[GHC] #13768: Incorrect warnings generated by exhaustiveness checker with pattern synonyms / GADT combination

[GHC]

#13768: Incorrect warnings generated by exhaustiveness checker with pattern
synonyms / GADT combination
-------------------------------------+-------------------------------------
        Reporter:  kosmikus          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:
       Component:  Compiler          |              Version:  8.2.1-rc2
      Resolution:                    |             Keywords:
                                     |  PatternSynonyms
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"6b77914cd37b697354611bcd87897885c1e5b4a6/ghc" 6b77914/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="6b77914cd37b697354611bcd87897885c1e5b4a6"
 Fix instantiation of pattern synonyms

 In Check.hs (pattern match ovelap checking) we to figure out the
 instantiation of a pattern synonym from the type of the pattern. We
 were doing this utterly wrongly.  Trac #13768 demonstrated this
 bogosity.

 The fix is easy; and is described in PatSyn.hs
   Note [Pattern synonym result type]
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13768#comment:2>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler