Add -Wcompat warning flag group

Reviewers: hvr, austin, thomie, bgamari

Reviewed By: hvr, austin, thomie, bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1495

GHC Trac Issues: #11000

10 files changed, 133 insertions, 11 deletions

diff --git a/compiler/main/DynFlags.hs b/compiler/main/DynFlags.hs
index ad05ed5a4d..f386d8d40c 100644
--- a/compiler/main/DynFlags.hs
+++ b/compiler/main/DynFlags.hs
@@ -2570,13 +2570,15 @@ dynamic_flags = [
   , defGhcFlag "mavx512pf"    (noArg (\d -> d{ avx512pf = True }))
 
      ------ Warning opts -------------------------------------------------
-  , defFlag "W"      (NoArg (mapM_ setWarningFlag minusWOpts))
-  , defFlag "Werror" (NoArg (setGeneralFlag           Opt_WarnIsError))
-  , defFlag "Wwarn"  (NoArg (unSetGeneralFlag         Opt_WarnIsError))
-  , defFlag "Wall"   (NoArg (mapM_ setWarningFlag minusWallOpts))
-  , defFlag "Wnot"   (NoArg (do upd (\dfs -> dfs {warningFlags = IntSet.empty})
-                                deprecate "Use -w instead"))
-  , defFlag "w"      (NoArg (upd (\dfs -> dfs {warningFlags = IntSet.empty})))
+  , defFlag "W"       (NoArg (mapM_ setWarningFlag minusWOpts))
+  , defFlag "Werror"  (NoArg (setGeneralFlag           Opt_WarnIsError))
+  , defFlag "Wwarn"   (NoArg (unSetGeneralFlag         Opt_WarnIsError))
+  , defFlag "Wcompat" (NoArg (mapM_ setWarningFlag minusWcompatOpts))
+  , defFlag "Wno-compat" (NoArg (mapM_ unSetWarningFlag minusWcompatOpts))
+  , defFlag "Wall"    (NoArg (mapM_ setWarningFlag minusWallOpts))
+  , defFlag "Wnot"    (NoArg (do upd (\dfs -> dfs {warningFlags = IntSet.empty})
+                                 deprecate "Use -w instead"))
+  , defFlag "w"       (NoArg (upd (\dfs -> dfs {warningFlags = IntSet.empty})))
 
         ------ Plugin flags ------------------------------------------------
   , defGhcFlag "fplugin-opt" (hasArg addPluginModuleNameOption)
@@ -3416,6 +3418,7 @@ optLevelFlags -- see Note [Documenting optimisation flags]
 --
 --  * docs/users_guide/using.xml
 
+-- | Warnings enabled unless specified otherwise
 standardWarnings :: [WarningFlag]
 standardWarnings -- see Note [Documenting warning flags]
     = [ Opt_WarnOverlappingPatterns,
@@ -3441,8 +3444,8 @@ standardWarnings -- see Note [Documenting warning flags]
         Opt_WarnTabs
       ]
 
+-- | Things you get with -W
 minusWOpts :: [WarningFlag]
--- Things you get with -W
 minusWOpts
     = standardWarnings ++
       [ Opt_WarnUnusedTopBinds,
@@ -3455,8 +3458,8 @@ minusWOpts
         Opt_WarnDodgyImports
       ]
 
+-- | Things you get with -Wall
 minusWallOpts :: [WarningFlag]
--- Things you get with -Wall
 minusWallOpts
     = minusWOpts ++
       [ Opt_WarnTypeDefaults,
@@ -3469,6 +3472,16 @@ minusWallOpts
         Opt_WarnUntickedPromotedConstructors
       ]
 
+-- | Things you get with -Wcompat.
+--
+-- This is intended to group together warnings that will be enabled by default
+-- at some point in the future, so that library authors eager to make their
+-- code future compatible to fix issues before they even generate warnings.
+minusWcompatOpts :: [WarningFlag]
+minusWcompatOpts
+    = [ Opt_WarnMissingMonadFailInstance
+      ]
+
 enableUnusedBinds :: DynP ()
 enableUnusedBinds = mapM_ setWarningFlag unusedBindsFlags
 
diff --git a/docs/users_guide/7.12.1-notes.rst b/docs/users_guide/7.12.1-notes.rst
index 511111f1a0..4456454503 100644
--- a/docs/users_guide/7.12.1-notes.rst
+++ b/docs/users_guide/7.12.1-notes.rst
@@ -152,6 +152,17 @@ Compiler
 -  When printing an out-of-scope error message, GHC will give helpful advice if
    the error might be caused by too restrictive imports.
 
+-  Added the ``-Wcompat`` warning group, along with its opposite
+   ``-Wno-compat``. Turns on warnings that will be enabled by default in the
+   future, but remain off in normal compilations for the time being. This
+   allows library authors eager to make their code future compatible to adapt
+   to new features before they even generate warnings.
+
+-  Added the ``-fwarn-missing-monadfail-instance`` flag. When enabled, this
+   will issue a warning if a failable pattern is used in a context that does
+   not have a ``MonadFail`` constraint. This flag represents phase 1 of the
+   MonadFail proposal (MFP).
+
 GHCi
 ~~~~
 
diff --git a/docs/users_guide/using-warnings.rst b/docs/users_guide/using-warnings.rst
index 5118168a8c..5bec157b06 100644
--- a/docs/users_guide/using-warnings.rst
+++ b/docs/users_guide/using-warnings.rst
@@ -45,6 +45,23 @@ standard “packages” of warnings:
     ``-fwarn-missing-exported-sigs``, ``-fwarn-missing-import-lists``
     and ``-fwarn-identities``.
 
+``-Wcompat``
+    .. index::
+       single: -Wcompat
+
+    Turns on warnings that will be enabled by default in the future, but remain
+    off in normal compilations for the time being. This allows library authors
+    eager to make their code future compatible to adapt to new features before
+    they even generate warnings.
+
+    This currently enables only ``-fwarn-missing-monadfail-instance``.
+
+``-Wno-compat``
+    .. index::
+       single: -Wno-compat
+
+    Disables all warnings enabled by ``-Wcompat``.
+
 ``-w``
     .. index::
        single: -w
@@ -223,8 +240,9 @@ command line.
     Warn when a failable pattern is used in a do-block that does not have a
     ``MonadFail`` instance.
 
-    This option is off by default, but will be switched on in a future GHC
-    release, as part of the MFP (MonadFail Proposal).
+    Being part of the ``-Wcompat`` option group, this warning is off by
+    default, but will be switched on in a future GHC release, as part of
+    the MFP (MonadFail Proposal).
 
 ``-fwarn-deprecated-flags``
     .. index::
diff --git a/testsuite/tests/wcompat-warnings/WCompatWarningsNotOn.hs b/testsuite/tests/wcompat-warnings/WCompatWarningsNotOn.hs
new file mode 100644
index 0000000000..2f86d46bc2
--- /dev/null
+++ b/testsuite/tests/wcompat-warnings/WCompatWarningsNotOn.hs
@@ -0,0 +1,12 @@
+-- Test purpose:
+-- Ensure that not using -Wcompat does not enable its warnings
+
+-- {-# OPTIONS_GHC -Wcompat #-}
+-- {-# OPTIONS_GHC -Wno-compat #-}
+
+module WCompatWarningsNotOn where
+
+monadFail :: Monad m => m a
+monadFail = do
+    Just _ <- undefined
+    undefined
diff --git a/testsuite/tests/wcompat-warnings/WCompatWarningsOff.hs b/testsuite/tests/wcompat-warnings/WCompatWarningsOff.hs
new file mode 100644
index 0000000000..727a4e7600
--- /dev/null
+++ b/testsuite/tests/wcompat-warnings/WCompatWarningsOff.hs
@@ -0,0 +1,12 @@
+-- Test purpose:
+-- Ensure that using -Wno-compat does not switch on warnings
+
+-- {-# OPTIONS_GHC -Wcompat #-}
+{-# OPTIONS_GHC -Wno-compat #-}
+
+module WCompatWarningsOff where
+
+monadFail :: Monad m => m a
+monadFail = do
+    Just _ <- undefined
+    undefined
diff --git a/testsuite/tests/wcompat-warnings/WCompatWarningsOn.hs b/testsuite/tests/wcompat-warnings/WCompatWarningsOn.hs
new file mode 100644
index 0000000000..29fcc9eeb3
--- /dev/null
+++ b/testsuite/tests/wcompat-warnings/WCompatWarningsOn.hs
@@ -0,0 +1,12 @@
+-- Test purpose:
+-- Ensure that -Wcompat switches on the right warnings
+
+{-# OPTIONS_GHC -Wcompat #-}
+-- {-# OPTIONS_GHC -Wno-compat #-}
+
+module WCompatWarningsOn where
+
+monadFail :: Monad m => m a
+monadFail = do
+    Just _ <- undefined
+    undefined
diff --git a/testsuite/tests/wcompat-warnings/WCompatWarningsOn.stderr b/testsuite/tests/wcompat-warnings/WCompatWarningsOn.stderr
new file mode 100644
index 0000000000..03fc4e26c1
--- /dev/null
+++ b/testsuite/tests/wcompat-warnings/WCompatWarningsOn.stderr
@@ -0,0 +1,21 @@
+
+WCompatWarningsOn.hs:11:5: warning:
+    Could not deduce (MonadFail m)
+      arising from the failable pattern ‘Just _’
+      (this will become an error a future GHC release)
+    from the context: Monad m
+      bound by the type signature for:
+                 monadFail :: Monad m => m a
+      at WCompatWarningsOn.hs:9:14-27
+    Possible fix:
+      add (MonadFail m) to the context of
+        the type signature for:
+          monadFail :: Monad m => m a
+    In a stmt of a 'do' block: Just _ <- undefined
+    In the expression:
+      do { Just _ <- undefined;
+           undefined }
+    In an equation for ‘monadFail’:
+        monadFail
+          = do { Just _ <- undefined;
+                 undefined }
diff --git a/testsuite/tests/wcompat-warnings/WCompatWarningsOnOff.hs b/testsuite/tests/wcompat-warnings/WCompatWarningsOnOff.hs
new file mode 100644
index 0000000000..26d3973702
--- /dev/null
+++ b/testsuite/tests/wcompat-warnings/WCompatWarningsOnOff.hs
@@ -0,0 +1,12 @@
+-- Test purpose:
+-- Ensure that -Wno-compat disables a previously set -Wcompat
+
+{-# OPTIONS_GHC -Wcompat #-}
+{-# OPTIONS_GHC -Wno-compat #-}
+
+module WCompatWarningsOnOff where
+
+monadFail :: Monad m => m a
+monadFail = do
+    Just _ <- undefined
+    undefined
diff --git a/testsuite/tests/wcompat-warnings/all.T b/testsuite/tests/wcompat-warnings/all.T
new file mode 100644
index 0000000000..4447f994cb
--- /dev/null
+++ b/testsuite/tests/wcompat-warnings/all.T
@@ -0,0 +1,4 @@
+test('WCompatWarningsOn',    normal, compile, [''])
+test('WCompatWarningsOff',   normal, compile, [''])
+test('WCompatWarningsNotOn', normal, compile, [''])
+test('WCompatWarningsOnOff', normal, compile, [''])
diff --git a/utils/mkUserGuidePart/Options/Warnings.hs b/utils/mkUserGuidePart/Options/Warnings.hs
index e6c6333108..563ce94ab1 100644
--- a/utils/mkUserGuidePart/Options/Warnings.hs
+++ b/utils/mkUserGuidePart/Options/Warnings.hs
@@ -19,6 +19,13 @@ warningsOptions =
          , flagType = DynamicFlag
          , flagReverse = "-w"
          }
+  , flag { flagName = "-Wcompat"
+         , flagDescription =
+           "enable future compatibility warnings " ++
+           "(details in :ref:`options-sanity`)"
+         , flagType = DynamicFlag
+         , flagReverse = "-Wno-compat"
+         }
   , flag { flagName = "-Werror"
          , flagDescription = "make warnings fatal"
          , flagType = DynamicFlag