[GHC] #10195: GHC forgets constraints when matching on GADTs

[GHC]

#10195: GHC forgets constraints when matching on GADTs
-------------------------------------+-------------------------------------
        Reporter:  crockeea          |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.8.4
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
-------------------------------------+-------------------------------------

Comment (by crockeea):

 Can you confirm in 7.10?

 Since we have no idea what's causing the issue, nor exactly what "this
 situation" is, it's possible this behavior could occur for a less generic
 instance in a situation where the instance constraint can't just be
 trivially promoted to a superclass constraint.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10195#comment:2>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler