[GHC] #14366: Type family equation refuses to unify wildcard type patterns

[GHC]

#14366: Type family equation refuses to unify wildcard type patterns
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.2.1
  checker)                           |             Keywords:  TypeFamilies,
      Resolution:                    |  TypeInType
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):

 I could go either way on this one. (Where I'm choosing between "that's
 correct behavior" and "that's a bug".) I interpret `_` in a type family
 equation to mean the same as a fresh type variable. And I also think that
 `Cast a b Refl x = x` is suspect. On the other hand, this is a bit silly
 of GHC not to unify.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14366#comment:1>
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