The nested coroutine is specified to return generator<yielded, ranges::range_value_t<R>, Alloc> which can be problematic as the value type of R is really irrelevant to yielded, unnecessarily violating the generator's Mandates (demo):

#include <generator>
#include <vector>

std::generator<std::span<int>> f() {
  std::vector<int> v;
  co_yield v; // ok
}

std::generator<std::span<int>> g() {
  std::vector<std::vector<int>> v;
  co_yield std::ranges::elements_of(v); // hard error
}

This proposed resolution is to change the second template parameter from range_value_t<R> to void since that type doesn't matter to us.