[GHC] #13381: "opt_univ fell into a hole" panic from rewrite rule

[GHC]

#13381: "opt_univ fell into a hole" panic from rewrite rule
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        Reporter:  dfeuer            |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
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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Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"af6ed4a62c77e57f544243aa72bba51a1cff6808/ghc"
 af6ed4a6/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="af6ed4a62c77e57f544243aa72bba51a1cff6808"
 Fix constraint simplification in rules

 Trac #13381 showed that we were losing track of a wanted constraint
 when simplifying the LHS constraints for a RULE.

 This patch fixes it, makes the code a bit simpler, and better
 documented.
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13381#comment:4>
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