[GHC] #13658: Assertion failure on HEAD: "optCoercion changed types!"

[GHC]

#13658: Assertion failure on HEAD: "optCoercion changed types!"
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        Reporter:  bgamari           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by slyfox):

 The same error is also reproducible on a ghc-HEAD with this tiny
 '''mk/build.mk''':
 {{{
 SRC_HC_OPTS += -g -dcore-lint -DDEBUG
 }}}

 I'll try to extract something smaller.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13658#comment:2>
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