[GHC] #15220: ScopedTypeVariables binds a non-existent variable

[GHC]

#15220: ScopedTypeVariables binds a non-existent variable
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           Reporter:  goldfire       |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.5
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 GHC accepts this definition:

 {{{#!hs
 f :: (forall a. a -> a) -> ()
 f (x :: b -> b) = x (undefined :: b) `seq` ()
 }}}

 If a scoped type variable is bound always to a variable, then what
 variable is `b` bound to? In the desugared Core, it looks like `b` is
 bound to `Any`. Note that I couldn't find any valid argument for `x` here
 other than `undefined`.

 I think this definition should be rejected.

 In terms of implementation, the `zonkTcTypeToType` function should never
 encounter a unfilled-in `SigTv`. If it does, error. I don't like putting
 the error there, but I'm not sure I know another place to do it.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15220>
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