Type parameter nullability mismatch when using @Nullable T with ? extends @Nullable T

[sdeleuze]

I am working on enabling JSpecify mode in Spring Framework, and so far the most frequent source of issues has been the handling of ? extends @Nullable T versus @Nullable of T.

See for example this simple code sample:

Supplier<? extends @Nullable String> stringSupplier = () -> null;

Which generates the following error:

error: [NullAway] Cannot assign from type Supplier<@Nullable String> to type Supplier<? extends @Nullable String> due to mismatched nullability of type parameters

It looks like a value use case to me, if not, please let me know.I have also the same issue with nullable method reference.

[sdeleuze]

I have also this use case where I have a isEmpty method defined as:

@Contract("null -> true")
public static boolean isEmpty(@Nullable Map<?, ? extends @Nullable Object> map) {
	return (map == null || map.isEmpty());
}

That I try to use like:

Map<String, @Nullable Object> variables = new HashMap<>();
if (CollectionUtils.isEmpty(variables)) {
	// ...
}

Which generates the following error:

error: [NullAway] Cannot pass parameter of type @Nullable Map<String, @Nullable Object>, as formal parameter has type @Nullable Map<?, ? extends @Nullable Object>, which has mismatched type parameter nullability

[agrieve]

I think this might be another example of the same:

@NullMarked
class Foo {
    static abstract class MyClass<T extends @Nullable Object>  {
        abstract T doThing(T value);
    }

    static void repro() {
      new MyClass<@Nullable Object>() {
        @Override
        @Nullable Object doThing(@Nullable Object value) {
            return value;
        }
      }.doThing(null);
    }
}

Foo.java:14: warning: [NullAway] method returns @Nullable, but superclass method Foo.MyClass.doThing(T) returns @NonNull
        @Nullable Object doThing(@Nullable Object value) {
                         ^
    (see http://t.uber.com/nullaway )

[msridhar]

I think this might be another example of the same:

@NullMarked
class Foo {
static abstract class MyClass<T extends @nullable Object> {
abstract T doThing(T value);
}

static void repro() {
  new MyClass<@Nullable Object>() {
    @Override
    @Nullable Object doThing(@Nullable Object value) {
        return value;
    }
  }.doThing(null);
}

}

Foo.java:14: warning: [NullAway] method returns @Nullable, but superclass method Foo.MyClass.doThing(T) returns @NonNull
       @Nullable Object doThing(@Nullable Object value) {
                        ^
   (see http://t.uber.com/nullaway )

@agrieve do you get this error with JSpecify mode enabled? Without JSpecify mode it is expected as NullAway treats all type variables as @NonNull. In any case I think it's a different issue than the one above.

[agrieve]

Yes, this is in jspecify mode.

[msridhar]

Yes, this is in jspecify mode.

Ok opened #1139 for that one

[sdeleuze]

To be sure we are all on the same page, this issue is also with the JSpecify mode enabled.

[msridhar]

Yes @sdeleuze I have a fix PR up for the original issue you reported in #1137

[msridhar]

@sdeleuze NullAway 0.12.4 is now released with the fix for this issue and a couple of the other issues you reported