Expected behavior

Given the following source and target models (immutable code generated by the immutables framework):

@Value.Immutable
public interface ASource {
    ImmutableBSource getB();
}

@Value.Immutable
public interface ATarget {
    Optional<ImmutableBTarget> getB();
}

@Value.Immutable
public interface BSource {
    String getS();
}

@Value.Immutable
public interface BTarget {
    String getS();
}

and the following mapper definition:

public interface Mapper {
    ImmutableATarget map(ImmutableASource value);

    ImmutableBTarget map(ImmutableBSource value);

    default <T> Optional<T> wrapAsOptional(T value) {
        return Optional.ofNullable(value);
    }
}

MapStruct 1.4.2 generated a correct mapper that uses the wrapOptional method for wrapping the b field in an optional after having converted it:

immutableATarget.b( wrapAsOptional( map( value.getB() ) ) );

Actual behavior

After upgrading to 1.5.3, MapStruct fails to generate a mapper and bails out with the following error message:

Can't map property "ImmutableBSource b" to "Optional<? extends com.bol.generics.ImmutableBTarget> b". Consider to declare/implement a mapping method: "Optional<? extends com.bol.generics.ImmutableBTarget> map(ImmutableBSource value)".

I expect it to generate a mapping method similar to the one generated by version 1.4.x.

Steps to reproduce the problem

See https://github.com/ntinnemeier/mapstruct-1.5-upgrade-bugs/tree/main/generics-with-immutables for a full example.
Switch between version 1.4.x and 1.5.x to see the difference in behaviour.

I couldn't reproduce this behaviour with vanilla Java domain objects. That is to say, domain objects that are not generated by the immutables framework. I haven't checked what happens when the domain classes are generated by Lombok.

MapStruct Version

MapStruct 1.5.3