Re: [committed] libstdc++: Fix filesystem::path comparisons for C++23

[Christophe Lyon <christophe.lyon@linaro.org>]

Hi Jonathan,

On Fri, 11 Jun 2021 at 20:25, Jonathan Wakely via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> In C++23 there is a basic_string_view(Range&&) constructor, which is
> constrained to take a range (specifically, a contiguous_range). When the
> filesystem::path comparison operators call lhs.compare(rhs) the overload
> taking a string_view is considered, which means checking whether path
> satisfies the range concept. That satisfaction result changes depending
> whether path::iterator is complete, which is ill-formed; no diagnostic
> required. To avoid the problem, this change ensures that the overload
> resolution is performed in a context where path::iterator is complete
> and the range concept is satisfied. (The result of overload resolution
> is always that the compare(const path&) overload is the best match, but
> we still have to consider the compare(basic_string_view<value_type>) one
> to decide if it even participates in overload resolution).
>
> For std::filesystem::path we can't define the comparison operators later
> in the file, because they are hidden friends, so a new helper is
> introduced that gets defined when everything else is complete.
>
> For std::experimental::filesystem::path we can just move the definitions
> of the comparison operators later in the file.
>
> Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
>
> libstdc++-v3/ChangeLog:
>
>         * include/bits/fs_path.h (operator==, operator<=>): Use new
>         _S_compare function.
>         (path::_S_compare): New function to call path::compare in a
>         context where path::iterator is complete.
>         * include/experimental/bits/fs_path.h (operator<, operator==):
>         Define after path::iterator is complete.
>         * testsuite/27_io/filesystem/path/native/conv_c++23.cc: New
>         test.
>         * testsuite/experimental/filesystem/path/native/conv_c++23.cc:
>         New test.
>
> Tested powerpc64le-linux. Committed to trunk.
>
> This also needs to be backported to gcc-11 where the string_view
> constructor is present for C++23 mode.
>

The new test experimental/filesystem/path/native/conv_c++23.cc
fails on arm-eabi and aarch64-elf:
/libstdc++-v3/testsuite/experimental/filesystem/path/native/conv_c++23.cc:4:
fatal error: experimental/filesystem: No such file or directory
(both trunk and gcc-11)

I guess that's easy to fix? :-)


Thanks,

Christophe