`[A] => ((x: A) => x)` is not handled correctly

[smarter]

It should be equivalent to [A] => (x: A) => x but instead we get an error:

scala> val id: [A] => A => A = [A] => (x: A) => x
val id: PolyFunction{apply: [A](x$1: A): A} = <function1>

scala> val id: [A] => A => A = [A] => ((x: A) => x)
1 |val id: [A] => A => A = [A] => ((x: A) => x)
  |                            ^
  |Implementation restriction: polymorphic function literals must have a value parameter