[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 RyanGlScott):

 Ah, you bring up a good point. I suppose the thing we really should be
 scrutinizng is `Cast a b Refl x = x`. I'm also a bit baffled that that
 doesn't work either—after all, something quite similar works at the term
 level!

 {{{#!hs
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeOperators #-}

 import Data.Type.Equality

 cast :: a :~: b -> a -> b -> Int
 cast Refl (_ :: a) (_ :: b) = 42
 }}}

 I'm aware that this is a bit of a strawman, since comparing type families
 and scoped type pattern variables is a bit apples-and-oranges. But surely
 you can see the point I'm driving at here—if `a` and `b` are equated by
 the pattern match on `Refl`, should we reject the use of two syntactically
 different type variables for the second and third arguments of `cast`?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14366#comment:2>
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