try to catch using - instead of _ in filenames so the compiler can give a better error message for people who don't know that you need to use _ in file names

clj-1297-v5.patch should be the one. I can only attempt to blame the previous bone-headed failure on lack of sleep, but even then ...

The bad end colon is not fixed and the optional message is not included in the format string.

Patch clj-1297-v4.patch modifies only RT.load, including an extra message only if the file name in the argument contains a '_' character in it, with the message suggested by Alex Miller in his last comment before this.

With latest:

That looks goofy due to the base message in RT.load(): throw new FileNotFoundException(String.format("Could not locate %s or %s on classpath: ", classfile, cljfile));

I would like to:
1) Fix that RT.load() message to not end with a colon: "Could not locate %s or %s on classpath."
2) Instead of changing load-one, just add the additional sentence in RT.load(). Second optional sentence could be: "Please check that namespaces with dashes use underscores in the Clojure file name."

Final message would then look like:

"Could not locate a_b__init.class or a_b.clj on classpath. Please check that namespaces with dashes use underscores in the Clojure file name."

clj-1297-v3.patch is nearly identical to clj-1297-v2.patch, described in the previous comment, except it eliminates an unnecessary let expression.