Error(s):

source_file.cpp(26): error C2668: 'fma': ambiguous call to overloaded function
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE\cmath(448): note: could be 'long double fma(long double,long double,long double) noexcept'
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE\cmath(151): note: or       'float fma(float,float,float) noexcept'
C:\Program Files (x86)\Windows Kits\10\include\10.0.10240.0\ucrt\math.h(494): note: or       'double fma(double,double,double)'
source_file.cpp(21): note: or       'ns::foo ns::fma<ns::foo,ns::foo,ns::foo,0>(T &&,U &&,V &&)' [found using argument-dependent lookup]
        with
        [
            T=ns::foo,
            U=ns::foo,
            V=ns::foo
        ]
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE\xtgmath.h(146): note: or       'float fma<ns::foo,ns::foo,ns::foo>(_Ty1,_Ty2,_Ty3)'
        with
        [
            _Ty1=ns::foo,
            _Ty2=ns::foo,
            _Ty3=ns::foo
        ]
source_file.cpp(26): note: while trying to match the argument list '(ns::foo, ns::foo, ns::foo)'
Microsoft (R) C/C++ Optimizing Compiler Version 19.00.23506 for x64