Teach Ycheck to spot references to Java packages

[DarkDimius]

Ycheck should have complained here: #2396 (comment)

but it didn't.

[abeln]

@DarkDimius can you add a bit more context here?

1. in which AST nodes can references to Java packages appear/not appear?
2. is the answer to 1) true for all phases, or only after a specific phase?

[DarkDimius]

@abeln,

1. is hard to answer because we use Idents & Selects to represent Imports. So It's not about "in what trees", but instead about "in what context". Eg. packages should be passed as arguments or be assigned to variables.

The nice way to do it would be to introduce a phase after Pickler that gets rid of all references to packages in Select\Ident's. This phase should have a post-conditions that those trees don't re-appear.

[abeln]

Thanks. How do we "get rid" of references to Java packages? What should they be replaced with?

[DarkDimius]

@abeln by replacing all verbose references that include packages by Idents.
Eg. Select(Select(Select(_root_, scala), Predef) println) => Ident(println)
After this, there should be no references to packages through RefTrees left.

[abeln]

Here's my understanding of what needs to be done:

1. Declare a new MiniPhaseTransform, to be run together with Pickler (or after?)
2. override transformSelect in the new mini-phase
3. Within transformSelect, detect whether the qualifier stands for a package
4. Simplify the Select node as you wrote above: (p1, p2, ..., pn).Ident(i) => Ident(i)

Is that reasonable?
If yes, then what's a good way to do step 3 (detecting whether a qualifier stands for a package)? The type of the qualifier is a TermRef (because packages are objects, right?), but how do I tell it apart from a "regular" object (otherwise I'll turn x.y.T into T)?

[DarkDimius]

Is that reasonable?

Yes 👍

1. after Pickler.
2. yes.
3. yes
4. yes. Because the thing you refer to is top-level you can always refer to it through tpd.ref

If yes, then what's a good way to do step 3 (detecting whether a qualifier stands for a package)?

Take the term symbol of the qualifier. Check if it has both flags Package and JavaDefined set. I fou don't check for JavaDefined you can also get package objects.

[abeln]

Thanks. I took a first stab at this in #2528