UBSan, clang -fsanitize=undefined, when formatting a time point with a floating-point rep:

__chrono/duration.h:75:63: runtime error: 1e+300 is outside the range of representable values of type 'long'
__chrono/duration.h:75:63: runtime error: nan is outside the range of representable values of type 'long long'

Repro:

using fsec = std::chrono::duration<double>;
fmt::format("{:%T}", fmt::sys_time<fsec>(fsec(1e300)));
fmt::format("{:%T}", fmt::sys_time<fsec>(fsec(NAN)));  // prints "00:00:00an"

The mixed integer <-> float overload of detail::duration_cast delegates to std::chrono::duration_cast, so to_time_t and the sys_time / local_time formatters convert the seconds count with an unchecked static_cast. Only the float to integral direction can overflow, so the overload is split: integer to float keeps the plain cast, and float to integral range-checks the count before casting (which also rejects NaN) and throws like the other checked overloads, with the same boundary handling as to_nonnegative_int (#4802). is_similar_arithmetic_type had no remaining users and is removed.