[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:  8.4.1
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  GHC rejects       |            Test Case:
  valid program                      |  typecheck/should_compile/T12919
      Blocked By:                    |             Blocking:
 Related Tickets:  #13643            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 Does this patch fix the panic in #14024?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12919#comment:15>
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