[GHC] #12432: TypeInType: open type family application as type family return kind fails to compile

[GHC]

#12432: TypeInType: open type family application as type family return kind fails
to compile
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        Reporter:  j6carey           |                Owner:
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:  duplicate         |             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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Comment (by goldfire):

 That last test case is an unrelated issue, not yet fixed. The problem is
 that `-XTypeInType` can't use a kind equality "right away". Equality
 constraints in Haskell are ''lifted'', meaning that we must make sure they
 are not bottom before using them to cast anything. Normally, this is all
 optimized away. But it does mean that we can't immediately use a kind
 equality in a type unless there is a place to put a check for bottom. Your
 code has no such place.

 This is all a bit of a dark corner, and I'm afraid you shouldn't hold your
 breath for a solution. On the flip side, it would be very enlightening if
 you have a realistic scenario where this limitation bites.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12432#comment:3>
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