[Bug libstdc++/110860] std::format("{:f}",2e304) invokes undefined behaviour

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110860

--- Comment #10 from Jonathan Wakely <redi at gcc dot gnu.org> ---
GCC's ubsan doesn't seem to diagnose this rule:

"A prvalue of a floating-point type can be converted to a prvalue of an integer
type. The conversion truncates; that is, the fractional part is discarded. The
behavior is undefined if the truncated value cannot be represented in the
destination type."

There's no ubsan diagnostic for:

unsigned long i = -HUGE_VAL;

So it doesn't complain about adding log10(0) to a size_t.

I can reproduce it with Clang though, and have tested a fix.