[committed] libstdc++: Add feature test macro for heterogeneous lookup in unordered containers

[Jonathan Wakely]

On 04/06/21 16:01 +0100, Jonathan Wakely wrote:
>Also update the C++20 status docs.
>
>Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
>
>libstdc++-v3/ChangeLog:
>
>	* doc/xml/manual/status_cxx2020.xml:
>	* doc/html/*: Regenerate.
>	* include/bits/hashtable.h (__cpp_lib_generic_unordered_lookup):
>	Define.
>	* include/std/version (__cpp_lib_generic_unordered_lookup):
>	Define.
>	* testsuite/23_containers/unordered_map/operations/1.cc: Check
>	feature test macro.
>	* testsuite/23_containers/unordered_set/operations/1.cc:
>	Likewise.
>
>Tested powerpc64le-linux. Committed to trunk.
>
>I'll also add a note to the GCC 11 release notes, and I've updated the
>compiler support page at cppreference.com

Oh, and this should be backported to gcc-11 too.