gsliepen

    auto [value, text, goose] = std::scan<int, std::string, Goose>(input, "{} {} {}");

    std::optional<int> value;
    std::optional<std::string> text;
    std::optional<Goose> goose;
    /* auto result = */ std::scan(input, "{} {} {}", value, text, goose);

    template<>
    struct std::scanner<Goose> {
        constexpr auto parse(std::format_parse_context& ctx) {…}
        auto scan(std::format_context& ctx) const -> std::optional<Goose> {…}
    };

    if (sscanf(user_input, "%4u-%2u-%2u", &year, &month, &day) != 3) {
        // return an error
    }

    if (sscanf(user_input, "%4u-%2u-%2u%c", &year, &month, &day, &dummy) != 3) {
        // return an error
    }