[GHC] #12239: Dependent type family does not reduce

[GHC]

#12239: Dependent type family does not reduce
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        Reporter:  int-index         |                Owner:  goldfire
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |             Keywords:  TypeInType,
      Resolution:                    |  TypeFamilies
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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Changes (by goldfire):

 * owner:   => goldfire


Comment:

 I was wondering if someone would find a way to tickle this. I'm pretty
 sure you've hit the shortcoming detailed
 [https://github.com/ghc/ghc/blob/e368f3265b80aeb337fbac3f6a70ee54ab14edfd/compiler/typecheck/TcFlatten.hs#L982
 here].

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12239#comment:3>
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