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3894. generator::promise_type::yield_value(ranges::elements_of<Rng, Alloc>) should not be noexcept

Section: 26.8.5 [coro.generator.promise] Status: Tentatively Ready Submitter: Tim Song Opened: 2023-02-25 Last modified: 2023-03-22

Priority: Not Prioritized

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Discussion:

The overload of yield_value for yielding elements of arbitrary ranges does so by creating a nested generator, but to do so it needs to:

• call ranges::begin/ranges::end on the range

• allocate a new coroutine frame (unless the allocation is elided by the compiler, which isn't guaranteed)

• copy/move the iterator and sentinel into the coroutine frame

All of these are allowed to throw, so this overload should not be noexcept.

[2023-03-22; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

Proposed resolution:

This wording is relative to N4928.

1. Modify 26.8.5 [coro.generator.promise] as indicated:

   namespace std {
     template<class Ref, class V, class Allocator>
     class generator<Ref, V, Allocator>::promise_type {
     public:
       […]
       auto yield_value(const remove_reference_t<yielded>& lval)
         requires is_rvalue_reference_v<yielded> &&
           constructible_from<remove_cvref_t<yielded>, const remove_reference_t<yielded>&>;
       
       template<class R2, class V2, class Alloc2, class Unused>
         requires same_as<typename generator<R2, V2, Alloc2>::yielded, yielded>
           auto yield_value(ranges::elements_of<generator<R2, V2, Alloc2>&&, Unused> g) noexcept;
       
       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) noexcept;
       […]
      };
   }
   

   […]

   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) noexcept;
   

   -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>, 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))));
   

   […]