Overload that differs only in parameter type is mistaken as override, crashes with a ClassCastException at runtime

[julienrf]

Compiler version

3.0.1-RC1

Minimized code

trait Foo[A]:
  def foo(x: A): Unit

trait Bar[A] extends Foo[A]:
  def foo(x: A & String): Unit = println(x.toUpperCase)

object Baz extends Bar[Int]

@main def run() = Baz.foo(42)

Output

The code compiles, but fails at run-time:

java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String
at Baz$.foo(main.scala:7)

Expectation

The code should not compile. It should produce the following error:

object creation impossible, since def foo(x: A): Unit in trait Foo is not defined
(Note that
parameter A in def foo(x: A): Unit in trait Foo does not match
parameter A & String in def foo(x: A & String): Unit in trait Bar
type A & String is a subtype of trait A, but method parameter types must match exactly.)

Note: adding the override keyword raises a compile-time error:

trait Foo[A]:
  def foo(x: A): Unit

trait Bar[A] extends Foo[A]:
  override def foo(x: A & String): Unit = println(x.toUpperCase)
//                    ^^^^^^^^^^
// method foo has a different signature than the overridden declaration

[smarter]

The override check is supposed to happen in https://github.com/lampepfl/dotty/blob/4ad0c8f2a73f12e58750bcd4570616cbf50a3011/compiler/src/dotty/tools/dotc/typer/RefChecks.scala#L499-L521
But even though the opc.matches method returns true on the symbols we'd like to compare, ops.hasNext returns false, so this seems to be an issue in how OverridingPairs.Cursor works and I have to admit that I don't fully understand that class (but I know we're missing some fixes from scala 2 there: #7551), do you think you could have a look @odersky ?