[GHC] #12814: Should GND infer an instance context when deriving method-free classes?

[GHC]

#12814: Should GND infer an instance context when deriving method-free classes?
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        Reporter:  RyanGlScott       |                Owner:  RyanGlScott
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #11369, #12810    |  Differential Rev(s):  Phab:D2692
       Wiki Page:                    |
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Comment (by Ben Gamari <ben@…>):

 In [changeset:"03e8d26fe0a8f7981a76550118f3c584cad76c47/ghc"
 03e8d26f/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="03e8d26fe0a8f7981a76550118f3c584cad76c47"
 Prevent GND from inferring an instance context for method-less classes

 When `GeneralizedNewtypeDeriving` is used with a type class that has no
 methods, it will generate a redundant context, and as a result, it can
 trigger warnings when compiled with `-Wredundant-constraints`. This is a
 simple change in behavior to check beforehand if a class has methods
 when deriving it with GND, and if it has no methods, avoid inferring the
 redundant context.

 Beware that the test for #6088, which used to be expected to fail, now
 compiles without issue since it doesn't infer a problematic instance
 context.

 Thanks to Simon Peyton Jones for doing the necessary refactoring in
 f05d685ae05ec293083f2fa7ec7ba057fbe64869.

 Fixes #12814.

 Test Plan: ./validate

 Reviewers: goldfire, rwbarton, simonpj, austin, bgamari

 Reviewed By: simonpj

 Subscribers: thomie

 Differential Revision: https://phabricator.haskell.org/D2692

 GHC Trac Issues: #12814
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12814#comment:15>
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