[GHC] #13337: GHC doesn't think a type is of kind *, despite having evidence for it

[GHC]

#13337: GHC doesn't think a type is of kind *, despite having evidence for it
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.2
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         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 RyanGlScott):

 Forgive me if this is a silly question, but why shouldn't the first
 example work? I know that the implementation details would significantly
 complicate the story, but my perspective, being able to write the first
 example would make a lot of code easier to write. An example of this that
 came up in real code is when [https://mail.haskell.org/pipermail/ghc-
 devs/2017-February/013850.html Edward Kmett tried to sketch out a
 solution] to #13327 on the ghc-devs mailing list, and attempted to write
 this code:

 {{{#!hs
 class (Typeable t, Typeable k) => DataIfStar (k :: t) where
   dataIf :: (t ~ Type) => proxy k -> (Data k => r) -> r
 }}}

 But this won't work, because `Data :: Type -> Constraint`, and GHC isn't
 able to conclude that `k :: t` is actually `k :: Type`, since the evidence
 that `t ~ Type` isn't being used when typechecking.

 If programs like these aren't supposed to typecheck by design, what is the
 workaround?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13337#comment:2>
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