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