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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].

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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.

1. 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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