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

[GHC]

#9291: Don't reconstruct sum types if the type subtly changes
----------------------------+----------------------------------------------
        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:           |
----------------------------+----------------------------------------------

Comment (by goldfire):

 That's an interesting idea, perhaps somewhat saner than dependently-typed
 coercions. But, proving such a thing type-safe would be quite a bear, I
 think. It seems to hinge on parametricity, which is not yet proven for
 Haskell or Core.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9291#comment:7>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler