[GHC] #5821: SPECIALISE fails with a cryptic warning

[GHC]

#5821: SPECIALISE fails with a cryptic warning
-------------------------------------+-------------------------------------
        Reporter:  rl                |                   Owner:  ezyang
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:  7.10.1
       Component:  Compiler          |                 Version:  7.5
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  warning at compile-time            |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:  Phab:D610
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Changes (by ezyang):

 * differential:   => Phab:D610


Comment:

 The ideal resolution is to teach the RULE engine how to deal with type
 families, but until then I've just augmented the error message with a
 little note.

 It might even be possible to look at type and realize that type families
 are involved (and maybe even suggest a better type) but that seems way
 more involved than just fixing it.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/5821#comment:11>
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