gistfile1.txt

Prelude> data T =  forall a. MkT a
Prelude> let x = MkT 1
Prelude> :t case x of { MkT y -> y }

<interactive>:1:22:
    Couldn't match expected type ‘t’ with actual type ‘a’
      because type variable ‘a’ would escape its scope
    This (rigid, skolem) type variable is bound by
      a pattern with constructor
        MkT :: forall a. a -> T,
      in a case alternative
      at <interactive>:1:13-17
    Relevant bindings include y :: a (bound at <interactive>:1:17)
    In the expression: y
    In a case alternative: MkT y -> y
Prelude>