Implement new `-fwarn-noncanonical-monoid-instances`

This is similiar to the `-fwarn-noncanonical-monad-instances` warning
implemented via #11128, but applies to `Semigroup`/`Monoid` instead
and the `(<>)`/`mappend` methods (of which `mappend` is planned to move
out of `Monoid` at some point in the future being redundant and thus
error-prone).

This warning is contained in `-Wcompat` but not in `-Wall`.

This addresses #11150

Reviewed By: quchen

Differential Revision: https://phabricator.haskell.org/D1553

5 files changed, 61 insertions, 3 deletions

diff --git a/testsuite/tests/wcompat-warnings/WCompatWarningsNotOn.hs b/testsuite/tests/wcompat-warnings/WCompatWarningsNotOn.hs
index 24cab851c9..64a19e5cf9 100644
--- a/testsuite/tests/wcompat-warnings/WCompatWarningsNotOn.hs
+++ b/testsuite/tests/wcompat-warnings/WCompatWarningsNotOn.hs
@@ -6,9 +6,21 @@
 
 module WCompatWarningsNotOn where
 
+import qualified Data.Semigroup as Semi
+
 monadFail :: Monad m => m a
 monadFail = do
     Just _ <- undefined
     undefined
 
 (<>) = undefined -- Semigroup warnings
+
+-- -fwarn-noncanonical-monoid-instances
+newtype S = S Int
+
+instance Semi.Semigroup S where
+  (<>) = mappend
+
+instance Semi.Monoid S where
+  S a `mappend` S b = S (a+b)
+  mempty = S 0
diff --git a/testsuite/tests/wcompat-warnings/WCompatWarningsOff.hs b/testsuite/tests/wcompat-warnings/WCompatWarningsOff.hs
index 4c53a1e4ea..6ed25f1ef7 100644
--- a/testsuite/tests/wcompat-warnings/WCompatWarningsOff.hs
+++ b/testsuite/tests/wcompat-warnings/WCompatWarningsOff.hs
@@ -6,9 +6,21 @@
 
 module WCompatWarningsOff where
 
+import qualified Data.Semigroup as Semi
+
 monadFail :: Monad m => m a
 monadFail = do
     Just _ <- undefined
     undefined
 
 (<>) = undefined -- Semigroup warnings
+
+-- -fwarn-noncanonical-monoid-instances
+newtype S = S Int
+
+instance Semi.Semigroup S where
+  (<>) = mappend
+
+instance Semi.Monoid S where
+  S a `mappend` S b = S (a+b)
+  mempty = S 0
diff --git a/testsuite/tests/wcompat-warnings/WCompatWarningsOn.hs b/testsuite/tests/wcompat-warnings/WCompatWarningsOn.hs
index 3b2586aff8..c155f37f42 100644
--- a/testsuite/tests/wcompat-warnings/WCompatWarningsOn.hs
+++ b/testsuite/tests/wcompat-warnings/WCompatWarningsOn.hs
@@ -6,9 +6,21 @@
 
 module WCompatWarningsOn where
 
+import qualified Data.Semigroup as Semi
+
 monadFail :: Monad m => m a
 monadFail = do
     Just _ <- undefined
     undefined
 
 (<>) = undefined -- Semigroup warnings
+
+-- -fwarn-noncanonical-monoid-instances
+newtype S = S Int
+
+instance Semi.Semigroup S where
+  (<>) = mappend
+
+instance Semi.Monoid S where
+  S a `mappend` S b = S (a+b)
+  mempty = S 0
diff --git a/testsuite/tests/wcompat-warnings/WCompatWarningsOn.stderr b/testsuite/tests/wcompat-warnings/WCompatWarningsOn.stderr
index 7b6b501708..5de8745544 100644
--- a/testsuite/tests/wcompat-warnings/WCompatWarningsOn.stderr
+++ b/testsuite/tests/wcompat-warnings/WCompatWarningsOn.stderr
@@ -1,12 +1,12 @@
 
-WCompatWarningsOn.hs:11:5: warning:
+WCompatWarningsOn.hs:13:5: 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:9:1-27
+        at WCompatWarningsOn.hs:11:1-27
       Possible fix:
         add (MonadFail m) to the context of
           the type signature for:
@@ -20,6 +20,16 @@ WCompatWarningsOn.hs:11:5: warning:
             = do { Just _ <- undefined;
                    undefined }
 
-WCompatWarningsOn.hs:14:1: warning:
+WCompatWarningsOn.hs:16:1: warning:
     Local definition of ‘<>’ clashes with a future Prelude name.
     This will become an error in a future release.
+
+WCompatWarningsOn.hs:22:3: warning:
+    Noncanonical ‘(<>) = mappend’ definition detected
+    in the instance declaration for ‘Semigroup S’.
+    Move definition from ‘mappend’ to ‘(<>)’
+
+WCompatWarningsOn.hs:25:3: warning:
+    Noncanonical ‘mappend’ definition detected
+    in the instance declaration for ‘Monoid S’.
+    Define as ‘mappend = (<>)’
diff --git a/testsuite/tests/wcompat-warnings/WCompatWarningsOnOff.hs b/testsuite/tests/wcompat-warnings/WCompatWarningsOnOff.hs
index 2f4aedff23..44f554ee47 100644
--- a/testsuite/tests/wcompat-warnings/WCompatWarningsOnOff.hs
+++ b/testsuite/tests/wcompat-warnings/WCompatWarningsOnOff.hs
@@ -6,9 +6,21 @@
 
 module WCompatWarningsOnOff where
 
+import qualified Data.Semigroup as Semi
+
 monadFail :: Monad m => m a
 monadFail = do
     Just _ <- undefined
     undefined
 
 (<>) = undefined -- Semigroup warnings
+
+-- -fwarn-noncanonical-monoid-instances
+newtype S = S Int
+
+instance Semi.Semigroup S where
+  (<>) = mappend
+
+instance Semi.Monoid S where
+  S a `mappend` S b = S (a+b)
+  mempty = S 0