https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66318
--- Comment #3 from Martin Sebor <msebor at gcc dot gnu.org> --- You're right that the text of diagnostics is outside the scope of the C standard. But as you note, even diagnostics are relied on by tools to manipulate the referenced files, and changing the file names in the compiler output would break such tools. For example, in Emacs Compilation Mode (http://www.gnu.org/software/emacs/manual/html_node/emacs/Compilation-Mode.html) if diagnostics in a file named "beep\a.c" were introduced by the string "beep<BEL>.c:" instead, Emacs would no longer be able to open the file. On the other hand, "beep\a.c" seems like a corner case not worth worrying about, and since most multibyte encodings (with the exception of ISO 10646) seem to take care to avoid using control characters for the second and subsequent bytes, maybe the impact of such filtering wouldn't be as bad as might at first seem, especially if an option were provided to disable the filtering. David Wheeler presents some good arguments in favor of restricting the set of valid file name characters in his paper on Fixing Unix/Linux/POSIX Filenames: http://www.dwheeler.com/essays/fixing-unix-linux-filenames.html
