[GHC] #15592: Type families without CUSKs cannot be given visible kind variable binders

[GHC]

#15592: Type families without CUSKs cannot be given visible kind variable binders
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.3
      Resolution:                    |             Keywords:
                                     |  TypeApplications, TypeFamilies,
                                     |  CUSKs
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 > Bottom line: any use of visible kind application should be considered to
 be an instance of polymorphic recursion.

 Yes, and we (presumably) have exactly same at the term level
 {{{
 reverse (xs :: [a]) = ...(reverse @a ys)....
 }}}
 This is illegal. When type checking `reverse`'s RHS we have `reverse ::
 alpha` in the envt, and you can't type-apply that to anything.  If we had
 a signature
 {{{
 reverse :: [b] -> [b]
 reverse (xs :: [a]) = ...(reverse @a ys)....
 }}}
 that's fine, because the signature puts `reverse :: forall b. [b] -> [b]`
 into the envt.

 This might be a useful example to add to the user-manual documetation for
 visible type application.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15592#comment:3>
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