JSpecify: handle stream values mapped to `@Nullable`

[msridhar]

A self-contained test (to separate from library modeling issues):

import org.jspecify.annotations.*;
@NullMarked
class Test {
    interface List<T extends @Nullable Object> {
        Stream<T> stream();
    }
    interface Stream<T extends @Nullable Object> {
        <R extends @Nullable Object> Stream<R> map(Function<? super T, ? extends R> mapper);
        void forEach(Consumer<? super T> action);
    }
    interface Function<T extends @Nullable Object, R extends @Nullable Object> {
        R apply(T t);
    }
    interface Consumer<T extends @Nullable Object> {
        void accept(T t);
    }
    static @Nullable String mapToNull(String s) {
        return null;
    }
    static void test(List<String> list) {
        // legal, should infer R -> @Nullable String
        list.stream().map(Test::mapToNull).forEach(s -> {
            // BUG: Diagnostic contains: dereferenced expression s is @Nullable
            s.hashCode();
        });
    }
}

Right now, even in JSpecify mode, we don't report a warning on the s.hashCode() call. Hopefully, better handling of wildcards will get us there.

[beikov]

Is this topic on anyone's radar already? The Hibernate ORM project is considering to switch from CheckerFramework to JSpecify with Nullaway, but we're a bit unsure how this limitation might affect our code base.
I just want to understand if this is something you want to support in the future or if you draw a line here and say that this can't be implemented efficiently, so you abstain from supporting it.

[msridhar]

@beikov this feature is definitely on our roadmap and we plan to support it. We've been a bit resource constrained lately, and I can't make promises, but I would hope in the next couple of months we could make some progress on it. I will comment here when we have made some progress. Thanks!