Why does the Java 9 compiler warn "requires directive for an automatic module" if `module-info.java` references automatic modules with "Automatic-Module-Name" set? What's the risk of such modules?

This question isn't an exact duplicate of [What is an automatic module?](https://stackoverflow.com/q/46741907/14731) because the latter does not address the specific reasons behind the compiler warning I have referenced (the context of the question matters). That said, it is a useful link for follow-up reading.