[Bug c/55406] New: Again: pasting xxx and xxx does not give a valid preprocessing token

[Bug c/55406] New: Again: pasting xxx and xxx does not give a valid preprocessing token

[zhouzhenghui at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55406

             Bug #: 55406
           Summary: Again: pasting xxx and xxx does not give a valid
                    preprocessing token
    Classification: Unclassified
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: zhouzhenghui@gmail.com


I have konwn that there are too many "SPAM" about this title. Most of them are
list in BUG [7976], http://gcc.gnu.org/bugzilla/show_bug.cgi?id=7976. The
compiler requirement of a valid preprocessing token is correct due to the
standard definition.

But when taking the the preprocessor as a meta programming language, there is
chance to running into such situation. One thing is the BOOST_PP_IS_EMPTY
assertion. The example may be:


#include <boost/preprocessor/facilities/is_empty.hpp>

BOOST_PP_IS_EMPTY() /* result: 1 */
BOOST_PP_IS_EMPTY(a) /* result: 0 */
BOOST_PP_IS_EMPTY(+a) /* error report here, for '+' cannot be pasted */

And more information I have posted to comp.std.c, take a look at
https://groups.google.com/forum/?fromgroups=#!topic/comp.std.c/7rgt8k6ILMM.

Since in most simple cases, the cocmpiler just does the same thing of pasting
when omitting the ## operator, so may be it is a option to allow pasting tokens
here.

[Bug c/55406] Again: pasting xxx and xxx does not give a valid preprocessing token

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55406

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> 2012-11-20 01:51:39 UTC ---
Undefined at compile time rather than undefined at runtime.  Anyways GCC is
correct.  BOOST_PP_IS_EMPTY is broken plain and simple, report this to boost
instead.

[Bug c/55406] Again: pasting xxx and xxx does not give a valid preprocessing token

[zhouzhenghui at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55406

--- Comment #2 from Zhenghui Zhou <zhouzhenghui at gmail dot com> 2012-11-20 02:19:34 UTC ---
The BOOST interface is just one thing may be dealed with, it may be good to
find the potential power of the preprocessor. IMHO, the undefined behaviour is
less harmful in preprocessor. after all, they can be language extensions.