[PATCH] libstdc++: Fix filesystem::path constraints for volatile [PR 100630]

[Jonathan Wakely]

On 17/05/21 15:02 +0100, Jonathan Wakely wrote:
>The constraint check for filesystem::path construction uses
>decltype(__is_path_src(declval<Source>())) which mean it considers
>conversion from an rvalue.  When Source is a volatile-qualified type
>it cannot use is_path_src(const Unknown&) because a const lvalue
>reference can only bind to a non-volatile rvalue.
>
>Since the relevant path members all have a const Source& parameter,
>the constraint should be defined in terms of declval<const Source&>(),
>not declval<Source>(). This avoids the problem of volatile-qualified
>rvalues, because we no longer use an rvalue at all.
>
>libstdc++-v3/ChangeLog:
>
>	PR libstdc++/100630
>	* include/bits/fs_path.h (__is_constructible_from): Test
>	construction from a const lvalue, not an rvalue.
>	* include/experimental/bits/fs_path.h (__is_constructible_from):
>	Likewise.
>	* testsuite/27_io/filesystem/path/construct/100630.cc: New test.
>	* testsuite/experimental/filesystem/path/construct/100630.cc:
>	New test.
>
>Tested x86_64-linux, pushed to gcc-10 (this isn't needed for gcc-11 or
>trunk, but I also plan to backport it to gcc-9).

Oh actually this is needed for experimental::filesystem::path on trun
kand gcc-11 (as I found when I added to the new tests to trunk) so
I'll fix it there too.