[Bug c++/101510] New: std::filesystem::create_directory on an existing symlink to a directory

["enometh at meer dot net" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101510

            Bug ID: 101510
           Summary: std::filesystem::create_directory  on an existing
                    symlink to a directory
           Product: gcc
           Version: 11.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: enometh at meer dot net
  Target Milestone: ---

cppreference.com states

```     
bool create_directory( const std::filesystem::path& p );

Creates the directory p as if by POSIX mkdir() with a second argument of
static_cast<int>(std::filesystem::perms::all) (the parent directory must
already exist). If the function fails because p resolves to an existing
directory, no error is reported. Otherwise on failure an error is reported.
```

This should accomodate situations when `p' resolves to an existing directory
when `p' it is a symbolic link.

However create_directory(p) fails when p is a symbolic link which points
to an existing directory.

The standard usage pattern is to call mkdir(p) - and if it fails on EEXIST
to stat(2) p - following symlinks, and check if is a directory. The pattern
is used to ensure that `p' can be treated as a directory for further
operations and this includes p being a symbolic link to a directory)

This use of pattern is defeated if user code is replaced with a call to
std::create_directory without considering the symlink following
semantics of the pattern. (eg. replacing glib g_mkdir_with_parents with
std::fs::create_directories)

I think libstdc++ follow symlinks when resolving names for the create_directory
and create_directory functions.