[GHC] #11642: Heterogeneous type equality evidence ignored

[GHC]

#11642: Heterogeneous type equality evidence ignored
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        Reporter:  bgamari           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.0.1
       Component:  Compiler (Type    |              Version:  7.10.3
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by bgamari):

 Judging by the `-ddump-tc-trace` output (for the fully expanded `case`
 version
 given in comment:2), the problem is that this wanted is unsolved,
 {{{
 (~#) k_a1Lg[ssk] k_a1Lg[ssk]
      x_a1Li[ssk] ((TYPE |> <Levity>_N -> Sym cobox_a1Mi) 'Lifted)
 (CNonCanonical)}
 }}}

 given these givens,
 {{{
 (~#) * *
      k_a1L4[ssk] (f_a1Ld[ssk] x_a1Le[ssk])

 (~#) (k_a1Lc[ssk] -> *) (k_a1Lc[ssk] -> *)
      f_a1Ld[ssk] (f_a1Lh[ssk] x_a1Li[ssk])

 (~#) * *
      (* -> * -> *) (k_a1Lg[ssk] -> k_a1Lc[ssk] -> *)

 (~#) (* -> * -> *) (k_a1Lg[ssk] -> k_a1Lc[ssk] -> *)
      (->) f_a1Lh[ssk]

 (~#) * *
      k_a1Lg[ssk] k_a1L6[ssk]

 (~#) k_a1Lg[ssk] k_a1L6[ssk]
      x_a1Li[ssk] a_a1L7[ssk]
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11642#comment:5>
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