[GHC] #10897: Incorrect ASSERT for buildPatSyn

[GHC]

#10897: Incorrect ASSERT for buildPatSyn
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        Reporter:  ezyang            |                   Owner:  cactus
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler (Type    |                 Version:  7.11
  checker)                           |                Keywords:
      Resolution:                    |  PatternSynonyms
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash                              |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Comment (by cactus):

 Hmm, it's starting to come back -- all those `ifPat*` type fields of
 `IfacePatSyn` are needed so that we can implement `pprIfaceDecl` and
 `freeNamesIfDecl` without the ability to turn the `IfExtName` of
 `ifPatMatcher` into a proper `Id`. That suggests we should look into it a
 bit more to see if it's valid when these self-confessed redundant fields
 aren't actually the same as the types recovered from the `Id` of the
 matcher.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10897#comment:3>
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