[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):

 The type system of Core is actually _better_ than that of Haskell (for
 some value of better). And having a typed intermediate language has many
 benefits, e.g. it is easier to spot bugs in the core-to-core
 transformations.

 An example for what happens when you use unsafeCoerce, even internally:
 GeneralizedNewtypeDeriving used to be implemented this way, and together
 with Type Families or GADTS it was suddenly possible to crash a Haskell
 program.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9291#comment:4>
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