[GHC] #12919: Equality not used for substitution

[GHC]

#12919: Equality not used for substitution
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        Reporter:  int-index         |                Owner:  goldfire
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:  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):

 While I don't see what's going wrong here -- and something surely is --
 I'm not as concerned about `flatten_co`. The second return value from the
 function is always ignored and should indeed be removed. It is a coercion
 relating the output coercion to the input, as constructed by
 `mkProofIrrelCo`. (Yes, this is a coercion relating two coercions.) These
 coercions are cheap to make, as any two coercions proving the same
 proposition are considered equal.

 Is this holding you up today? Or is it OK to wait a few weeks? I see the
 "highest" priority and will surely fix for 8.2.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12919#comment:5>
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