[GHC] #14938: Pattern matching on GADT does not refine type family parameters

[GHC]

#14938: Pattern matching on GADT does not refine type family parameters
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        Reporter:  kcsongor          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.2.2
  checker)                           |             Keywords:  GADTs,
      Resolution:                    |  TypeFamilies
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:                    |
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Comment (by goldfire):

 But if you have a type-level `Refl` here, then `x` and `y` really will be
 the same. The equation you want is `R x x Refl = Refl`, like in those
 other dependently typed languages. If you write a larger example, I can
 show you how the equation I suggest will work.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14938#comment:1>
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