[GHC] #9725: Constraint deduction failure

[GHC]

#9725: Constraint deduction failure
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              Reporter:  heisenbug   |            Owner:
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:
             Component:  Compiler    |          Version:  7.8.3
  (Type checker)                     |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:  GHC         |  Related Tickets:
  rejects valid program              |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by simonpj):

 Ah I see. You are right. But this is pretty coincidental.  If the wanted
 was `[W] ent ~ M` we'd be out of luck.  And taking account of the
 coincidence is far from straightforward; it would be a significant
 complication in the solver, which will be redundant when your stuff lands.

 So I propose to wait.  Sorry Gabor, but Richard's stuff is itself a major
 increment, and I just don't know what a simple cut-down version might look
 like.  ETA is up to Richard.  Meanwhile, `usafeCoerce` may be needed.

 Simon

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9725#comment:11>
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