[GHC] #15791: typeKind confuses Type and Constraint

[GHC]

#15791: typeKind confuses Type and Constraint
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        Reporter:  goldfire          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.6.1
      Resolution:                    |             Keywords:
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):

 I tried this, but got stuck here, in `GHC.Err`:
 {{{
 undefined :: forall (r :: RuntimeRep). forall (a :: TYPE r).
              HasCallStack => a
 }}}
 What is the kind of `undefined`'s type?  With `typeKind` we get `Type`.
 But with `tcTypeKind` we dive into the body of the `=>`, and take the kind
 of `a`; but alas that mentions the universally quantified `r`, so we get a
 panic.

 And indeed the type `forall r (a::TYPE r). a` is indeed ill-formed.  But
 with the `HasCallStack` it is actually fine.

 What to do?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15791#comment:1>
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