[clang-tidy] false positive readability-container-size-empty

[iillyyaa]

std::regex_result type has two methods: difference_type length(size_type n = 0) const and bool empty() const
However, they are not an exact match for each other with respect to the readability-container-size-empty check, in particular, when the index n is supplied, since there is no corresponding empty(size_type n) overload.

Yet, this (trimmed) code is claimed to violate the readability-container-size-empty check:

#include <regex>
#include <string>

bool test(const std::string& val) {
    static const std::regex re("([0-9]+)/([A-B]+)");
    if (std::smatch m; std::regex_match(val, m, re)) {
        return m.length(2) > 0;
    }
    return false;
}

Here is how it was originally reported against a similar fragment in our code base:

/home/ilyak/veo/crux/app/bbr/key.cpp:37:9: error: the 'empty' method should be used to check for emptiness instead of 'length' [readability-container-size-empty,-warnings-as-errors]
   37 |     if (m.length(4) > 0) {
      |         ^~~~~~~~~~~~~~~
      |         !m.empty()
/home/ilyak/veo/oe-scarthgap/tmp/build-glibc/sysroots/intel-skylake-64/usr/lib/x86_64-oe-linux/13.2.0/../../../include/c++/13.2.0/bits/regex.h:1887:7: note: method 'match_results'::empty() defined here
 1887 |       empty() const noexcept
      |       ^

Reproducible in clang-tidy (master) via: https://godbolt.org/z/eocdWo7j4