From e3ca78f3767176dd9870509aa334505d0146d94c Mon Sep 17 00:00:00 2001
From: Krzysztof Gogolewski <krzysztof.gogolewski@tweag.io>
Date: Thu, 9 Nov 2023 15:37:08 +0100
Subject: [PATCH] Deprecate -Wsemigroup
This warning was used to prepare for Semigroup becoming a superclass
of Monoid, and for (<>) being exported from Prelude. This happened in
GHC 8.4 in 8ae263ceb3566 and feac0a3bc69fd3.
The leftover logic for (<>) has been removed in GHC 9.8, 4d29ecdfcc79.
Now the warning does nothing at all and can be deprecated.
---
compiler/GHC/Driver/Flags.hs | 5 ++---
compiler/GHC/Driver/Session.hs | 2 +-
docs/users_guide/using-warnings.rst | 20 ++++++++-----------
testsuite/tests/ghci/scripts/ghci024.stdout | 1 -
.../tests/ghci/scripts/ghci024.stdout-mingw32 | 1 -
testsuite/tests/wcompat-warnings/Template.hs | 2 --
.../wcompat-warnings/WCompatWarningsOn.stderr | 12 +++++------
7 files changed, 17 insertions(+), 26 deletions(-)
diff --git a/compiler/GHC/Driver/Flags.hs b/compiler/GHC/Driver/Flags.hs
index 0aadea7c3d38..a819caf7a0d3 100644
--- a/compiler/GHC/Driver/Flags.hs
+++ b/compiler/GHC/Driver/Flags.hs
@@ -623,7 +623,7 @@ data WarningFlag =
| Opt_WarnRedundantRecordWildcards
| Opt_WarnDeprecatedFlags
| Opt_WarnMissingMonadFailInstances -- since 8.0, has no effect since 8.8
- | Opt_WarnSemigroup -- since 8.0
+ | Opt_WarnSemigroup -- since 8.0, has no effect since 9.8
| Opt_WarnDodgyExports
| Opt_WarnDodgyImports
| Opt_WarnOrphans
@@ -1002,8 +1002,7 @@ minusWeverythingOpts = [ toEnum 0 .. ]
-- code future compatible to fix issues before they even generate warnings.
minusWcompatOpts :: [WarningFlag]
minusWcompatOpts
- = [ Opt_WarnSemigroup
- , Opt_WarnNonCanonicalMonoidInstances
+ = [ Opt_WarnNonCanonicalMonoidInstances
, Opt_WarnNonCanonicalMonadInstances
, Opt_WarnCompatUnqualifiedImports
, Opt_WarnTypeEqualityOutOfScope
diff --git a/compiler/GHC/Driver/Session.hs b/compiler/GHC/Driver/Session.hs
index 6f330707bb9d..f700610037ed 100644
--- a/compiler/GHC/Driver/Session.hs
+++ b/compiler/GHC/Driver/Session.hs
@@ -2215,7 +2215,7 @@ wWarningFlagsDeps = [minBound..maxBound] >>= \x -> case x of
Opt_WarnMissingMethods -> warnSpec x
Opt_WarnMissingMonadFailInstances
-> depWarnSpec x "fail is no longer a method of Monad"
- Opt_WarnSemigroup -> warnSpec x
+ Opt_WarnSemigroup -> depWarnSpec x "Semigroup is now a superclass of Monoid"
Opt_WarnMissingSignatures -> warnSpec x
Opt_WarnMissingKindSignatures -> warnSpec x
Opt_WarnMissingPolyKindSignatures -> warnSpec x
diff --git a/docs/users_guide/using-warnings.rst b/docs/users_guide/using-warnings.rst
index 720d3114e341..c759bed50fea 100644
--- a/docs/users_guide/using-warnings.rst
+++ b/docs/users_guide/using-warnings.rst
@@ -165,7 +165,6 @@ as ``-Wno-...`` for every individual warning in the group.
.. hlist::
:columns: 3
- * :ghc-flag:`-Wsemigroup`
* :ghc-flag:`-Wnoncanonical-monoid-instances`
* :ghc-flag:`-Wnoncanonical-monad-instances`
* :ghc-flag:`-Wcompat-unqualified-imports`
@@ -654,8 +653,9 @@ of ``-W(no-)*``.
a ``MonadFail`` instance.
.. ghc-flag:: -Wsemigroup
- :shortdesc: warn when a ``Monoid`` is not ``Semigroup``, and on non-
- ``Semigroup`` definitions of ``(<>)``?
+ :shortdesc: *(deprecated)*
+ Warn when a ``Monoid`` is not ``Semigroup``, and on non-``Semigroup``
+ definitions of ``(<>)``
:type: dynamic
:reverse: -Wno-semigroup
:category:
@@ -665,15 +665,11 @@ of ``-W(no-)*``.
.. index::
single: semigroup; warning
- Warn when definitions are in conflict with the future inclusion of
- ``Semigroup`` into the standard typeclasses.
-
- 1. Instances of ``Monoid`` should also be instances of ``Semigroup``
- 2. The ``Semigroup`` operator ``(<>)`` will be in ``Prelude``, which
- clashes with custom local definitions of such an operator
-
- Being part of the :ghc-flag:`-Wcompat` option group, this warning is off by
- default, but will be switched on in a future GHC release.
+ This warning is deprecated. It no longer has any effect since GHC 9.8.
+ It was used during the transition period of the semigroup proposal,
+ to warn when an instance of ``Monoid`` was not an instance of ``Semigroup``,
+ or when a custom local operator ``(<>)`` could clash with `(<>)`,
+ now exported from ``Prelude``.
.. ghc-flag:: -Wdeprecated-flags
:shortdesc: warn about uses of commandline flags that are deprecated
diff --git a/testsuite/tests/ghci/scripts/ghci024.stdout b/testsuite/tests/ghci/scripts/ghci024.stdout
index e82226aff141..8af7066534f9 100644
--- a/testsuite/tests/ghci/scripts/ghci024.stdout
+++ b/testsuite/tests/ghci/scripts/ghci024.stdout
@@ -16,7 +16,6 @@ other dynamic, non-language, flag settings:
-fprefer-byte-code
-fbreak-points
warning settings:
- -Wsemigroup
-Wcompat-unqualified-imports
-Wtype-equality-out-of-scope
-Wimplicit-rhs-quantification
diff --git a/testsuite/tests/ghci/scripts/ghci024.stdout-mingw32 b/testsuite/tests/ghci/scripts/ghci024.stdout-mingw32
index c7658f986c2f..8914a08aeefc 100644
--- a/testsuite/tests/ghci/scripts/ghci024.stdout-mingw32
+++ b/testsuite/tests/ghci/scripts/ghci024.stdout-mingw32
@@ -15,7 +15,6 @@ other dynamic, non-language, flag settings:
-fprefer-byte-code
-fbreak-points
warning settings:
- -Wsemigroup
-Wcompat-unqualified-imports
-Wtype-equality-out-of-scope
-Wimplicit-rhs-quantification
diff --git a/testsuite/tests/wcompat-warnings/Template.hs b/testsuite/tests/wcompat-warnings/Template.hs
index 86c1449c6405..1a6c328f6c04 100644
--- a/testsuite/tests/wcompat-warnings/Template.hs
+++ b/testsuite/tests/wcompat-warnings/Template.hs
@@ -2,8 +2,6 @@ module WCompatWarningsOnOff where
import qualified Data.Semigroup as Semi
-(<>) = undefined -- Semigroup warnings
-
-- -fwarn-noncanonical-monoid-instances
newtype S = S Int
diff --git a/testsuite/tests/wcompat-warnings/WCompatWarningsOn.stderr b/testsuite/tests/wcompat-warnings/WCompatWarningsOn.stderr
index 4ef1b425b119..db1e5c990979 100644
--- a/testsuite/tests/wcompat-warnings/WCompatWarningsOn.stderr
+++ b/testsuite/tests/wcompat-warnings/WCompatWarningsOn.stderr
@@ -1,12 +1,12 @@
-Template.hs:11:3: warning: [GHC-50928] [-Wnoncanonical-monoid-instances (in -Wdefault, -Wcompat)]
+Template.hs:9:3: warning: [GHC-50928] [-Wnoncanonical-monoid-instances (in -Wdefault, -Wcompat)]
Noncanonical ‘(<>) = mappend’ definition detected
in the instance declaration for ‘Semigroup S’.
Suggested fix:
Move definition from ‘mappend’ to ‘(<>)’
See also: https://gitlab.haskell.org/ghc/ghc/-/wikis/proposal/semigroup-monoid
-Template.hs:14:3: warning: [GHC-50928] [-Wnoncanonical-monoid-instances (in -Wdefault, -Wcompat)]
+Template.hs:12:3: warning: [GHC-50928] [-Wnoncanonical-monoid-instances (in -Wdefault, -Wcompat)]
Noncanonical ‘mappend’ definition detected
in the instance declaration for ‘Monoid S’.
‘mappend’ will eventually be removed in favour of ‘(<>)’
@@ -14,21 +14,21 @@ Template.hs:14:3: warning: [GHC-50928] [-Wnoncanonical-monoid-instances (in -Wde
Either remove definition for ‘mappend’ (recommended) or define as ‘mappend = (<>)’
See also: https://gitlab.haskell.org/ghc/ghc/-/wikis/proposal/semigroup-monoid
-Template.hs:24:3: warning: [GHC-22705] [-Wnoncanonical-monad-instances (in -Wdefault, -Wcompat)]
+Template.hs:22:3: warning: [GHC-22705] [-Wnoncanonical-monad-instances (in -Wdefault, -Wcompat)]
Noncanonical ‘pure = return’ definition detected
in the instance declaration for ‘Applicative M’.
Suggested fix:
Move definition from ‘return’ to ‘pure’
See also: https://gitlab.haskell.org/ghc/ghc/-/wikis/proposal/monad-of-no-return
-Template.hs:25:3: warning: [GHC-22705] [-Wnoncanonical-monad-instances (in -Wdefault, -Wcompat)]
+Template.hs:23:3: warning: [GHC-22705] [-Wnoncanonical-monad-instances (in -Wdefault, -Wcompat)]
Noncanonical ‘(*>) = (>>)’ definition detected
in the instance declaration for ‘Applicative M’.
Suggested fix:
Move definition from ‘(>>)’ to ‘(*>)’
See also: https://gitlab.haskell.org/ghc/ghc/-/wikis/proposal/monad-of-no-return
-Template.hs:28:3: warning: [GHC-22705] [-Wnoncanonical-monad-instances (in -Wdefault, -Wcompat)]
+Template.hs:26:3: warning: [GHC-22705] [-Wnoncanonical-monad-instances (in -Wdefault, -Wcompat)]
Noncanonical ‘return’ definition detected
in the instance declaration for ‘Monad M’.
‘return’ will eventually be removed in favour of ‘pure’
@@ -36,7 +36,7 @@ Template.hs:28:3: warning: [GHC-22705] [-Wnoncanonical-monad-instances (in -Wdef
Either remove definition for ‘return’ (recommended) or define as ‘return = pure’
See also: https://gitlab.haskell.org/ghc/ghc/-/wikis/proposal/monad-of-no-return
-Template.hs:30:3: warning: [GHC-22705] [-Wnoncanonical-monad-instances (in -Wdefault, -Wcompat)]
+Template.hs:28:3: warning: [GHC-22705] [-Wnoncanonical-monad-instances (in -Wdefault, -Wcompat)]
Noncanonical ‘(>>)’ definition detected
in the instance declaration for ‘Monad M’.
‘(>>)’ will eventually be removed in favour of ‘(*>)’
--
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