[Bug libstdc++/68984] error: ‘constexpr bool std::isinf(double)’ conflicts with a previous declaration

[glisse at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68984

--- Comment #1 from Marc Glisse <glisse at gcc dot gnu.org> ---
This is really a dup of PR60407, PR48891, but it is true that the error message
looks a bit different when you write things this way.