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copied!<p>I've tried this code out on a compiling web site (Coliru), with GCC 4.8, and the results look bleak. I don't know if it's GCC in particular, or if all the other compilers out there do something similar. So can people with other compilers (Clang, Visual C++, Intel, etc.) try this out?</p> <pre><code>#include <cstdarg> #include <iostream> #include <ostream> #include <utility> template < typename ...Args > int super_vaprintf( long, std::va_list &, Args &&... ) { return 17; } template < typename ...Args > int super_printf( long x, Args &&...a, ... ) { std::va_list args2; int result; va_start( args2, a ); // (A) try { result = super_vaprintf( x, args2, std::forward<Args>(a)... ); } catch ( ... ) { va_end( args2 ); throw; } va_end( args2 ); return result; } int main() { std::cout << super_printf<int, int>( 0L, 1, 2, 3, 4, 5 ) << std::endl; // (B) return 0; } </code></pre> <p>The call to <code>super_printf</code> on line (B) explicitly sets the C++ varargs to two <code>int</code> entries. This will make the function use arguments <code>1</code> and <code>2</code> as C++ varargs and the latter three as C varargs.</p> <p>On line (A), the compiler insists that the code with <code>a</code> in it have a "<code>...</code>" somewhere. So I change it to:</p> <pre><code>va_start( args2, a... ); // (A) </code></pre> <p>I get another error about having the wrong number of arguments. It makes sense since <code>a</code> expands to two arguments. If I change line (B) to one C++ vararg:</p> <pre><code>std::cout << super_printf<int>( 0L, 1, 2, 3, 4, 5 ) << std::endl; // (B) </code></pre> <p>it works just fine. If I remove the C++ varargs entirely:</p> <pre><code>std::cout << super_printf<>( 0L, 1, 2, 3, 4, 5 ) << std::endl; // (B) </code></pre> <p>we get the wrong-number-or-arguments error again, because <code>a</code> has length of zero). If we do this when <code>a</code> is empty:</p> <pre><code>va_start( args2, x /*a...*/ ); // (A) </code></pre> <p>the code works again, although there is a warning about <code>x</code> not being the last named parameter.</p> <p>We can approach the example in another way. Let's reset to:</p> <pre><code>va_start( args2, a... ); // (A) //... std::cout << super_printf( 0L, 1, 2, 3, 4, 5 ) << std::endl; // (B) </code></pre> <p>where all the arguments after the first are grouped as C++ varargs. We get the same too-many-arguments error in <code>va_start</code>, of course. I progressively comment out trailing arguments. It works when there are exactly two arguments left (which makes <code>a</code> have exactly one argument).</p> <p>There is also an error when there's only one argument left, but the error message changes to explicitly say "too few" arguments instead of "wrong amount." Like before, I switched out "<code>a...</code>" for "<code>x</code>" in line (A), and the code was accepted, but there was no warning. So it seems that when I explicitly include "<code><Whatever></code>" for <code>super_printf</code> in line (B) I get a different parser error path than when I don't include them, although both paths go to the same conclusion.</p> <p>Time to tell the committee that they overlooked something....</p>

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