[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:  closed
        Priority:  normal            |            Milestone:  8.0.1
       Component:  Compiler          |              Version:  7.10.2
      Resolution:  fixed             |             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:"fd6dd41c67f3bd23bbf074357219cfd251eb53d6/ghc"
 fd6dd41c/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="fd6dd41c67f3bd23bbf074357219cfd251eb53d6"
 Implement `-Wnoncanonical-monadfail-instances` warning

 The MonadFail proposal implemented so far via #10751 only warns about
 missing `MonadFail` instances based on existence of failible pattern
 matches in `do`-blocks.

 However, based on the noncanonical Monad warnings implemented via #11150
 we can provide a different mechanism for detecting missing `MonadFail`
 instances quite cheaply. That is, by checking for canonical `fail`
 definitions.

 In the case of `Monad`/`MonadFail`, we define the canonical implementation
 of
 `fail` to be such that the soft-deprecated method shall (iff overridden)
 be
 defined in terms of the non-deprecated method. Consequently, in case of
 `MonadFail`, the `Monad(fail)` method shall be defined as alias of
 the `MonadFail(fail)` method.

 This allows us at some distant point in the future to remove `fail` from
 the `Monad` class, while having GHC ignore/tolerate such literal canonical
 method definitions.

 Reviewed By: bgamari, RyanGlScott

 Differential Revision: https://phabricator.haskell.org/D1838
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11150#comment:4>
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