[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 Ben Gamari <ben@…>):

 In [changeset:"f40e122b7709c11d4ad20fd5cb26bf719235dbf1/ghc" f40e122/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="f40e122b7709c11d4ad20fd5cb26bf719235dbf1"
 Fix typechecking for pattern synonym signatures

 Various tickets have revealed bad shortcomings in the typechecking of
 pattern type synonyms.  Discussed a lot in (the latter part of)
 Trac #11224.

 This patch fixes the most complex issues:

 - Both parser and renamer now treat pattern synonyms as an
   ordinary LHsSigType.  Nothing special.  Hooray.

 - tcPatSynSig (now in TcPatSyn) typechecks the signature, and
   decomposes it into its pieces.
   See Note [Pattern synonym signatures]

 - tcCheckPatSyn has had a lot of refactoring.
   See Note [Checking against a pattern signature]

 The result is a lot tidier and more comprehensible.
 Plus, it actually works!

 NB: this patch doesn't actually address the precise
     target of #11224, namely "inlining pattern synonym
     does not preserve semantics".  That's an unrelated
     bug, with a separate patch.

 ToDo: better documentation in the user manual

 Test Plan: Validate

 Reviewers: austin, hvr, goldfire

 Subscribers: goldfire, mpickering, thomie, simonpj

 Differential Revision: https://phabricator.haskell.org/D1685

 GHC Trac Issues: #11224
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11224#comment:26>
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