There is some case both BeanMapping#unmappedSourcePolicy and BeanMapping#unmappedTargetPolicy will be helpful.
let take an example of 2 class (class Student with more than 20 properties and StudentDto with 2 attributes just name and firstname)

So in our mapper we are looking for 2 methods :

1. a first method Student toStudent(StudentDto studentDto) that can create a new Student and just fill the name and the firstname.
2. a second method StudenDto toStudentDto(Student student) that can map the name and firstname of a student to studentDto

For me, in order to achieve that goal and also take advantage of typesafe warning, it will be easier for me to do something like this :

1. in the first case

@BeanMapping(unmappedTargetPolicy = ReportingPolicy.IGNORE, unmappedSourcePolicy = ReportingPolicy.WARN)
Student toStudent(StudentDto studentDto);

2. in the second case

@BeanMapping(unmappedTargetPolicy = ReportingPolicy.WARN, unmappedSourcePolicy = ReportingPolicy.IGNORE)
StudentDto toStudentDto(Student student);

But i don't know why BeanMapping#unmappedSourcePolicy does not exists ?
So i'm wondering why and also wondering if you plan another best way to achieve that goal.
thanks