Unchecked overridden opaque type

[prolativ]

Compiler version

3.0.0...3.1.2-RC2

Minimized code

trait Foo {
  opaque type Out = Int
  def out1: Out
  def out2: Out = out1
}

object Bar extends Foo {
  override opaque type Out = String
  override def out1 = "abc"
}

@main def run() =
  val x = Bar.out2

Output

Exception in thread "main" java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Integer
        at scala.runtime.BoxesRunTime.unboxToInt(BoxesRunTime.java:99)
        at Bar$.out1(Opaque.scala:9)
        at Foo.out2(Opaque.scala:4)
        at Foo.out2$(Opaque.scala:1)
        at Bar$.out2(Opaque.scala:7)
        at Opaque$package$.run(Opaque.scala:13)
        at run.main(Opaque.scala:12)

Expectation

This should not compile. Instead a compilation error should be reported just like this happens for non-opaque types, e.g.

trait Foo {
  type Out = Int
  def out1: Out
  def out2: Out = out1
}

object Bar extends Foo {
  override type Out = String
  override def out1 = "abc"
}

[error] error overriding type Out in trait Foo, which equals Int;
[error]   type Out, which equals String has incompatible type
[error]   override type Out = String
[error]                 ^

Alternatively maybe we should consider making opaque types unoverridable? It seems quite strange to me that currently it's possible to override a concrete opaque type although you cannot define an abstract one. Additionally it looks like redefining an opaque type in a subtype works as a hack to leak the opaque type's internals, e.g. the code below compiles successfully unless you comment out the marked line:

trait Foo {
  opaque type Out = Int
  def out1: Out
  def out2: Out = out1
}

object Bar extends Foo {
  override opaque type Out = Int // compilation breaks if this line gets commented out
  override def out1 = 1
}

[Swoorup]

If overriding is disallowed I am wondering if there is any alternative mechanism to reuse the transparent alias mechanism. I have asked a question here: https://contributors.scala-lang.org/t/good-way-to-reuse-opaque-types-transparentness/5642