What do these numbers mean? "923:12"
I tried line numbers, but that doesn't work.
It means character 12 on line 923, after expanding all macros. This code was written before macro support existed, and the actual macro expansion happens before the parsing that generates such error messages, so the parsing code doesn't know anything about what line numbers were before macros expansion occurred.
I've considered this issue, but don't have any great ideas about how to deal with the problem. Outputting the whole macro-substituted file into the log would be a huge dump of text. I suppose there could be a way to output the substituted text in a separate file...