A.scala

class A[T]{
  def foo(t: T): Unit = {}
}

class B extends A[Y] {
  def foo(t: X): Unit = {}
}
 
class X

class Y extends X


object Main {

  (new B).foo(new Y) // compile error.  "ambiguous reference to overloaded definition"

  (new B).foo(new Y: X) // success
  
}

error.txt

error: ambiguous reference to overloaded definition,
both method foo in class B of type (t: X)Unit
and  method foo in class A of type (t: Y)Unit
match argument types (Y)
                (new B).foo(new Y)
                        ^