[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 nomeata):

 Do you intentionally disallow `return x = pure x`, which some people might
 write? (No strong opinion here, having precisely one official way is
 probably desirable, but let’s at least document that it has been given
 thought.)

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11128#comment:2>
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