This is my attempt to merge D98077 (bugfix the format strings for Windows paths, which use wchar_t not char) and D96986 (replace C++ variadic templates with C-style varargs so that __attribute__((format(printf))) can be applied, for better safety). Those two patches merge-conflict with each other, and also I had some pretty heavy "requested changes" on D96986, so I decided to just show what I mean directly.
The one intentional functional change here is in __create_what. It now prints path1 and path2 in square-brackets and double-quotes, rather than just square-brackets. Prior to this patch, it would print either path double-quoted if-and-only-if it was the empty string. Now the double-quotes are always present. I doubt anybody's code is relying on the current format, right?
(Added @EricWF as the original author of this code, in 9158bfd32ebd4)