[GHC] #9291: Don't reconstruct sum types if the type subtly changes

[GHC]

#9291: Don't reconstruct sum types if the type subtly changes
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        Reporter:  schyler  |            Owner:
            Type:  feature  |           Status:  new
  request                   |        Milestone:
        Priority:  normal   |          Version:  7.8.2
       Component:           |         Keywords:
  Compiler                  |     Architecture:  Unknown/Multiple
      Resolution:           |       Difficulty:  Moderate (less than a day)
Operating System:           |       Blocked By:
  Unknown/Multiple          |  Related Tickets:
 Type of failure:           |
  None/Unknown              |
       Test Case:           |
        Blocking:           |
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Comment (by nomeata):

 Are Core’s coercions expressible enough for this? I don’t think so. So one
 would have to resort to `unsafeCoerce` here, which wouldn’t be nice.

 Or this can be done at the STG stage, but then we might miss further
 optimizations.

 Tricky.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9291#comment:2>
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