[GHC] #9725: Constraint deduction failure

[GHC]

#9725: Constraint deduction failure
-------------------------------------+-------------------------------------
              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 goldfire):

 But you don't have to decompose the given equality to get the wanted. We
 have

 {{{
  [G] e1 :: ent k1 ~ M b
  [G] e2 :: Kn (M b)

  [W] e3 :: Kn (ent k1)
 }}}

 Solve with

 {{{
   e3 := e2 |> Kn (sym e1)
 }}}

 The problem is that GHC tries to decompose the given, fails utterly, and
 then has no way forward. So, I'd say this shows a completeness bug in the
 solver. It's one we may not want to address, as I agree my kind equality
 stuff makes this straightforward and I don't see an easy way of solving
 this problem.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9725#comment:10>
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