idea: check and fix use of declaration-site type variance

[vlsi]

See:

• @In/@Out annotations for declaration-site variance -- kind of jspecify/jspecify#72

Currently, Java requires use-site type variance, so if someone has Function<IN, OUT> method parameter, it should rather be Function<? super IN, ? extends OUT>.

Unfortunately, it is not easy to notice that ? super and ? extends is missing, so it would be nice if there was a tool that could detect missing variance and suggest adding it.

The list of well-known classes could be hard-coded within OpenRewrite: Function, Predicate, BiFunction, Consumer, Supplier, and so on.

Here is a recent case:

• Make nestable condition factory accept conditions on super types assertj/assertj#2905

WDYT?

Corner cases:

• https://github.com/JetBrains/intellij-community/blob/cd82cab9aff47787e684b4a58be902d1b917f264/plugins/InspectionGadgets/src/com/siyeh/ig/style/BoundedWildcardInspection.java
• https://youtrack.jetbrains.com/issue/IDEA-235069/False-positive-in-Can-use-bounded-wildcard
• https://github.com/JetBrains/intellij-community/blob/cd82cab9aff47787e684b4a58be902d1b917f264/plugins/InspectionGadgets/src/com/siyeh/ig/style/VarianceUtil.java#L125-L133

See also:

• Discussion in Error Prone: idea: check proper use of declaration-site type variance google/error-prone#3711
• Discussion in forbidden-apis: idea: check proper use of declaration-site type variance policeman-tools/forbidden-apis#216

[jkschneider]

I'm a big fan of this for a set of hard-coded well-known classes.

[jkschneider]

Are there also a series of types for which we wouldn't add variance? For example: Function<String, Car> should be Function<String, ? extends Car> not Function<? super String, ? extends Car> right?

[vlsi]

I think so. However, I suggest discussing meta-issues in jspecify issue (the first link) to avoid splitting the questions/findings into several issues ;)

[jkschneider]

Automatically added variance should skip type parameters that are final classes as well.

[jkschneider]

Note that when a method overrides another method, it cannot use declaration-site type variance without changing the overridden method. Nor can on overridding method remove declaration-site type variance when overriding one that has it.

interface Test {
    void test(Function<List, List> f);
}

class TestImpl implements Test {
    @Override
    public void test(Function<? super List, ? extends List> f) {
    }
}

[vlsi]

A class can become non-final, so having variance for final classes might be helpful.
However, it is unlikely String can become open, so ? extends String is probably useless