[Bug libstdc++/69893] [6 Regression] Conflicting declarations in <math.h> and <parallel/algorithm>

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69893

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
And this already failed with GCC 5 and -std=c++11

#include <cmath>
using std::acosh;
#include <tr1/cmath>