Add noncanonical mappend (#497)

* Add noncanonical mappend

* Add noncanonical mappend error message

Author: noughtmare

message-index/messages/GHC-50928/index.md

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+---
+title: Noncanonical `mappend` definition
+summary: A Monoid instance uses a custom `mappend` implementation rather than re-using the default `(<>)` operation from its Semigroup instance.
+severity: warning
+flag: -Wnoncanonical-monoid-instances
+introduced: 9.8.1
+---
+
+As part of the Semigroup Monoid proposal, the Semigroup class is now a
+superclass of Monoid. Now, the `mappend` function is redundant and should
+always do exactly the same as the `(<>)` function of the Semigroup instance.
+In the future, the `mappend` function may even be removed completely.
+
+So, you should adapt to these changes by either:
+
+* Changing the definition of `mappend` to use `(<>)`. This is backwards compatible, but may break in the future.
+* Removing the definition of `mappend` and rely on the default instance which uses `(<>)`. This is forwards compatible, but breaks your code for GHC 8.2 or earlier.
+

message-index/messages/GHC-50928/noncanonical-mappend/after/NoncanonicalMappend.hs

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+module NoncanonicalMappend where
+
+data A = A
+
+instance Semigroup A where
+  A <> A = A
+
+instance Monoid A where
+  mempty = A
+  mappend = (<>)
+

message-index/messages/GHC-50928/noncanonical-mappend/before/NoncanonicalMappend.hs

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+module NoncanonicalMappend where
+
+data A = A
+
+instance Semigroup A where
+  A <> A = A
+
+instance Monoid A where
+  mempty = A
+  mappend A A = A
+

message-index/messages/GHC-50928/noncanonical-mappend/index.md

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+---
+title: Noncanonical `mappend` definition.
+---
+
+## Error Message
+
+```
+NoncanonicalMappend.hs:10:3: warning: [GHC-50928] [-Wnoncanonical-monoid-instances]
+    Noncanonical ‘mappend’ definition detected
+    in the instance declaration for ‘Monoid A’.
+    ‘mappend’ will eventually be removed in favour of ‘(<>)’
+    Suggested fix:
+      Either remove definition for ‘mappend’ (recommended) or define as ‘mappend = (<>)’
+      See also: https://gitlab.haskell.org/ghc/ghc/-/wikis/proposal/semigroup-monoid
+   |
+10 |   mappend A A = A
+   |   ^^^^^^^^^^^^^^^
+```