Fix LWG-3699 lexically_relative on UNC drive paths (\\?\C:\...) results in a default-constructed value

[strega-nil-ms]

Fixes #2256 and LWG-3699

This is one choice of how to implement this; it makes lexically_relative treat \\?\<drive letter>:\ as equivalent to <drive letter>:\; another option would just be to allow \\?\<drive letter>:\ and not have them be equivalent.

[strega-nil-ms]

Marking decision needed as we need to decide whether \\?\C: and C: are "the same" for purposes of lexically_relative

link to discussion with @BillyONeal

[StephanTLavavej]

Looks good to me - all I noticed were very minor issues/questions.

[strega-nil-ms]

@StephanTLavavej fixed :)

[StephanTLavavej]

We talked about this at the weekly maintainer meeting and decided that the strategy of "UNC paths can be lexically relative to one another, but not UNC and DOS" is reasonable given the Standard's wording and our behavior here (where we consider C: and c: to be different for the purposes of lexically relative).

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

Thanks for improving filesystem behavior on Windows! 🗄️ 🗂️ 💾

[ZubairRafiq]

`#include` <filesystem> #include <vector)
#include <string> #include <iostream>

struct Result {
      std::string s1;
      std::string s2;
      }
Result test () {
      std::string test = {R" (\\?\C:\temp\test-file.txt)");
      std::filesystem::path const p(test);
      
      Result res;
      res.s1.append(p.root_name ().string()); 
      res.s2.append(p.root_path().string());

      std:: cout <‹ std::endl <‹ test <‹ std::endl;
      std::cout <‹ "root_name\t" <‹ p.root_name ().string() <‹ std:: endl;
      std::wcout <‹ L" absolute: " <‹ std::filesystem::absolute (p).wstring() <‹ std::endl; 
      std::cout <‹ "root_directory\t" <‹ p.root_directory().string() <‹ std: :end1; 
      std::cout <‹ "root_path\t" <‹ p.root_path().string() <‹ std::endl;
      std::cout <‹ "relative_path\t" <‹ p.relative_path ().string() <‹ std:: end1; 
      std::cout <‹ "parent_path\t" <‹ p.parent_path().string() ‹‹ std::endl;
      return res;
}

The output:
root name
absolute: w:?\C: \temp\test-file.txt
root directory \
root_path \
relative_path ?\C:\temp\test-file.txt
parent_path ?\C:\temp

The “root name” component extracted from the path is reported as an empty string (””). However, the expected value was “C:”, which represents the drive letter of the path.

The root directory output is "\", however it should be "C:\".
What is the reason for this behavior of std::filesystem::path?