[GHC] #11224: Program doesn't preserve semantics after pattern synonym inlining.

[GHC]

#11224: Program doesn't preserve semantics after pattern synonym inlining.
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        Reporter:  anton.dubovik     |                Owner:
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.0.1
       Component:  Compiler          |              Version:  7.10.2
      Resolution:                    |             Keywords:
                                     |  PatternSynonyms
Operating System:  Windows           |         Architecture:  x86_64
 Type of failure:  Incorrect result  |  (amd64)
  at runtime                         |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #11225            |  Differential Rev(s):  Phab:D1632
       Wiki Page:                    |
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Comment (by goldfire):

 But how should we tell universals from existentials? If I say

 {{{
 pattern type P :: a -> String
 }}}

 is `a` universal or existential? It seems mpickering wants the answer to
 be "it depends on the definition for `P`". That may well be implementable,
 but it negates a critical property of type signatures: that they give a
 specification of how a pattern synonym behaves absent its definition. We
 somehow have to be able to specify this in the signature itself.

 I don't have much perspective on Simon's comment:22. It's about
 implementation, and I'm still stuck on design.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11224#comment:23>
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