[GHC] #12187: Clarify the scoping of existentials for pattern synonym signatures

[GHC]

#12187: Clarify the scoping of existentials for pattern synonym signatures
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        Reporter:  mpickering        |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
                                     |  PatternSynonyms
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #12108            |  Differential Rev(s):
       Wiki Page:                    |
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Description changed by simonpj:

@@ -24,1 +24,1 @@
- This kind of problem is related to #12108
+ This kind of problem is related to #12108, #12179

New description:

 What are scoping rules for type variables in pattern synonym signatures?

 We said that existentials variables are scoped as follows:

 1. Existentials scope over only the provided constraints and the
 arguments, not over the result type.


 {{{
 type Disguised a b = b -> RP a
 pattern Q :: () => Eq b => Disguised a b
 }}}

 However, Simon argues that `Q` should be accepted as if we expand the
 type synonym then the existentially quantified `b` is no longer in the
 result type.

 Richard disagrees, he considers the result type to be `Disguised a b` and
 so `b` is in the result type and not in-scope. Therefore he wishes to
 reject this example.

 The purpose of this ticket is to decide the fate of `Q`.

 This kind of problem is related to #12108, #12179

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12187#comment:5>
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