[GHC] #11128: New `-fwarn-noncanonical-monad-instances` warning

[GHC]

#11128: New `-fwarn-noncanonical-monad-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:                    |  Differential Rev(s):  Phab:D1516
       Wiki Page:                    |
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Comment (by Herbert Valerio Riedel <hvr@…>):

 In [changeset:"f09f2470a76bb08b7f51d2f5663daa672b86f618/ghc"
 f09f2470/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="f09f2470a76bb08b7f51d2f5663daa672b86f618"
 Implement new `-fwarn-noncanonical-monad-instances`

 Warn about incoherent/non-canonical 'Applicative'/'Monad' instance
 declarations. Specifically the following invariants are checked:

 In 'Monad' instances declarations warn if the any of the following
 conditions does not hold:

  * If `return` is overridden it must be canonical (i.e. `return = pure`).
  * If `(>>)` is overridden it must be canonical (i.e. `(>>) = (*>)`).

 In 'Applicative' instance declarations:

  * Warn if 'pure' is defined backwards (i.e. `pure = return`).
  * Warn if '(*>)' is defined backwards (i.e. `(*>) = (>>)`).

 NB, this warning flag is not enabled via `-Wall` nor `-Wcompat`.

 This addresses #11128

 Reviewers: quchen, austin, bgamari

 Reviewed By: bgamari

 Differential Revision: https://phabricator.haskell.org/D1516
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11128#comment:4>
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