Dotty emits unnecessary forwarders for trait methods

[lrytz]

While working on scala/scala-dev#98 to eliminate unnecessary forwarders for inherited trait methods I basically followed dotty's approach (https://github.com/lampepfl/dotty/blob/master/src/dotty/tools/dotc/transform/MixinOps.scala#L45).

I realized that dotty still emits unnecesary forwarders:

trait T1 { def f = 1 }
trait T2 extends T1 { override def f = 2 }
trait T3 { self: T1 => override def f = 2 }
class C1 extends T1 with T2 // no forwarder needed
class C2 extends T1 with T3 // forwarder needed

Dotty emits an unnecessary forwarder in C1.

See scala/scala#5085 for my patch to scala/scala.

[DarkDimius]

@lrytz, Are you sure that in C1 no forwarder is needed?

http://docs.oracle.com/javase/specs/jvms/se8/html/jvms-5.html#jvms-5.4.3.3

Otherwise, if any superinterface of C declares a method with the name and descriptor specified by the method reference that has neither its ACC_PRIVATE flag nor its ACC_STATIC flag set, one of these is arbitrarily chosen and method lookup succeeds.

Afaik, the spec says that you need it here. Am I missing something?

[lrytz]

it's covered by the point above, IUC.

If the maximally-specific superinterface methods of C for the name and descriptor specified by the method reference include exactly one method that does not have its ACC_ABSTRACT flag set, then this method is chosen and method lookup succeeds.

see jvms-5.4.3.3, "maximally-specific method"

[smarter]

I believe this is now moot since scalac ended up having -Xmixin-force-forwarders default to true for performance reasons (and dotty should imitate this behavior and emit more forwarders, not less), feel free to reopen if I'm missing something.