Inline methods with by-name arguments break debugging information

[Blaisorblade]

In the snippet below, setting breakpoints in isSubType is hard if it's compiled by Dotty, because all of isSubType appears written on line 100.

transparent def track(f: => T): T = { ... f ... }

def isSubType(...) = track { // line 100
  body
]

Probably should be addressed before bootstrap, hence tentatively targeting milestone 0.10.

[Blaisorblade]

@odersky can you take a look?

[nicolasstucki]

The same issue happens at runtime with the stack trace

object Foo {
  transparent def track[T](f: => T): T = f

  def main(args: Array[String]): Unit = {
    track {
      val a = 9
      ???
    }
  }

}

Exception in thread "main" scala.NotImplementedError: an implementation is missing
	at scala.Predef$.$qmark$qmark$qmark(Predef.scala:284)
	at Foo$.main(Foo.scala:5)
	at Foo.main(Foo.scala)

[smarter]

@nicolasstucki That's a separate (and much harder to solve) issue, if your code is inlined in a method, you can't have the stacktrace show the methods that have been inlined. JSR-45 (that the Scala Center was supposed to implement) should help in theory but I don't think it's used for stacktraces.

[smarter]

Actually JSR-45 is relevant for the original issue from @Blaisorblade too: without implementing it, there is no way to preserve line information for debugging when inlining across files.

[nicolasstucki]

No, in this case we lose the position of code that was in the current method (not inlined code). The position of the tree of ??? should not be lost while inlining it in the code of track.

[nicolasstucki]

The position is changed in erasure when the Inline tree is dropped.