[Bug libstdc++/48406] New: <algorithm> #undefs isfinite() from math.h in C++0x mode

[jyasskin at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48406

           Summary: <algorithm> #undefs isfinite() from math.h in C++0x
                    mode
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: jyasskin@gcc.gnu.org


$ cat test.cc
#include <math.h>
#include <algorithm>

bool foo(double d) {
  return isfinite(d);
}
$ ~/opensource/gcc/git/install/bin/g++ -c test.cc 
$ ~/opensource/gcc/git/install/bin/g++ -c test.cc -std=gnu++0x
test.cc: In function ‘bool foo(double)’:
test.cc:5:20: error: ‘isfinite’ was not declared in this scope
test.cc:5:20: note: suggested alternative:
.../include/c++/4.7.0/cmath:528:21: note:   ‘std::isfinite’
$

This is a regression since gcc-4.4:

$ g++-4.4 -c test.cc -std=gnu++0x
$ 

It happens because <stl_algo.h> includes <random> in C++0x mode only, and
<random> includes <cmath>, which #undefs all the C99 math functions that
<math.h> defined, without 'using' the std functions back into the global
namespace.

As far as I can tell, [depr.c.headers] says that <math.h> puts the <cmath>
names into the global namespace, and it doesn't allow a later include of
<cmath> to take them back out. [depr.c.headers] also allows <cmath> to
unconditionally put its declarations and definitions into the global namespace,
which is what I'd propose as a fix.