Paraphrased from email from Maurizio:

When you pass a bad argument to javac - such as "--modulesourcepath", what you get is this:

javac: invalid flag: --modulesourcepath
Usage: javac <options> <source files>
use --help for a list of possible options

Now, the output looks good, but, as this goes through Log.printRawLines, the condition is not really treated as an error. Is there any reason as to why we don't emit something like:

error: invalid flag: --modulesourcepath
Usage: javac <options> <source files>
use --help for a list of possible options

instead? [e.g. a proper Log.error]

This discrepancy is particularly evident in this case:

javac: option --module-source-path not allowed with target 1.8
error: no class output directory specified

I believe either both should be errors or none should?