Missing `@Nullable` annotation in `TypedArgumentConverter`

[cyril-steimer]

JUnit Jupiter 6.0.0 added @Nullable annotations.

I believe TypedArgumentConverter is missing such an annotation for the return type of its protected convert method.

Currently the signature is:

protected abstract T convert(@Nullable S source) throws ArgumentConversionException;

I believe this should be:

protected abstract @Nullable T convert(@Nullable S source) throws ArgumentConversionException;

This would simplify writing Kotlin code using TypedArgumentConverter.

The following Kotlin code does not compile against JUnit Jupiter 6.0.0:

class DummyConverter : TypedArgumentConverter<String, String>(String::class.java, String::class.java) {
    override fun convert(source: String?): String? = source
}

I believe that with an additional @Nullable annotation as shown above this code would compile.

The current workaround in Kotlin is somewhat ugly as it requires an unchecked cast:

class DummyConverter : TypedArgumentConverter<String, String?>(String::class.java, String::class.java as Class<String?>) {
    override fun convert(source: String?): String? = source
}

[sbrannen]

I believe TypedArgumentConverter is missing such an annotation for the return type of its protected convert method.

I thought that the T extends @Nullable Object declaration in class TypedArgumentConverter<S, T extends @Nullable Object> covered that already. 🤔

[marcphilipp]

Whether or not T is nullable is determined by the type parameter because T is declared as T extends @Nullable Object (see JSpecify docs). Changing the convert method's return type from T to @Nullable T would make it impossible to specify that the converter is for a non-nullable type and therefore never returns null.

One thing we could do, though, is to change the Class<T> targetType constructor parameter to Class<@NonNull T> targetType. I'll investigate.