Deprecate -Wmissing-monadfail-instances (#17875)wip/T17875

Also document deprecation of Wnoncanonical-monadfail-instances
and -Wimplicit-kind-vars

7 files changed, 26 insertions, 47 deletions

diff --git a/compiler/GHC/Driver/Session.hs b/compiler/GHC/Driver/Session.hs
index aa761325d3..8b760a1b08 100644
--- a/compiler/GHC/Driver/Session.hs
+++ b/compiler/GHC/Driver/Session.hs
@@ -3172,7 +3172,8 @@ wWarningFlagsDeps = mconcat [
               "it is replaced by -Wmissing-local-signatures",
   warnSpec    Opt_WarnMissingLocalSignatures,
   warnSpec    Opt_WarnMissingMethods,
-  warnSpec    Opt_WarnMissingMonadFailInstances,
+  depWarnSpec Opt_WarnMissingMonadFailInstances
+              "fail is no longer a method of Monad",
   warnSpec    Opt_WarnSemigroup,
   warnSpec    Opt_WarnMissingSignatures,
   warnSpec    Opt_WarnMissingKindSignatures,
diff --git a/docs/users_guide/using-warnings.rst b/docs/users_guide/using-warnings.rst
index dc21cd468f..840c66eef6 100644
--- a/docs/users_guide/using-warnings.rst
+++ b/docs/users_guide/using-warnings.rst
@@ -517,32 +517,18 @@ of ``-W(no-)*``.
     This option is off by default.
 
 .. ghc-flag:: -Wnoncanonical-monadfail-instances
-    :shortdesc: warn when ``Monad`` or ``MonadFail`` instances have
+    :shortdesc: *(deprecated)*
+        warn when ``Monad`` or ``MonadFail`` instances have
         noncanonical definitions of ``fail``.
-        See flag description in :ref:`options-sanity` for more details.
     :type: dynamic
     :reverse: -Wno-noncanonical-monadfail-instances
     :category:
 
-    Warn if noncanonical ``Monad`` or ``MonadFail`` instances
-    declarations are detected.
-
-    When this warning is enabled, the following conditions are verified:
-
-    In ``Monad`` instances declarations warn if any of the following
-    conditions does not hold:
-
-     * If ``fail`` is defined it must be canonical
-       (i.e. ``fail = Control.Monad.Fail.fail``).
-
-    Moreover, in ``MonadFail`` instance declarations:
-
-     * Warn if ``fail`` is defined backwards
-       (i.e. ``fail = Control.Monad.fail``).
-
-    See also :ghc-flag:`-Wmissing-monadfail-instances`.
-
-    This option is off by default.
+    This warning is deprecated. It no longer has any effect since GHC 8.8.
+    It was used during the transition period of the MonadFail proposal,
+    to detect when an instance of the ``Monad`` class was not defined
+    via ``MonadFail``, or when a ``MonadFail`` instance was defined
+    backwards, using the method in ``Monad``.
 
 .. ghc-flag:: -Wnoncanonical-monoid-instances
     :shortdesc: warn when ``Semigroup`` or ``Monoid`` instances have
@@ -571,25 +557,17 @@ of ``-W(no-)*``.
     :ghc-flag:`-Wcompat` option group.
 
 .. ghc-flag:: -Wmissing-monadfail-instances
-    :shortdesc: Warn when a failable pattern is used in a do-block that does
+    :shortdesc: *(deprecated)*
+        Warn when a failable pattern is used in a do-block that does
         not have a ``MonadFail`` instance.
     :type: dynamic
     :reverse: -Wno-missing-monadfail-instances
     :category:
 
-    .. index::
-       single: MFP
-       single: MonadFail Proposal
-
-    Warn when a failable pattern is used in a do-block that does not have a
-    ``MonadFail`` instance.
-
-    See also :ghc-flag:`-Wnoncanonical-monadfail-instances`.
-
-    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, as part of
-    the `MonadFail Proposal (MFP)
-    <https://gitlab.haskell.org/haskell/prime/-/wikis/libraries/proposals/monad-fail>`__.
+    This warning is deprecated. It no longer has any effect since GHC 8.8.
+    It was used during the transition period of the MonadFail proposal,
+    to warn when a failable pattern is used in a do-block that does not have
+    a ``MonadFail`` instance.
 
 .. ghc-flag:: -Wsemigroup
     :shortdesc: warn when a ``Monoid`` is not ``Semigroup``, and on non-
@@ -836,23 +814,27 @@ of ``-W(no-)*``.
     ``toInteger``, ``toRational``, ``fromIntegral``, and ``realToFrac``.
 
 .. ghc-flag:: -Wimplicit-kind-vars
-    :shortdesc: warn when kind variables are implicitly quantified over.
+    :shortdesc: *(deprecated)* warn when kind variables are
+        implicitly quantified over.
     :type: dynamic
     :reverse: -Wno-implicit-kind-vars
     :category:
 
-    .. index::
-       single: implicit prelude, warning
-
-    Have the compiler warn if a kind variable is not explicitly quantified
+    This warning is deprecated. It no longer has any effect since GHC 8.10.
+    It was used to detect if a kind variable is not explicitly quantified
     over. For instance, the following would produce a warning: ::
 
         f :: forall (a :: k). Proxy a
 
-    This can be fixed by explicitly quantifying over ``k``: ::
+    This is now an error and can be fixed by explicitly quantifying
+    over ``k``: ::
 
         f :: forall k (a :: k). Proxy a
 
+    or ::
+
+        f :: forall {k} (a :: k). Proxy a
+
 .. ghc-flag:: -Wimplicit-lift
     :shortdesc: warn about implicit ``lift`` in Template Haskell quotes
     :type: dynamic
diff --git a/testsuite/tests/rebindable/rebindable2.hs b/testsuite/tests/rebindable/rebindable2.hs
index 3858d2b9f3..5729c4ef34 100644
--- a/testsuite/tests/rebindable/rebindable2.hs
+++ b/testsuite/tests/rebindable/rebindable2.hs
@@ -1,5 +1,4 @@
 {-# LANGUAGE RebindableSyntax, NPlusKPatterns #-}
-{-# OPTIONS -Wno-error=missing-monadfail-instances #-}
 
 module Main where
         {
diff --git a/testsuite/tests/rebindable/rebindable3.hs b/testsuite/tests/rebindable/rebindable3.hs
index c5fd3b255b..0434c1d0fd 100644
--- a/testsuite/tests/rebindable/rebindable3.hs
+++ b/testsuite/tests/rebindable/rebindable3.hs
@@ -1,5 +1,4 @@
 {-# LANGUAGE RebindableSyntax, NPlusKPatterns #-}
-{-# OPTIONS -Wno-error=missing-monadfail-instances #-}
 
 module Main where
         {
diff --git a/testsuite/tests/rebindable/rebindable4.hs b/testsuite/tests/rebindable/rebindable4.hs
index 26adc87e21..f657683a08 100644
--- a/testsuite/tests/rebindable/rebindable4.hs
+++ b/testsuite/tests/rebindable/rebindable4.hs
@@ -1,5 +1,4 @@
 {-# LANGUAGE RebindableSyntax, NPlusKPatterns #-}
-{-# OPTIONS -Wno-error=missing-monadfail-instances #-}
 
 module Main where
         {
diff --git a/testsuite/tests/rebindable/rebindable5.hs b/testsuite/tests/rebindable/rebindable5.hs
index 2ce735b670..3120ea1a90 100644
--- a/testsuite/tests/rebindable/rebindable5.hs
+++ b/testsuite/tests/rebindable/rebindable5.hs
@@ -1,6 +1,5 @@
 {-# LANGUAGE RebindableSyntax, NPlusKPatterns, FlexibleInstances,
              MultiParamTypeClasses, FunctionalDependencies #-}
-{-# OPTIONS -Wno-error=missing-monadfail-instances #-}
 
 module Main where
         {
diff --git a/testsuite/tests/typecheck/should_compile/all.T b/testsuite/tests/typecheck/should_compile/all.T
index 9cd7c732ca..968aeb1aa7 100644
--- a/testsuite/tests/typecheck/should_compile/all.T
+++ b/testsuite/tests/typecheck/should_compile/all.T
@@ -663,7 +663,7 @@ test('T15473', normal, compile_fail, [''])
 test('T15499', normal, compile, [''])
 test('T15586', normal, compile, [''])
 test('T15368', normal, compile, ['-fdefer-type-errors'])
-test('T15645', normal, compile, ['-Wwarn=missing-monadfail-instances'])
+test('T15645', normal, compile, [''])
 test('T15646',
      compile_timeout_multiplier(0.01),
      # 0.01 may seem tiny (1 is timeout after 300s, so this is 3 seconds),