How does Scala handle Any to AnyVal covariance in method return types?

Question

An example is worth a thousand words:

class A { def foo: Any = new Object }

class B extends A {
  override def foo: AnyVal = 42
}

In Java, the signature @Override public int foo() wouldn't even be allowed, and the overridden method foo in B could only return the wrapper integer type (@Override java.lang.Integer foo()).

Is Scala able to avoid the boxing/unboxing of AnyVal values in the overridden def foo: AnyVal method above?

Answer 1

No, it does not. Scala has to adhere to emitting the correct bytecode:

λ scalac -Xprint:jvm Bar.scala
[[syntax trees at end of                       jvm]] // Bar.scala
package yuval.tests {
  class A extends Object {
    def foo(): Object = new Object();
    def <init>(): yuval.tests.A = {
      A.super.<init>();
      ()
    }
  };
  class B extends yuval.tests.A {
    override def foo(): Object = scala.Int.box(42);
    def <init>(): yuval.tests.B = {
      B.super.<init>();
      ()
    }
  }
}

You can see that although AnyVal was permitted in Scala, the actual method signature for the emitted foo is Object and not AnyVal, and Int is boxed.