src/jdk.compiler/share/classes/com/sun/tools/javac/main/JavaCompiler.java

*** 1448,1457 ****
--- 1448,1462 ----
          if (implicitSourcePolicy == ImplicitSourcePolicy.NONE
                  && !inputFiles.contains(env.toplevel.sourcefile)) {
              return;
          }
  
+         if (!modules.multiModuleMode && env.toplevel.modle != modules.getDefaultModule()) {
+             //can only generate classfiles for a single module:
+             return;
+         }
+ 
          if (compileStates.isDone(env, CompileState.LOWER)) {
              results.addAll(desugaredEnvs.get(env));
              return;
          }