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

[Jonathan Wakely]

On 17/05/21 15:25 +0100, Jonathan Wakely wrote:
>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.

Here's the patch for trunk and gcc-11.

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