hask

data Test where
  Test :: (forall a. (Typeable a) => a -> ()) -> Test

callTest :: Test -> ()
callTest (Test fn) = case cast 'c' of   <-- Error here
  Just b -> fn b

||     • No instance for (Typeable a0) arising from a use of ‘cast’
||     • In the expression: cast 'c'
||       In the expression: case cast 'c' of { Just b -> fn b }
||       In an equation for ‘callTest’:
||           callTest (Test fn) = case cast 'c' of { Just b -> fn b }
||     |
|| 210 | callTest (Test fn) = case cast 'c' of
||     |                           ^^^^^^^^