Fix selecting terms using _root_

[dwijnand]

No description provided.

[odersky]

I find this deeply inelegant. Checking for _root_ when parsing every ident is terrible.

We should disallow definitions of _root_ in valdefs or defdefs instead. That's seems like the cleaner fix to me.

[odersky]

Or do nothing and let the chips fall, which was the state before. I believe it is much preferable to just leave _root_ as a regular identifier and live with malicious code that defines alternates than to mess up the parser like this.

That's why it was like this in the first place. The suspicion was that it's not a bug worth fixing, i.e. the fix would likely be worse than the bug. The change in this PR confirms that suspicion for me.

So if we do anything, we can try to rule out definitions. Or revert to the status quo before we decided to change things with _root_.

[som-snytt]

Ginseng is a select root.

Scala 2 does the opposite and merely lints the usage, placing no additional restrictions. It's only a backstop.

scala> object x { object _root_ { def y = 42 } }; x._root_.y
object x
val res0: Int = 42

scala> object x { object _root_ { def y = 42 }; def z = _root_.y }; x._root_.y
                                                               ^
       error: object y is not a member of package <root>
                                                        ^
       warning: _root_ in root position of qualifier refers to the root package, not object _root_ in object x, which is in scope

scala>

[dwijnand]

We only check val/def definitions now.

[odersky]

I thought we could not burden Parser with this, but check later at Typer? is there a reason not to do this?

[dwijnand]

I thought we could not burden Parser with this, but check later at Typer? is there a reason not to do this?

It seemed like logic that is simple enough it could be done in Parser. I'll have a look at what it looks like done in Typer.

[som-snytt]

Comment from ten years ago (as I was spelunking today):

@adriaanm said:
Optimization == evil.root

I propose we update the pun to _root_.evil.

That may also be theologically more sound.

[som-snytt]

Tests failures of the form

 Test 'tests/run-staging/i3876-b.scala' failed with output:                      

  exception while retyping package _root_ {
  class Generated$Code$From$Quoted() extends Object() {
    def apply: Any =
      {
        def f(x: Int): Int = x.+(x)
        f(3)
      }
  }
} of class PackageDef # -1

but I'm rooting for you!

[odersky]

Interesting that we need that for reflection. But yes, makes sense.

[Kordyjan]

I assume it is a follow-up to #18187, so leaving it out of lts unless there is significant interest.