[GHC] #11405: Incorrect failure of type-level skolem escape check

[GHC]

#11405: Incorrect failure of type-level skolem escape check
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        Reporter:  goldfire          |                Owner:  goldfire
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.0.1
       Component:  Compiler          |              Version:  8.0.1-rc1
      Resolution:                    |             Keywords:  TypeInType
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 gridaphobe):

 Is it just a problem because of `x0`, ie would

 {{{
 x = \ @(v :: Levity) @(a :: TYPE v) ($dict :: (?callStack :: CallStack))
 ->
     let $dict' = ... in
     undefined @v @a $dict'
 }}}

 be OK? If so, can we change the desugarer to not generate the intermediate
 binder when there's only one use and no recursion? I'm not sure that would
 be a general fix, but it would fix the immediate problem.

 (Also, why are levity-polymorphic binders not allowed? Is this explained
 somewhere? Just curious.)

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11405#comment:5>
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