});
        sourcesPath.setCanBeConsumed(false);
        sourcesPath.setCanBeResolved(true);
        sourcesPath.extendsFrom(runtimeClasspath);

        extension.getClasspath().from(runtimeClasspath);
        extension.getSourceRoots().from(sourcesPath.getIncoming().artifactView(v -> v.lenient(true)).getFiles());

            String existing = sourceState.get();
            if (!Files.exists(target) || !Objects.equals(current, existing)) {
                pomArtifactTransformer.transform(project, session, src, target);
                Files.setLastModifiedTime(target, Files.getLastModifiedTime(src.getPath()));
            }
            result = current;
        }
        sourceState.set(result);
        return result;
    }

                    https://issues.apache.org/jira/browse/MCOMPILER-174
                    (Compare what guava-gwt has to do for maven-javadoc-plugin.)
                -->
                <arg>-sourcepath</arg>
                <arg>doesnotexist</arg>
              </compilerArgs>
            </configuration>
          </execution>
          <execution>
            <id>compile-java9</id>
            <goals>

                the base compiler settings (in the root `pom.xml`) to enable MRJAR output.
              -->
              <compilerArgs combine.self="override">
                <arg>-sourcepath</arg>
                <arg>${project.basedir}/src</arg>
                <arg>--add-reads=com.google.common=ALL-UNNAMED</arg>
                <arg>--add-reads=com.google.common.testlib=ALL-UNNAMED</arg>

                base compile settings (in the root `pom.xml`) to enable MRJAR output.
              -->
              <compilerArgs combine.self="override">
                <arg>-sourcepath</arg>
                <arg>${project.basedir}/src</arg>
                <arg>--add-reads=com.google.common=ALL-UNNAMED</arg>
                <!-- https://errorprone.info/docs/installation#maven -->