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.