I think there may be a mistake in the Reference Guide documentation. In section 2.4 it describes the mapstruct.unmappedSourcePolicy and states it has a default value of WARN. But in practice, it seems that the default value is actually IGNORE. That is the behavior I get just setting up a new Mapper with no other configuration.

The source code for Mapper.java seems to support the idea that IGNORE is the intended default: ReportingPolicy unmappedSourcePolicy() default ReportingPolicy.IGNORE;

I believe this was a simple copy-paste error introduced in #2572