[Bug preprocessor/56825] Preprocessor does not expand macro correctly if it is an argument and argument of a macro contains ##

["stelek at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56825

--- Comment #4 from Evgeny Televitckiy <stelek at gmail dot com> 2013-04-04 06:41:38 UTC ---
Ye, I thought it was something along those lines. Thanks for the clarification,
now I understand the reason why this paragraph appear in standard.
Yet, appealing to the end of your comment. I would expect ,##x construct to be
an exception, especially that it has a totally different meaning then
concatenating # or stringifying ##.
Clearly one would expect:
#define E3N(n, ar...) E3NI(_, ##ar, n, n, n)
#define E3NI(a0, a1, a2, a3, ...) a1, a2, a3

#define USE_E3_ARG_R(n, ar...) (E3N(n, ar))
#define USE_E3_ARG_W(n, ar...) (E3N(n, ##ar))

and used:
 USE_E3_ARG_R(5, USE_E3_ARG_R(6, 7));
 USE_E3_ARG_W(5, USE_E3_ARG_W(6, 7));

To produce the same result...

In addition I wanted to add that it is not some virtual problem. I actually
spent 3 hours of my life in that macro hell, trying to understand what went
wrong with that macro,