Discrepancy in behavior between fractional second parsing between std::chrono::parse and scnlib

[lenard-mosys]

Observed behavior

consider the following:

#include <chrono>
#include <sstream>
#include <iostream>

#include <scn/scan.h>
#include <scn/chrono.h>

int main() {
    const auto datetime = std::string_view("2025-02-05T16:00:01.010101");
    std::chrono::sys_time<std::chrono::nanoseconds> tp;
    auto is = std::istringstream(std::string(datetime));
    is >> std::chrono::parse("%Y-%m-%dT%H:%M:%S", tp);
    if (is.fail()) { return 1; }
    std::cout << tp.time_since_epoch().count() << std::endl;

    auto result = scn::scan<std::chrono::system_clock::time_point>(
        datetime, "{:%Y-%m-%dT%H:%M:%S}");
    if (!result) { return 2; }
    std::cout << result->value().time_since_epoch().count() << std::endl;

    return 0;
}

This program outputs:

1738771201010101000
1738771201000000000

https://godbolt.org/z/vPfxP9bvG

Here std::chrono::parse and scn::scan use the same chrono format specifier for the datetime, but scnlib parses up to the decimal point, and std::chrono::parse parses up to the end of the input string.

Otherwise scn::scan can parse datetimes fine, including the fractional seconds if they include some terminator, like "2025-02-05T16:00:01.010101Z" with the format string %Y-%m-%dT%H:%M:%SZ.

It looks like the difference is that std::chrono::parse eagerly parses, while scnlib lazily.

Expected behavior

1. std::chrono::parse and scnlib::scan to behave consistently when parsing date and time.
2. Overall eager seems to be preferable in this case, regardless of what std::chrono::parse is doing.

Notes

Changing this possibly breaks existing user code. As a compromise a distinct eager seconds format specifier could be added (if it doesn't exist already, I didn't look too hard), and a change in behavior could be considered for %S in a major version bump.