[Bug libstdc++/89991] Complex numbers: Calculation of imaginary part is not correct

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89991

--- Comment #14 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Which is unsurprising because std::sqrt(z) just calls
__builtin_csqrt(z.__rep())