Traits do not have static forwarders in bytecode

[bishabosha]

minimized code

compile with dotc 19.0-RC1

trait Foo { def foo: String = "foo" }

compile with scalacenter/scala@a8f11ef

object TestFoo {
  def main(args: Array[String]): Unit = {
    assert(new Foo {}.foo == "foo")
  }
}

runtime error:

Exception in thread "main" java.lang.NoSuchMethodError: Foo.foo$(LFoo;)Ljava/lang/String;
	at TestFoo$$anon$1.foo(TestFoo.scala:5)
	at TestFoo$.main(TestFoo.scala:5)
	at TestFoo.main(TestFoo.scala)
        ...

expectation

Anonymous subclasses of traits generated by Scala 2.13.x implement methods by calling a static forwarder on the generated Interface.

[smarter]

Dotty and Scala 2 use a some what different trait encoding. For differences in forwarders see #5928, for differences in initialization see scala/scala-dev#642. It's likely that we'll end up changing both scalac and dotty to converge to a single representation, but until that happens feel free to make a PR adding static forwarders to Dotty if that makes your work easier.

[bishabosha]

So I believe this should be closed in favour of an issue to move the whole scala 2 scheme over, as seen here field initialisation and access just isn't compatible

The following trait is compiled with Dotty

trait TraitWithSideEffects {
  val map: mutable.AnyRefMap[String, Boolean] = mutable.AnyRefMap.empty

  map += "True"  -> true
  map += "False" -> false
}

an anonymous subclass for TraitWithSideEffects looks like this in scalac with tasty reader: (using CFR 0.146)

public final class TraitWithSideEffects$.anon.1
implements TraitWithSideEffects {

    // default ctor does nothing

    @Override
    public AnyRefMap<String, Object> map() {
        return TraitWithSideEffects.map$((TraitWithSideEffects)this);
    }
}

and the correct implementation produced by dotty:

private static final class TraitWithSideEffects$.anon.1
implements TraitWithSideEffects {
    private final AnyRefMap map = TraitWithSideEffects.super.initial$map();

    // default ctor also calls $init$

    public AnyRefMap map() {
        return this.map;
    }
}

[sjrd]

Fixed in #8652.