I don't see how this breaks binary compatibility:

• Define a package private class C with a public static method M that returns interface F, release that as version N
• For version N+1:
  • Change the return type of M to a concrete implementation of F (the kind return type change should not matter)
  • Run JApiCmp and all is well, obviously, since this C is a package-private class
  • Change the visibility of C from package-private to public
  • JApiCmp now fails.

The failure does not make sense since no one outside the package could call C in version N.
For example:

• git clone -n https://gitbox.apache.org/repos/asf/commons-io.git
• git checkout 877b9e3f5e553f8ad988b89b8adb6b93fa97a964
• Make the class org.apache.commons.io.StreamIterator public
• Run mvn clean install -DskipTests japicmp:cmp -Dcommons.japicmp.version=0.18.1

[ERROR] Failed to execute goal com.github.siom79.japicmp:japicmp-maven-plugin:0.18.1:cmp (default-cli) on project commons-io: There is at least one incompatibility: org.apache.commons.io.filefilter.PathMatcherFileFilter:METHOD_ABSTRACT_ADDED_IN_IMPLEMENTED_INTERFACE,org.apache.commons.io.function.Uncheck.get(org.apache.commons.io.function.IOSupplier,java.util.function.Supplier):CLASS_GENERIC_TEMPLATE_CHANGED,org.apache.commons.io.StreamIterator.iterator(java.util.stream.Stream):METHOD_RETURN_TYPE_CHANGED -> [Help 1]