[GHC] #11368: Pattern synonym name is mangled when patterns are non-exhaustive

[GHC]

#11368: Pattern synonym name is mangled when patterns are non-exhaustive
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        Reporter:  Iceland_jack  |                Owner:  mpickering
            Type:  bug           |               Status:  new
        Priority:  low           |            Milestone:
       Component:  Compiler      |              Version:  8.1
      Resolution:                |             Keywords:  PatternSynonyms
Operating System:  Linux         |         Architecture:  x86
 Type of failure:  Other         |            Test Case:
      Blocked By:                |             Blocking:
 Related Tickets:  #11367        |  Differential Rev(s):
       Wiki Page:                |
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Comment (by mpickering):

 This actually looks a bit more fiddly than I thought. After typechecking
 the pat syn is split into the builder and the matcher. Thus in the
 desugaring, the buider is just treated like any other function bind (line
 128 in DsBinds) which is the cause of the bad error message.

 A way to fix this would be to add a new constructor to IdDetails called
 PatSynBuilder which carries the PatSyn which was used to build the
 relevant Id. FunBind could then be changed to check whether the Id comes
 from a pat syn and modify the error message as appropriate.

 Does anyone else see an easier way? I guess the error messages should
 refer to the right things so I'll have to make these changes.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11368#comment:3>
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