[GHC] #12564: Type family in type pattern kind

[GHC]

#12564: Type family in type pattern kind
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        Reporter:  int-index         |                Owner:  goldfire
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.4.1
       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:  14119             |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 Current error message is a bit better
 ```
 T12564.hs:18:3: error:
     • Illegal type synonym family application ‘Len @a _1’ in instance:
         At @a ((':) @a x _1) ('FZ @(Len @a _1))
     • In the equations for closed type family ‘At’
       In the type family declaration for ‘At’
    |
 18 |   At (x ': _) FZ = x
    |   ^^^^^^^^^^^^^^^^^^
 ```

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12564#comment:12>
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