[GHC] #11150: New `-fwarn-noncanonical-monoid-instances` warning

[GHC]

#11150: New `-fwarn-noncanonical-monoid-instances` warning
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        Reporter:  hvr               |                Owner:  hvr
            Type:  feature request   |               Status:  patch
        Priority:  normal            |            Milestone:  8.0.1
       Component:  Compiler          |              Version:  7.10.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #11128, #11139    |  Differential Rev(s):  Phab:D1553
       Wiki Page:                    |
  prime:Libraries/Proposals/SemigroupMonoid|
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Comment (by Herbert Valerio Riedel <hvr@…>):

 In [changeset:"986ceb1679b501414b996c520b08ce929a40f94c/ghc"
 986ceb16/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="986ceb1679b501414b996c520b08ce929a40f94c"
 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
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11150#comment:2>
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