[GHC] #13770: HEAD: Type mentioned in error won't show up in pattern signature

[GHC]

#13770: HEAD: Type mentioned in error won't show up in pattern signature
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        Reporter:  heisenbug         |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by heisenbug):

 I believe the `r4` unification-variable originates from the two wildcard
 arguments
 (https://ghc.haskell.org/trac/ghc/attachment/ticket/13770/T13770.hs#L27)
 hidden in `Fun{}`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13770#comment:2>
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