[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'm pretty sure Agda used to reject having two different variables here,
 though it seems to be accepting it now. (Any Agda experts out there?)

 I think the pattern-signature version is a red herring, because those
 variables are meant to stand in for others -- indeed, that's their whole
 point.

 But I'm still not terribly bothered by this rejection. Yes, I suppose it's
 better to accept here, but I don't think that goal is easily achieved.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14366#comment:4>
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