[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 request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.8.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by nomeata):

 Uh. Good point. I guess we’d need a linear type system to prevent your
 illegal double-use of `eta_B1` here :-)
 (It is not really “case-of-case” that you are doing there, but your point
 still holds.)

 Note that this would be an illegal transformation not only for
 `writeMutVar#` but for anything that is not provabley total, even a pure
 but non-terminating function.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9291#comment:38>
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