<div dir="ltr">Consider the following small test application:<div> </div><div><pre class="" style="font-size:13px;margin-top:0px;margin-bottom:1em;padding:5px;border:0px;width:auto;max-height:600px;overflow:auto;font-family:Consolas,Menlo,Monaco,"Lucida Console","Liberation Mono","DejaVu Sans Mono","Bitstream Vera Sans Mono","Courier New",monospace,sans-serif;color:rgb(57,51,24);word-wrap:normal;background-color:rgb(239,240,241)"><code style="margin:0px;padding:0px;border:0px;font-family:Consolas,Menlo,Monaco,"Lucida Console","Liberation Mono","DejaVu Sans Mono","Bitstream Vera Sans Mono","Courier New",monospace,sans-serif;white-space:inherit">#include <iostream> #include <cmath> #include <math.h> int main() { double d = NAN; std::cout << std::isnan(d) << '\n'; std::cout << isnan(d) << '\n'; return 0; }</code></pre></div><div>Compiling on Ubuntu 16.04 with g++-5.4.0 using C++11 standard fails:</div><div> </div><div><pre class="" style="margin-top:0px;margin-bottom:0px;padding:5px;border:0px;font-size:13px;width:auto;max-height:600px;overflow:auto;font-family:Consolas,Menlo,Monaco,"Lucida Console","Liberation Mono","DejaVu Sans Mono","Bitstream Vera Sans Mono","Courier New",monospace,sans-serif;color:rgb(57,51,24);word-wrap:normal;background-color:rgb(239,240,241)"><code style="margin:0px;padding:0px;border:0px;font-family:Consolas,Menlo,Monaco,"Lucida Console","Liberation Mono","DejaVu Sans Mono","Bitstream Vera Sans Mono","Courier New",monospace,sans-serif;white-space:inherit">$ g++ -std=c++11 main.cpp main.cpp: In function ‘int main()’: main.cpp:10:25: error: ‘isnan’ was not declared in this scope std::cout << isnan(d) << '\n'; ^ main.cpp:10:25: note: suggested alternative: In file included from main.cpp:3:0: /usr/include/c++/5/cmath:641:5: note: ‘std::isnan’ isnan(_Tp __x) ^ </code></pre></div><div><code style="margin:0px;padding:0px;border:0px;font-family:Consolas,Menlo,Monaco,"Lucida Console","Liberation Mono","DejaVu Sans Mono","Bitstream Vera Sans Mono","Courier New",monospace,sans-serif;white-space:inherit"> </code></div><div><code style="margin:0px;padding:0px;border:0px;font-family:Consolas,Menlo,Monaco,"Lucida Console","Liberation Mono","DejaVu Sans Mono","Bitstream Vera Sans Mono","Courier New",monospace,sans-serif;white-space:inherit">I've compiled this on Centos gcc-4.9.1 and gcc-5.2.1 and it works on both. </code></div><div><code style="margin:0px;padding:0px;border:0px;font-family:Consolas,Menlo,Monaco,"Lucida Console","Liberation Mono","DejaVu Sans Mono","Bitstream Vera Sans Mono","Courier New",monospace,sans-serif;white-space:inherit">I've compiled this on Ubuntu gcc-4.9.3 and that too works.</code></div><div><code style="margin:0px;padding:0px;border:0px;font-family:Consolas,Menlo,Monaco,"Lucida Console","Liberation Mono","DejaVu Sans Mono","Bitstream Vera Sans Mono","Courier New",monospace,sans-serif;white-space:inherit">However, compiling on Ununtu </code>gcc-5.4.0 doesn't.</div><div> </div><div>I've also tried several online compilers, with mixed results.</div><div> </div><div><div><a href="http://melpon.org/wandbox/">Wandbox</a> has gcc-6.3.0, gcc-5.2.0, gcc-5.1.0 and they all compiled fine. </div><div><a href="http://gcc.godbolt.org/#">Godbolt</a> broke for gcc-4.9.2 through gcc-5.3, but then works on 6.1.</div></div><div> </div><div>Given it works for gcc-5.2.0 on Wandbox and breaks for gcc-5-2.0 on Godbolt makes me thing one must be using a patched standard library.</div><div> </div><div>Not sure how to report this bug?</div></div>