Issue 4119: generator::promise_type::yield_value(ranges::elements_of<R, Alloc>)'s nested generator may be ill-formed (original) (raw)

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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 #include

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

std::generator<std::span> g() { std::vector<std::vector> 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, yielded>
   auto yield_value(ranges::elements_of<R, Alloc> r);

   -13- Effects: Equivalent to:

   auto nested = [](allocator_arg_t, Alloc, ranges::iterator_t i, ranges::sentinel_t s)
   -> generator<yielded, ranges::range_value_tvoid, 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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