecheck/should_fail/all.T b/testsuite/tests/typecheck/should_fail/all.T
index 19f331f357738b3734c80f2d9e15fb8dc203d54f..82661689a1d4f3f7e31b4974e6ac89ca2a3fa0ae 100644
--- a/testsuite/tests/typecheck/should_fail/all.T
+++ b/testsuite/tests/typecheck/should_fail/all.T
@@ -706,7 +706,7 @@ test('VisFlag5', normal, compile_fail, [''])
test('T22684', normal, compile_fail, [''])
test('T23514a', normal, compile_fail, [''])
test('T22478c', normal, compile_fail, [''])
-test('T23776', normal, compile, ['']) # to become an error in GHC 9.12
+test('T23776', normal, compile_fail, ['']) # error due to -Werror=compat, scheduled to become an actual error in GHC 9.14
test('T17940', normal, compile_fail, [''])
test('ErrorIndexLinks', normal, compile_fail, ['-fprint-error-index-links=always'])
test('T24064', normal, compile_fail, [''])