[committed] libstdc++: Fix std::get<T> for std::tuple [PR101427]

[Jonathan Wakely]

On 15/07/21 16:26 +0100, Jonathan Wakely wrote:
>The std::get<T> functions relied on deduction failing if more than one
>base class existed for the type T.  However the implementation of Core
>DR 2303 (in r11-4693) made deduction succeed (and select the
>more-derived base class).
>
>This rewrites the implementation of std::get<T> to explicitly check for
>more than one occurrence of T in the tuple elements, making it
>ill-formed again. Additionally, the large wall of overload resolution
>errors described in PR c++/101460 is avoided by making std::get<T> use
>__get_helper<I> directly instead of calling std::get<I>, and by adding a
>deleted overload of __get_helper<N> for out-of-range N.
>
>Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
>
>libstdc++-v3/ChangeLog:
>
>	PR libstdc++/101427
>	* include/std/tuple (tuple_element): Improve static_assert text.
>	(__get_helper): Add deleted overload.
>	(get<i>(tuple<T...>&&), get<i>(const tuple<T...>&&)): Use
>	__get_helper directly.
>	(__get_helper2): Remove.
>	(__find_uniq_type_in_pack): New constexpr helper function.
>	(get<T>): Use __find_uniq_type_in_pack and __get_helper instead
>	of __get_helper2.
>	* testsuite/20_util/tuple/element_access/get_neg.cc: Adjust
>	expected errors.
>	* testsuite/20_util/tuple/element_access/101427.cc: New test.
>
>Tested powerpc64le-linux. Committed to trunk.

This should be backported to gcc-11 in time for 11.2 as well. If you
see any problems with it please let me know ASAP.