[GHC] #15675: Type operators in existential context cannot be parsed

[GHC]

#15675: Type operators in existential context cannot be parsed
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        Reporter:  int-index         |                Owner:  int-index
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Compiler          |              Version:  8.4.3
  (Parser)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D5180
       Wiki Page:                    |
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Comment (by Ryan Scott <ryan.gl.scott@…>):

 In [changeset:"bd7898537768f936d05c0c83eef1cd9b00933347/ghc"
 bd789853/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="bd7898537768f936d05c0c83eef1cd9b00933347"
 Parse the (!) type operator and allow type operators in existential
 context

 Summary:
 Improve the way `(!)`, `(~)`, and other type operators are handled in the
 parser,
 fixing two issues at once:

 1. `(!)` can now be used as a type operator
    that respects fixity and precedence (#15457)
 2. Existential context of a data constructor
    no longer needs parentheses (#15675)

 In addition to that, with this patch it is now trivial to adjust
 precedence of
 the `{-# UNPACK #-}` pragma, as suggested in
 https://ghc.haskell.org/trac/ghc/ticket/14761#comment:7

 There was a small change to API Annotations. Before this patch, `(~)` was
 a
 strange special case that produced an annotation unlike any other type
 operator. After this patch, when `(~)` or `(!)` are used to specify
 strictness they
 produce AnnTilde and AnnBang annotations respectively, and when they are
 used
 as type operators, they produce no annotations.

 Test Plan: Validate

 Reviewers: simonpj, bgamari, alanz, RyanGlScott

 Reviewed By: RyanGlScott

 Subscribers: RyanGlScott, rwbarton, mpickering, carter

 GHC Trac Issues: #15457, #15675

 Differential Revision: https://phabricator.haskell.org/D5180
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15675#comment:2>
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