[GHC] #9376: More informative error messages when closed type families fail to simplify

[GHC]

#9376: More informative error messages when closed type families fail to simplify
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              Reporter:              |            Owner:
  MikeIzbicki                        |           Status:  new
                  Type:  feature     |        Milestone:
  request                            |          Version:  7.8.2
              Priority:  normal      |         Keywords:
             Component:  Compiler    |     Architecture:  Unknown/Multiple
  (Type checker)                     |       Difficulty:  Unknown
            Resolution:              |       Blocked By:
      Operating System:              |  Related Tickets:
  Unknown/Multiple                   |
       Type of failure:              |
  None/Unknown                       |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by goldfire):

 Replying to [comment:4 simonpj]:
 > Is the test for rhss-unify important?  How bad would it be if the
 compatibility check ignored the RHS?

 The crux of compatibility is that the RHSs unify under the substitution.
 Without that check, what would compatibility mean? Or, do I misunderstand
 your question?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9376#comment:5>
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