[GHC] #11367: [Regression] Only one clause allowed in (explicitly bidirectional) pattern synonyms

[GHC]

#11367: [Regression] Only one clause allowed in (explicitly bidirectional) pattern
synonyms
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        Reporter:  Iceland_jack      |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
                                     |  PatternSynonyms
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Iceland_jack):

 Replying to [comment:10 mpickering]:
 > I think this reply is a mixture of a new ticket, #393 and some more
 context for this ticket.

 Thanks mpickering, #393 looks promising. I have a keyboard macro for
 inserting `"undefined "`, if it got implemented I would use it
 aggressively:

 {{{
 >>> data X
 >>> data Y
 >>> f :: X → Y
 >>> :t map f
 map f :: [X] -> [Y]
 }}}

 looks great.

 Could [[ExplicitCallStack/ImplicitLocations]] be an alternative in #393 to
 `-fdefer-type-errors` + typed holes

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11367#comment:11>
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