Created attachment 50317 [details]
test case highlighting the issue

Paths such as \\?\C:\WINDOWS or \\?\UNC\LOCALHOST\c$\WINDOWS don't work properly with std::filesystem::path (e.g. exists() & is_directory() return false).

According to Microsoft's documentation those are valid.[1]

The corresponding variants starting with \\.\ seem to fare better (safe for the root name & absolute issues, but I've filed 99333 for that already).

boost::filesystem works fine with all of these paths.

The attached sample program outputs the following, showing how Boost's & std's implementation differ:

-----------------------------------------
std::filesystem::path for \\?\C:\WINDOWS
  is_aboslute 0
  has_root_path 1
  has_root_name 0
  exists 0
  is_directory 0
boost::filesystem::path for \\?\C:\WINDOWS
  is_aboslute 1
  has_root_path 1
  has_root_name 1
  exists 1
  is_directory 1
std::filesystem::path for \\?\UNC\localhost\c$\WINDOWS
  is_aboslute 0
  has_root_path 1
  has_root_name 0
  exists 0
  is_directory 0
boost::filesystem::path for \\?\UNC\localhost\c$\WINDOWS
  is_aboslute 1
  has_root_path 1
  has_root_name 1
  exists 1
  is_directory 1
std::filesystem::path for \\.\C:\WINDOWS
  is_aboslute 0
  has_root_path 1
  has_root_name 0
  exists 1
  is_directory 1
boost::filesystem::path for \\.\C:\WINDOWS
  is_aboslute 1
  has_root_path 1
  has_root_name 1
  exists 1
  is_directory 1
std::filesystem::path for \\.\UNC\localhost\c$\WINDOWS
  is_aboslute 0
  has_root_path 1
  has_root_name 0
  exists 1
  is_directory 1
boost::filesystem::path for \\.\UNC\localhost\c$\WINDOWS
  is_aboslute 1
  has_root_path 1
  has_root_name 1
  exists 1
  is_directory 1
-----------------------------------------

gcc mingw 10.2.0 from MXE (cross-compiling from Linux to Windows), Boost 1.74.0

[1] https://docs.microsoft.com/en-us/dotnet/standard/io/file-path-formats