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4119.generator::promise_type::yield_value(ranges::elements_of<R, Alloc>)'s nested generator may be ill-formed
Section: 25.8.5 [coro.generator.promise] Status: WP Submitter: Hewill Kang Opened: 2024-07-11 Last modified: 2024-11-28
Priority: Not Prioritized
View all other issues in [coro.generator.promise].
View all issues with WP status.
Discussion:
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.
[2024-08-02; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[Wrocław 2024-11-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4986.
Modify 25.8.5 [coro.generator.promise] as indicated:
template<ranges::input_range R, class Alloc> requires convertible_to<ranges::range_reference_t<R>, yielded> auto yield_value(ranges::elements_of<R, Alloc> r);-13- Effects: Equivalent to:
[…]auto nested = [](allocator_arg_t, Alloc, ranges::iterator_t<R> i, ranges::sentinel_t<R> s) -> generator<yielded, ranges::range_value_t<R>void, Alloc> { for (; i != s; ++i) { co_yield static_cast<yielded>(*i); } }; return yield_value(ranges::elements_of(nested( allocator_arg, r.allocator, ranges::begin(r.range), ranges::end(r.range))));
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