[PATCH 1/2] libstdc++: Support UNC paths on MinGW in fs::path

[Jonathan Wakely]

On Sat, 10 May 2025 at 12:49, Johannes Grunenberg
<johannes.grunenberg@stud.uni-hannover.de> wrote:
>
> UNC paths on Windows start with a root name "\\server" instead of a
> drive letter ("c:"). Support for parsing this was already implemented
> for cygwin, which (compared to MinGW) doesn't use backslash as the
> preferred separator.
> This mainly enables SLASHSLASH_IS_ROOTNAME on Windows and adjusts the
> tests accordingly.
> I aligned path::_M_append with path::operator/= to use has_root_directory
> and is_absolute from `this` instead of the appended path. Note that
> before this patch, `is_absolute && has_root_directory` was always
> `false`, as is_absolute implied has_root_directory. This isn't the case
> anymore (e.g. `\\foo` doesn't have a root directory).
>
>         PR libstdc++-v3/99333
>         PR libstdc++-v3/99430
>         PR libstdc++-v3/106127
>
> libstdc++-v3/ChangeLog:
>
>         * config/abi/pre/gnu.ver: Add fs::path::_M_is_unc_path() to ABI
>         * include/bits/fs_path.h: Add and use fs::path::_M_is_unc_path()
>         on Windows.
>         * src/c++17/fs_path.cc (defined): Treat "//" as a root name on
>         Windows.
>         (path::_M_append): Align with operator/=.
>         (path::_M_is_unc_path): Added helper to check for
>         "(slash)(slash)(not-slash)".
>         * testsuite/27_io/filesystem/path/append/path.cc: Added tests for UNC
>         paths.
>         * testsuite/27_io/filesystem/path/append/source.cc: Added tests for
>         UNC paths.
>         * testsuite/27_io/filesystem/path/decompose/filename.cc: Switched
>         Windows FS to test for UNC paths.
>         * testsuite/27_io/filesystem/path/decompose/root_directory.cc:
>         Switched Windows FS to test for UNC paths.
>         * testsuite/27_io/filesystem/path/generation/normal.cc: Switched
>         Windows FS to test for UNC paths.
>         * testsuite/27_io/filesystem/path/generic/94242.cc: Added case for
>         double-slashes as root directories.
>         * testsuite/27_io/filesystem/path/generic/generic_string.cc: Added
>         case for double-slashes as root directories.
>         * testsuite/27_io/filesystem/path/generic/utf.cc: Added case for
>         double-slashes as root directories.
>         * testsuite/27_io/filesystem/path/generic/wchar_t.cc: Added case for
>         double-slashes as root directories.
>         * testsuite/27_io/filesystem/path/itr/traversal.cc: Switched Windows
>         FS to test for UNC paths.
>         * testsuite/27_io/filesystem/path/query/is_absolute.cc: Test that UNC
>         paths are absolute.
>
> Signed-off-by: Johannes Grunenberg <johannes.grunenberg@stud.uni-hannover.de>

Just to check, the Signed-off-by: trailer has a specific meaning which
is described at https://gcc.gnu.org/dco.html
Please confirm that you're using it that way (and not just adding it
because others do so).

I don't know much about UNC paths so this might take me a while to
review, but thanks for working on this and contributing the patches!