[GHC] #13410: GHC HEAD regression: Template variable unbound in rewrite rule

[GHC]

#13410: GHC HEAD regression: Template variable unbound in rewrite rule
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        Reporter:  RyanGlScott       |                Owner:  simonpj
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:  JoinPoints,
                                     |  SpecConstr
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by simonpj):

 * owner:  (none) => simonpj


Comment:

 I know exactly what is happening here.   Just starved of time to fix it --
 and next week looks little better.

 In brief: `TyCoRep.substCoVarBndr` is replacing occurrences of the binder
 with `Refl` in the LHS of a rule.  Solution: `mkCoVarCo` should not
 replace varaibles with `Refl`; but `optCoercion` should do so instead.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13410#comment:9>
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