Static forwarders for inherited polymorphic methods use erased type Object

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abstract class A[T] {
  def m(x: T): T = x
}
object Test extends A[String]

The static forwarder generated in Test has type m(Ljava/lang/Object;)Ljava/lang/Object, it should use Strings, and introduce casts where necessary.

The forwarder is generated in BCodeHelpers.addForwarder. The signature is taken "as-seen-from" the module, the code is val methodInfo = moduleClass.thisType.memberInfo(m), however we're at a late phase where T is already erased to Object.

We should take the signature enteringErasure, but then we have to manually run erasure, and possibly other transforms (lambdalift, mixin).

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Imported From: https://issues.scala-lang.org/browse/SI-5289?orig=1
Reporter: Petr Pudlak (petrp)
Affected Versions: 2.9.1, 2.10.0-M3
See #8293

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@adriaanm said:
Unassigning and rescheduling to M6 as previous deadline was missed.

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@lrytz said:
related to #8293

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@lrytz said:
This looks rather difficult to fix with the way forwarders are currently done. I think the better solution would be to generate static forwarders earlier in the pipeline and have them transformed normally. Probably the right way to go about this is to implement the @static pre-SIP: scala/docs.scala-lang#491.