Fix #11318: add regression tests

[anatoliykmetyuk]

No description provided.

[smarter]

The problem does not seem to be specific to polymorphic functions, it happens any time there's a class instance between the call and the extension method:

extension(a: Int)
  def b: Int = ???
  def h: Unit =
    new Function1[Int, Int] {
      def apply(r: Int): Int = b
    }

[anatoliykmetyuk]

Fixed by #13078.