The 2017-05-18 draft of JLS indicates that automatic modules are beyond the scope of JLS. I'm puzzled what that should mean for a compiler. At face value it seems to say that compilers need not care about automatic modules. Instead I'd expect JLS to state that the host system must be able to discover automatic modules "as if by invocation of ModuleFinder.of(Path...)." or similar.
Which is it? Stephan