[GHC] #13267: Constraint synonym instances

[GHC]

#13267: Constraint synonym instances
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        Reporter:  ezyang            |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC accepts       |  Unknown/Multiple
  invalid program                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"c7508083388a71d76a5b6f1e46adfbcffba74b96/ghc"
 c7508083/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="c7508083388a71d76a5b6f1e46adfbcffba74b96"
 Disallow class instances for synonyms

 See Trac #13267 and Note [Instances and constraint synonyms]
 in TcValidity.

 We can't easily do a perfect job, because the rename is really trying
 to do its lookup too early.  But this is at least an improvement.
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13267#comment:5>
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