[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 Simon Peyton Jones <simonpj@…>):

 In [changeset:"fb050a330ad202c1eb43038dc18cca2a5be26f4a/ghc" fb050a3/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="fb050a330ad202c1eb43038dc18cca2a5be26f4a"
 Do not bind coercion variables in SpecConstr rules

 Trac #14270 showed that SpecConstr could cause nasty Lint failures
 if it generates a RULE that binds coercion varables.  See

  * Note [SpecConstr and casts], and
  * the test simplCore/should_compile/T14270.

 This doesn't feel like the final word to me, because somehow the
 specialisation "ought" to work.  So I left in a debug WARN to yell if
 the new check acutally fires.

 Meanwhile, it stops the erroneous specialisation.

 binding coercion
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14270#comment:20>
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