Fix #4753: Direct method should keep Override if overriden nonempty

[liufengyun]

Fix #4753: Direct method should keep Override if overriden nonempty.

It is the case if the overriden symbol is in the same compilation unit.

[allanrenucci]

It seems to work but there is something fishy. If you compile the classes in the opposite order:

class Foo2 extends Foo1 {
  override def foo: implicit String => Int = 2
}

class Foo1 {
  def foo: implicit String => Int = 1
}

After ShortcutImplicits you get:

class Foo2 extends Foo1 { 
  override def foo: implicit String => Int = {
      def $anonfun(implicit evidence$1: String): Int = 
        this.foo$direct(evidence$1)
      closure($anonfun)
    }
  def foo$direct(implicit x$0: String): Int = 2
}

class Foo1 { 
  def foo: implicit String => Int = {
    def $anonfun(implicit evidence$2: String): Int = 
      this.foo$direct(evidence$2)
    closure($anonfun)
  }
  def foo$direct(implicit x$0: String): Int = 1
}

You can see that foo$direct in Foo2 is missing the override modifier, but RefChecks doesn't complain...

This issue derives from the fact that ShortcutImplicits doesn't have an InfoTransformer. Here @odersky explains why. Maybe the correct fix is to add more special cases to RefChecks.

Alternatively, we could try to move ShortcutImplicits after RefChecks, but we would need to move TailRec as well

[liufengyun]

Thanks a lot for looking into it @allanrenucci . I think the behavior is expected as when direct method in Foo2 is synthesized and checked in RefChecks, the direct method in Foo1 doesn't exist yet. The fix actually just restores the original logic, it avoids changes to overriding check in RefChecks.

[Blaisorblade]

After discussion in the meeting: If we rely on the Override flag to not mean anything after a certain phase (RefChecks?), which seemed the case, let's please mention this in Override's Scaladocs.

[allanrenucci]

@odersky Why was this closed?

[allanrenucci]

LGTM. We decided to get this in.

We should document that the Override modifier is meaningless after RefCheck since it is not consistently added by the compiler. This can be done as a separate PR