JSpecifyExperimental: false positive for addAll with ? element type

[unv-unv]

JSpecifyExperimental mode on.

@NullMarked
class Test {
    public Collection<Object> foo(List<?> list) {
        List<Object> result = new ArrayList<>();
        // BUG: List<?> cannot be converted to Collection<? extends Object> (List<?> is a subtype of Collection<?>)
        result.addAll(list);
        return result;
    }
}

Shows compilation error:

[ERROR]Test.java[6,22]: [NullAway] incompatible types: List<?> cannot be converted to Collection<? extends Object> (List<?> is a subtype of Collection<?>)

NullAway 0.14.1, Error Prone 2.50.0.

[msridhar]

The error message is bad on this one, but I think it's a real issue, since List<?> could contain null values. That's because its type variable has a nullable upper bound. Does that make sense? Sorry can provide links later if needed, I'm on my phone

[unv-unv]

But I have @NullMarked annotation. So List<?> should be non-nullable.

[msridhar]

See https://jspecify.dev/docs/user-guide/#wildcard-bounds. For unbounded wildcards in @NullMarked code, the default upper bound is the same as the upper bound of the corresponding type variable, which in this case is @Nullable Object, taking into account the JSpecify JDK model of List.

[unv-unv]

Is it possible to specify non-null question mark?

[msridhar]

Yes! Write List<? extends Object>. Object here is implicitly non-null