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#include <iostream>
#include <cmath>
 
namespace foo {
    template<class T> 
    inline bool isnan (T x)
    { //!< \brief return true if floating-point type t is NaN (Not A Number).
       std::cout << "foo::isnan" << std::endl;
       return true;
    }
}
 
using foo::isnan;
using std::isnan;
 
int main () {
    std::cout << isnan(5.5f) << std::endl;
    return 0;
}

prog.cpp:14:12: error: 'constexpr bool std::isnan(double)' conflicts with a previous declaration
 using std::isnan;
            ^
In file included from /usr/include/features.h:374:0,
                 from /usr/include/i386-linux-gnu/c++/5/bits/os_defines.h:39,
                 from /usr/include/i386-linux-gnu/c++/5/bits/c++config.h:482,
                 from /usr/include/c++/5/iostream:38,
                 from prog.cpp:1:
/usr/include/i386-linux-gnu/bits/mathcalls.h:234:1: note: previous declaration 'int isnan(double)'
 __MATHDECL_1 (int,isnan,, (_Mdouble_ __value)) __attribute__ ((__const__));
 ^

Standard output is empty