[GHC] #13430: Can't scope type variables when pattern matching on GADTs

[GHC]

#13430: Can't scope type variables when pattern matching on GADTs
-------------------------------------+-------------------------------------
        Reporter:  crockeea          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.2
      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):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by simonpj):

 BTW here is a workaround:
 {{{
 bar :: Bar x -> String
 bar x@(MkBar _)
   = case x of
       (x :: Bar (a, b)) -> show (typeRep (Proxy :: Proxy b))
 }}}
 Here, ''inside'' the pattern match for `MkBar` we make a new pattern-match
 on `x`, and give it a scoped type signature.

 Not beautiful but it works.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13430#comment:2>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler