[Bug libstdc++/85672] [9 Regression] error: redefinition of 'constexpr long double std::abs(long double)'

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85672

--- Comment #9 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #8)
> The new logic [...] comes before the automatically added parts.

And obviously you can't #undef something that hasn't been defined yet.