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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99430

            Bug ID: 99430
           Summary: std::filesystem::path: UNC device paths with \\?\=E2=80=
=A6 not
                    supported properly
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: moritz at bunkus dot org
  Target Milestone: ---

Created attachment 50317
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D50317&action=3Dedit
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 & st=
d'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=