[GHC] #13487: GHC panic with deferred custom type errors

[GHC]

#13487: GHC panic with deferred custom type errors
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        Reporter:  DimaSamoz         |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.4.1
       Component:  GHCi              |              Version:  8.0.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by DimaSamoz):

 Thank you! Yes, that looks similar to my definition of `:-:`:

 {{{
 #!haskell
 data Music :: forall n l. Partiture n l -> Type where
     -- | Parallel or harmonic composition of music.
     (:-:) :: ValidHarm m1 m2 => Music m1 -> Music m2 -> Music (m1 +-+ m2)
     ...
 }}}

 `ValidHarm` checks whether the two pieces of music can be composed and
 returns a `TypeError` if there is a composition mistake.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13487#comment:2>
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