[GHC] #9792: map/coerce rule never seems to fire

[GHC]

#9792: map/coerce rule never seems to fire
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              Reporter:  dfeuer      |            Owner:  ekmett
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:  7.10.1
             Component:  Core        |          Version:  7.9
  Libraries                          |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:  Runtime     |  Related Tickets:
  performance bug                    |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by nomeata):

 Tracking the stack depths is to avoid generating looping code at runtime.
 But we don’t have this guarantee in Core, and the desugarer + typechecker
 is too large a beast to give any guarantees besides best-effort, I’d say.

 Maybe it is possible to abstract over the constraints somehow, and write a
 constraint with value ⊥ in the general case, circumventing any special
 casing for Coercible?

 (But even if it is possible it includes so much malice that whoever does
 that waives any right to complain about `map coerce = coerce` :-))

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9792#comment:8>
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