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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - __VA_OPT__ is only supported by Clang's preprocessor in C++2a mode but GCC seems to support it in all language modes"
href="https://bugs.llvm.org/show_bug.cgi?id=40665">40665</a>
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<th>Summary</th>
<td>__VA_OPT__ is only supported by Clang's preprocessor in C++2a mode but GCC seems to support it in all language modes
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>Frontend
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>dan@su-root.co.uk
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org, neeilans@live.com, richard-llvm@metafoo.co.uk
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<p>
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<pre>The attached program can be compiled by GCC 8.2.1 in various language modes
```
gcc -x c -std=gnu89 varargs.c
gcc -x c -std=gnu99 varargs.c
gcc -x c -std=gnu11 varargs.c
gcc -x c -std=gnu89 varargs.c
gcc -x c -std=gnu99 varargs.c
gcc -x c -std=gnu11 varargs.c
g++ -x c++ -std=c++98 varargs.c
g++ -x c++ -std=c++11 varargs.c
g++ -x c++ -std=c++14 varargs.c
g++ -x c++ -std=c++17 varargs.c
g++ -x c++ -std=c++2a varargs.c
g++ -x c++ -std=gnu++98 varargs.c
g++ -x c++ -std=gnu++11 varargs.c
g++ -x c++ -std=gnu++14 varargs.c
g++ -x c++ -std=gnu++17 varargs.c
g++ -x c++ -std=gnu++2a varargs.c
```
but Clang fails to compile it because the `__VA_OPT__()` macro does not get
expanded for all of the above language variants except for `c++2a` and
`gnu++2a`.
I believe `__VA_OPT_()` is a C++2a feature so Clang is probably more correct
here.
However according to <a href="https://gcc.gnu.org/onlinedocs/cpp/Variadic-Macros.html">https://gcc.gnu.org/onlinedocs/cpp/Variadic-Macros.html</a>
`__VA_OPT_()` is supposed to be available for GNU C and GNU C++ too so we
probably ought to enable it when we are compiling for the GNU language variants
`-std=gnu*`.
The fact that GCC allows `__VA_OPT__()` in non-GNU language variants that isn't
`c++2a` seems like a bug but I am not currently in a position to report it.</pre>
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