libstdc++: Do not assume URBG::result_type exists [PR121919]

libstdc++: Do not assume URBG::result_type exists [PR121919]

The ranges::sample and ranges::shuffle algorithms are supposed to work
with types which model std::uniform_random_bit_generator, which means
they should not assume that G::result_type is present. That isn't needed
to satisfy the concept. Change the algorithms to use decltype(__g())
instead of using result_type.

This isn't sufficient to fix the bug though, because those algorithms
use std::uniform_int_distribution and that class template's operator()
overloads depend on the more restrictive uniform random bit generator
requirements, which do include the presence of a nested result_type
member.

We need to change std::uniform_int_distribution to also use decltype
instead of the nested result_type, even though the standard says that
std::uniform_int_distribution is allowed to assume that result_type
exists.

There's yet another problem, which is that a type that returns random
bool values can model the concept, but doesn't meet the named
requirements and can't be used with std::uniform_int_distribution. That
isn't addressed by this change.

libstdc++-v3/ChangeLog:

	PR libstdc++/121919
	* include/bits/ranges_algo.h (__sample_fn, __shuffle_fn): Use
	decltype(__g()) instead of remove_reference_t<_G>::result_type.
	* include/bits/uniform_int_dist.h
	(uniform_int_distribution::operator()): Use decltype(__urng())
	instead of _UniformRandomBitGenerator::result_type
	(uniform_int_distribution::__generate_impl): Likewise.
	* testsuite/25_algorithms/sample/121919.cc: New test.
	* testsuite/25_algorithms/shuffle/121919.cc: New test.

Reviewed-by: Nathan Myers <nmyers@redhat.com>