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

[GHC]

#12239: Dependent type family does not reduce
-------------------------------------+-------------------------------------
        Reporter:  int-index         |                Owner:  goldfire
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.2.1
       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:  #11348            |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by mpickering):

 * related:   => #11348


Comment:

 Int-index and I have been working through this together this afternoon and
 it is nothing to do with what Richard initially expected. Instead, the
 problem is that whilst checking the kind of the argument to `FieldType`,
 we need knowledge of the instances of `FieldCount` which only get added to
 the environment *after* all the type family instances have been type
 checked.

 So instead this ticket is related to #11348.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12239#comment:9>
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