Warn on unqualified top-level calls to Any or AnyRef methods

[mbovel]

Fixes #17266

[mbovel]

We should probably extend that to wait, notify and maybe other AnyRef methods.

[mbovel]

We should probably extend that to wait, notify and maybe other AnyRef methods.

Done ✅

[som-snytt]

As motivation, I just did this on Scala 2!

That was using eq from a value class. It doesn't root import from Predef, but the class had an explicit import.

The unreliable Scala 2 equality warning gave me a heads up, but linting all such selections seems safer. I haven't checked if eq is covered here. This Scala 2 ticket is just about the message:

scala/bug#12785

[dwijnand]

LGTM. Merge when you're happy to stop extending.

[odersky]

I'd like us to try a different warning. Predef is really not so special, after all. Consider this example:

object O

@main def foo =
  import O.*
  println(getClass)

This will print the O$, but it arguably should not.

I think the warning should instead be triggered by this condition:

When referring to a method from Any or AnyRef using a single identifier, one must be inside a named class or object (package objects don't count).

Note that if we put the code in a class or object like this:

object O
object C:
  @main def foo =
    import O.*
    println(getClass)

we get a different warning, which is what we want:

6 |    println(getClass)
  |            ^^^^^^^^
  |            Reference to getClass is ambiguous.
  |            It is both defined in object C
  |            and imported subsequently by import O._
  |
  | longer explanation available when compiling with `-explain`

[odersky]

By contrast I see much less of an issue if we write this.getClass at the toplevel. Excluding this without excluding use of this in package objects as a whole looks unsystematic.

So either disallow all explicit uses of this in package objects or allow them all.

[mbovel]

@odersky I changed to condition to:

if tree.symbol.exists
   && defn.topClasses.contains(tree.symbol.owner)
   && (!ctx.owner.enclosingClass.exists || ctx.owner.enclosingClass.isPackageObject) then
  report.warning(UnqualifiedCallToAnyRefMethod(tree, tree.symbol), tree)

Seems to do what we want. What do you think?

[odersky]

Otherwise LGTM

[odersky]

Maybe rename that to checkAnyMemberCall or something like that?