-Xlint:auxiliaryclass

Although legal, the use of multiple top level classes in the same file is somewhat questionable to begin with, but it is particularly bad when in some package class A in A.java refers to class B defined in C.java. This requires that at times the files must be compiled together, and prevents implicit compilation from locating such "hidden classes".

It would be a fine thing for javac to provide an optional warning to detect such references.

One suggestion is to add a new suboption to -Xlint, namely -Xlint:auxiliaryclass.