MapStruct can't map fields to an interface

[bidorffOL]

• Is this an issue (and hence not a question)?

There are some use cases where it would be great to be able to have an interface as a result type. One of those is the usage of Immutables beans. Because of there structure, it is not recommended to inherit from beans annotated with @Value.Immutables. Meaning that inheritance in this case is handled this way:
A ---> @Value.Immutable AbstractA ---> ImmutableA
A ---> B ---> @Value.Immutable AbstractB ---> ImmutableB

A consequence of this is that if we want to use @InheritConfiguration in our mappers, we need to use A and B as result types rather than AbstractA and AbstractB (leading to mapstruct not detecting the beans to be Immutables). A nice way to solving that would be for mapstruct to be able to "replace" the result type with the type provided as @BeanMapping#resultType allowing to implement mappers like this:

@BeanMapping(resultType = ImmutableA.Builder.class)
A convert(xxx);

An example of the issue (with a branch expected-behavior describing the proposed solution) is available here.