[Bug c/47781] warnings from custom printf format specifiers

["joseph at codesourcery dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47781

--- Comment #12 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
On Thu, 29 Jan 2015, tromey at gcc dot gnu.org wrote:

> E.g., firefox has a logging printf that accepts "%hs" to print char16_t*
> strings.  This extension means that printf checking can't be used here.
> Requiring a plugin to deal with this situation would also be difficult.
> However letting one write __attribute__((printf, 1, 2, "hs", char16_t*))
> would solve this nicely.

Do you then take this as being length modifier 'h' followed by format 
specifier 's', or is it a complete specifier on its own with everything 
that would otherwise be length and specifier being reparsed as an 
extension if it can't be parsed as a standard format?  Do the flags "-wp" 
and "cR" for %s formats apply to this format?