std::isnan<>() is broken on FreeBSD

[Lorenz Minder <lminder at gmx dot net>]

Hi,

On FreeBSD 5.3, if I run the following test program (compiled w/ system gcc,
which is 3.4.2; or a hand-compiled current CVS snapshot):

	#include <iostream>
	#include <cmath>

	int main(void)
	{
		std::cout << std::isnan(3.0) << '\n';
		return 0;
	}

I get a segfault:

	lorinder$ g++ x.cc
	lorinder$ ./a.out 
	Segmentation fault (core dumped)

There is no problem on Linux.  The segfault happens as well if I add
-D_GLIBCXX_USE_C99_MATH.

The problem is, that __gnu_cxx::isnan<>() calls __gnu_cxx::__capture_isnan<>(),
which in turn recursively calls __gnu_cxx::isnan<>().

Indeed, FreeBSD's math.h contains the following snippet:

	#define	isnan(x)					\
	    ((sizeof (x) == sizeof (float)) ? isnanf(x)		\
	    : (sizeof (x) == sizeof (double)) ? isnan(x)	\
	    : __isnanl(x))

So, isnan() is both a macro and a function taking a double. If my
understanding is correct, __capture_isnan() calls __gnu_cxx::isnan() instead of
::isnan().

I am well aware that std::isnan() is probably not strictly standard C++ (since
isnan is new in C99), but I'd consider it a quality of implementation issue to
have it working if it is available.

Should I file a PR?

Regards,
--Lorenz