[PATCH] PR libstdc++/80762 avoid ambiguous __constructible_from<void, void>

[Jonathan Wakely]

On 13/12/18 13:10 +0100, Christophe Lyon wrote:
>On Thu, 13 Dec 2018 at 12:00, Jonathan Wakely <jwakely@redhat.com> wrote:
>>
>> On 13/12/18 08:56 +0100, Christophe Lyon wrote:
>> >On Wed, 12 Dec 2018 at 17:13, Jonathan Wakely <jwakely@redhat.com> wrote:
>> >>
>> >> Ensure we don't try to instantiate __is_constructible_from<void, void>,
>> >> because there are two partial specializations that are equally good
>> >> matches.
>> >>
>> >>         PR libstdc++/80762
>> >>         * include/bits/fs_path.h (path::_Path): Use remove_cv_t and is_void.
>> >>         * include/experimental/bits/fs_path.h (path::_Path): Likewise.
>> >>         * testsuite/27_io/filesystem/path/construct/80762.cc: New test.
>> >>         * testsuite/experimental/filesystem/path/construct/80762.cc: New test.
>> >>
>> >
>> >Hi Jonathan,
>> >
>> >One of the new tests fails on bare-metal/newlib targets (aarch64-elf/arm-eabi):
>> >FAIL: experimental/filesystem/path/construct/80762.cc (test for excess errors)
>> >/libstdc++-v3/testsuite/experimental/filesystem/path/construct/80762.cc:20:
>> >fatal error: experimental/filesystem: No such file or directory
>> >
>> >I think there was a similar issue recently, but I don't remember how
>> >you fixed it?
>>
>> Like this. Sorry for forgetting it. Committed to trunk.
>OK thanks.
>Note that only the one in "experimental" failed, the 27_io one did pass.

Because of https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86756 :-)

The <filesystem> header is installed unconditionally, and that test
only needs the header not the library, so it happens to work.

The <experimental/filesystem> header is only installed when
libstdc++fs.a is available.

>>
>> Aside:
>> There's actually no reason the filesystem::path type can't be enabled
>> unconditionally for all targets, as it only relies on portable
>> features like std::string, but currently the definitions of its member
>> functions are in the libstdc++fs.a library which is only enabled
>> conditionally. Later today I plan to move the std::filesystem::path
>> type into the main libstdc++.so library, so only the tests for
>> std::experimental::filesystem::path will depend on libstdc++fs.a being
>> available. Until then, this patch is needed.
>>
>Great!