mod/pkg/X.java ->
public class X {
    public static void main(String[] args) {
        System.out.println("Module mod's pkg.X.main");
    }
}

mod/module-info.java ->
module mod {
}

javac -d mod/module-info.java mod/pkg/X.java
java -p mod -m mod/pkg.X --> "Module mod's pkg.X.main" (Works as expected)

Now try changing the mod's requires flag:
java --jar <asmtools.jar> jdec mod/module-info.class > mod/module-info.jcod
Modify from 'requires' table (jcod line:42) 0x8000 to 0x1000 (ACC_MANDATED to ACC_SYNTHETIC)
Recompile jcod to classfile:
java --jar <asmtools.jar> jcoder mod/module-info.jcod
cp module-info.class mod/

Now that module 'mod' has requires table as "java.base" and access flag as ACC_SYNTHETIC(0x1000). Below command is not expected to run successfully
java -p mod -m mod/pkg.X

As per jvms 4.7.25. The Module Attribute (assertion id: jvms-4.7.25-300-D-G.1-B), above illustrated example should throw error.