Avoid crash in erasure when reference cannot be emitted

[odersky]

Fixes #17391

[dwijnand]

Wait, why does can y.retainAll compile but x.f.retrainAll can't? Looks like either both should or both shouldn't - I don't see why they're different.

[odersky]

x.f's type is erased to Object and is then downcast to Bar in erasure. That cast cannot be expressed since Bar is not visible.

[dwijnand]

Bar is visible, it's only Base which isn't.

[odersky]

Bar is visible, it's only Base which isn't.

Indeed. I guess it tries to cast to Base, not Bar, since that's the owner of retainAll. What does Scala 2 do?

[dwijnand]

Just uses Bar, it looks like

[[syntax trees at end of                   erasure]] // test.scala
      val x: Foo = new Foo(new mypkg.Bar());
      val y: mypkg.Bar = new mypkg.Bar();
      y.retainAll("fd");
      x.f().$asInstanceOf[mypkg.Bar]().retainAll("fd")

[odersky]

Can you find out why it tried to refer to the inaccessible class?

[odersky]

You were right. There was another, deeper issue that caused the problem. See last commit.