[Bug libstdc++/123078] std::from_chars handles signedness of zero in underflow inconsistently with floating-point literals

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=123078

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
e.g. if you change the code to:

    constexpr char s[] = "-1e-1000";
    float x = 99.0f;
    const auto result = std::from_chars(s, std::end(s), x);
    // x = -1e-1000;
    std::cout << x;

Then it prints 99.

Or leave it uninitialized and enjoy a screenful of valgrind errors about the
uninitialised value.