[Bug libstdc++/83891] New: std::filesystem::path::is_absolute for Windows

[faithandbrave at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83891

            Bug ID: 83891
           Summary: std::filesystem::path::is_absolute for Windows
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: faithandbrave at gmail dot com
  Target Milestone: ---

Current is_absolute implementation is follow:

inline bool
path::is_absolute() const
{
#ifdef _GLIBCXX_FILESYSTEM_IS_WINDOWS
  return has_root_name();
#else
  return has_root_directory();
#endif
}

This means path("C:").is_absolute() == true. However, Windows GetFullPathName()
API says:

> If you specify "U:" the path returned is the current directory on the "U:\" 
> drive
https://msdn.microsoft.com/en-us/library/windows/desktop/aa364963(v=vs.85).aspx

I think path("C:").is_absolute() should return false.
As additional information, libc++/msvc experimental implementation returns
false.