[Bug preprocessor/44317] ,##__VA_ARGS__ comma not eaten with -std=c++0x

[Bug preprocessor/44317] ,##__VA_ARGS__ comma not eaten with -std=c++0x

[skannan at redhat dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44317

Shakthi Kannan <skannan at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |skannan at redhat dot com

--- Comment #1 from Shakthi Kannan <skannan at redhat dot com> ---
Confirmed.

Tested with cpp (GCC) 4.7.2 20121109.

[Bug preprocessor/44317] ,##__VA_ARGS__ comma not eaten with -std=c++0x

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44317

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
                 CC|                            |jakub at gcc dot gnu.org
         Resolution|---                         |INVALID

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Because the eating of comma is a GNU extension.  So it is enabled in GNU
compilation modes (the default, for C -std=gnu89, -std=gnu99, -std=gnu11, for
C++ -std=gnu++98, -std=gnu++03, -std=gnu++11, -std=gnu++0x, -std=gnu++1y) but
not in the more strict standard modes (-std=c{89,99,11},
-std=c++{98,03,11,0x,1y}).

[Bug preprocessor/44317] ,##__VA_ARGS__ comma not eaten with -std=c++0x

[manu at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44317

Manuel López-Ibáñez <manu at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |UNCONFIRMED
                 CC|                            |manu at gcc dot gnu.org
         Resolution|INVALID                     |---

--- Comment #3 from Manuel López-Ibáñez <manu at gcc dot gnu.org> ---
(In reply to Jakub Jelinek from comment #2)
> Because the eating of comma is a GNU extension.  So it is enabled in GNU
> compilation modes (the default, for C -std=gnu89, -std=gnu99, -std=gnu11,
> for C++ -std=gnu++98, -std=gnu++03, -std=gnu++11, -std=gnu++0x,
> -std=gnu++1y) but not in the more strict standard modes (-std=c{89,99,11},
> -std=c++{98,03,11,0x,1y}).

But g++ does not warn with -Wpedantic, as it should. Instead it gives a very
strange message about C99 in C++:

test.cc:7:17: warning: ISO C99 requires rest arguments to be used [enabled by
default]
   ASSERT( false );
                 ^

Clang++ does the right thing:

test.cc:2:39: warning: token pasting of ',' and __VA_ARGS__ is a GNU extension
[-Wgnu]
#define ASSERT( cnd, ... ) SomeClass(),##__VA_ARGS__
                                      ^
test.cc:3:39: warning: token pasting of ',' and __VA_ARGS__ is a GNU extension
[-Wgnu]
#define FAIL( ... )        SomeClass(),##__VA_ARGS__
                                      ^
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Subject: [Bug preprocessor/47254] ICE: unspellable token PRAGMA_EOL with #pragma redefine_extname and -fdirectives-only
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47254

--- Comment #4 from Manuel López-Ibáñez <manu at gcc dot gnu.org> ---
(In reply to Shakthi Kannan from comment #3)
> Sorry, I don't have a gcc.gnu.org account. Can you please change the status
> to "Confirmed"?

It is easy to get one and it is needed for commit access:

http://gcc.gnu.org/svnwrite.html#authenticated

so don't be shy! ;-)

NEW means confirmed, so done!
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http://gcc.gnu.org/bugzilla/show_bug.cgi?idV782

Dodji Seketeli <dodji at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED

--- Comment #4 from Dodji Seketeli <dodji at gcc dot gnu.org> ---
This has been committed to trunk and 4.8.

[Bug preprocessor/44317] ,##__VA_ARGS__ comma not eaten with -std=c++0x

[3dw4rd at verizon dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=44317

--- Comment #4 from Ed Smith-Rowland <3dw4rd at verizon dot net> ---
After the fix for PR61389 goes in we'll get:

[ed@localhost tr2]$ /home/ed/bin/bin/g++ -std=gnu++11 -Wpedantic -c
test_pr44317.cpp
test_pr44317.cpp:7:17: warning: ISO C++11 requires at least one argument for
the "..." in a variadic macro
   ASSERT( false );

It still doesn't point to the macro definition though.

[Bug preprocessor/44317] ,##__VA_ARGS__ comma not eaten with -std=c++0x

[manu at gcc dot gnu.org]

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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=44317

Manuel López-Ibáñez <manu at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2014-07-14
     Ever confirmed|0                           |1

--- Comment #5 from Manuel López-Ibáñez <manu at gcc dot gnu.org> ---
Confirmed. Perhaps the locations are added after the warning is emitted. Or the
incorrect location is passed to the warning. Needs further debugging.
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Subject: [Bug preprocessor/44317] ,##__VA_ARGS__ comma not eaten with -std=c++0x
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idD317

--- Comment #6 from emsr at gcc dot gnu.org ---
Created attachment 33119
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id3119&actioníit
Patch to pedwarn.

This doesn't have the right column pointed out.
Also, the message (and that given by clang) is, in my opinion, obscure.
At least this starts the ball rolling.

[Bug preprocessor/44317] ,##__VA_ARGS__ comma not eaten with -std=c++0x

[kkylheku at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=44317

Kaz Kylheku <kkylheku at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |kkylheku at gmail dot com

--- Comment #12 from Kaz Kylheku <kkylheku at gmail dot com> ---
(In reply to Harald van Dijk from comment #9)
> The comma should not be eaten, as the invocation of FAIL() is valid in
> standard C++. The rules of concatenation of empty pp-token sequences are
> well-defined: the empty sequences are replaced by placemarker preprocessor
> tokens, and concatenating any non-placeholder token with a placemarker
> preprocessor token produces the former, see [cpp.concat]. 

And thus, more directly to the point, here is the fix:

    cpp -std=gnu++0x test.cpp

The behavior does not reproduce then; the comma eating works.

If you want GNU extensions, request the GNU dialect which most closely
corresponds to the ISO dialect you want.

The issue here is that ## __VA_ARGS__ is a conforming extension in C,
and so it can work even when an ISO dialect is selected.
The documentation does not mention that ## __VA_ARGS__ might be
valid in some dialects of C++ and so has to be enabled as a nonconforming
extension.

Also this is an opton. The following workaround also works for me,
on gcc 10.x on Debian 11:

    #define ASSERT( cnd, ... ) SomeClass() __VA_OPT__(,) __VA_ARGS__
    #define FAIL( ... )        SomeClass() __VA_OPT__(,) __VA_ARGS__

This works with "cpp" with no options, with "-std=gnu++0x",
and with "-std=c++0x".

This may be a better workaround for someone who doesn't want to broadly
switch to the GNU dialect. __VA_OPT__ didn't exist when this bug was opened.