Error message mentions impredicative polymorphism

When we apply Lens to too many arguments in the :kind command we get a sensible error message

ghci> type Lens s a = forall f. Functor f => (a -> f a) -> (s -> f s)
ghci> :kind Lens _ _
Lens _ _ :: *
ghci> :kind Lens _ _ _

<interactive>:1:1: error:
    • Expecting one fewer argument to ‘Lens t0 t1’
      Expected kind ‘k0 -> k1’, but ‘Lens t0 t1’ has kind ‘*’
    • In the type ‘Lens _ _ _’

but use the same in a type signature or annotation and it complains about impredicative polymorphism

ghci> :t undefined :: Lens _ _ _

<interactive>:1:1: error:
    • Illegal polymorphic type: Lens _ _1
      GHC doesn't yet support impredicative polymorphism
    • In the expression: undefined :: Lens _ _ _
ghci> a :: Lens _ _ _; a = undefined

<interactive>:6:1: error:
    Illegal polymorphic type: Lens _ _1
    GHC doesn't yet support impredicative polymorphism

Trac metadata

Trac field	Value
Version	7.10.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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