[GHC] #14270: GHC HEAD's ghc-stage1 panics on Data.Typeable.Internal

[GHC]

#14270: GHC HEAD's ghc-stage1 panics on Data.Typeable.Internal
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        Reporter:  hvr               |                Owner:  bgamari
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.4.1
       Component:  Compiler          |              Version:  8.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #14236            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by bgamari):

 I tried extending the rule matching substitution with the CvSubst from the
 unification. Unfortunately, it seems as though the unifier returns an
 empty CvSubst. Perhaps this is because of the fact that the template
 variable appears both in the LHS of the rule and the argument being
 matched. Consequently the unifier may not extend its CvSubst when
 unifying, since doing so would mean adding a substitution `cobox_a3S9 :->
 cobox_a3S9` to the substitution. This would violate the invariant of
 matching that none of the template variables appear in the range of the
 resulting substitution (as described in, e.g., `Note [Matching in the
 presence of casts]`).


 Also, for future reference: The coercions in question come in to scope as
 the result of a pattern match on an `HRefl`,
 {{{#!hs
 case ds_d4wx of { HRefl cobox_a3S8 cobox_a3S9 ->
   ...
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14270#comment:4>
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