[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):

 If I understand correctly, Simon's comment:24 proposes precisely this,
 copied from comment:8:

   * Allow explicit quantification in two places, according to this schema:
 {{{
     pattern type Syn :: forall <<univs>>. <<req>> => forall <<exs>>.
 <<prov>> => <<args>> -> <<result>>
 }}}

   * In the absence of explicit quantification, universals are inferred to
 be the variables mentioned in either the result or the required context.
 Existentials are the remaining variables.

   * It is against the rules for existentials to shadow universals.

 Simon, do you agree that we're saying the same thing? If so, then let's do
 it!

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11224#comment:25>
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