[GHC] #12432: Use kind equality in instance context in type family instance (was: TypeInType: open type family application as type family return kind fails to compile)

[GHC]

#12432: Use kind equality in instance context in type family instance
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        Reporter:  j6carey           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Linux             |         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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Description changed by goldfire:

@@ -7,0 +7,4 @@
+
+ EDIT: This ticket got repurposed at comment:3 and below. The original
+ problem described above is fixed, but the commentary is useful to keep
+ around and on this ticket.

New description:

 With TypeInType enabled, I can use a closed type family to compute the
 return kind of a type family from its arguments.  But an equivalent open
 type family used in the same way triggers a compilation failure, as if the
 relevant instance of that open type family were not considered.  (Of
 course, there may be some subtle prohibition of which I am not aware at
 the present time.)  I will attach a test case.

 EDIT: This ticket got repurposed at comment:3 and below. The original
 problem described above is fixed, but the commentary is useful to keep
 around and on this ticket.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12432#comment:7>
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