Dependent method type referring to previous argument list

[scabug]

class A { 
  class B
  def b: B = new B 
}
def f(a: A)(b: a.B): a.B = b

val a = new A
f(a)(a.b)
f(new A)(a.b) // should not compile

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-3873?orig=1
Reporter: @ingoem

[scabug]

@adriaanm said:
(In r23111) closes #3873. fixed unsoundness in dependent method types reported by Ingo.

review by maier as no good deed goes unpunished