Hi,

maxim.cournoyer@gmail.com skribis:

> The following C++ snippet file reproduces it:
>
> // file: test.cpp
>
> #include <cmath>
> #include <iostream>
>
>
> int main() {
>     float x = 9.999;
>
>     std::cout << "rounding value: " << x << " --> " << std::round(x);
>
>     return 0;
> }
>
> // build with:
> // g++ -o test.o -std=c++17 test.cpp
>
>
> It affects only GCC 8 and newer, GCC 7 is fine.  The available "round"
> seems to be the one originating from tgmath.h (implementation detail of
> math.h AFAICT).
>
> The std::round function is conditionally defined in the C++ standard
> (since C++11) in the cmath header. It is supposed to also #undef the
> global "round" macro, but it doesn't, or perhaps the ordering of the
> includes is wrong and math.h gets included following cmath.
>
> In cmath, macros such as 'round' are undefined based on some
> conditions.  These conditions appear to be:
>
> __cplusplus >= 201103L and _GLIBCXX_USE_C99_MATH_TR1 and not _GLIBCXX_NO_C99_ROUNDING_FUNCS

See also
<https://lists.gnu.org/archive/html/guix-devel/2020-06/msg00096.html>,
which suggests rather a misconfiguration issue.

Comparing ‘c++config.h’ from GCC 7 (which works) and GCC 8 (the first
one that exhibits this problem), we see: