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 <jwak...@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.
