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4396. Improve inplace_vector(from_range_t, R&& rg)

Section: 23.2.4 [sequence.reqmts], 23.3.16.2 [inplace.vector.cons] Status: WP Submitter: Hewill Kang Opened: 2025-10-01 Last modified: 2025-11-11

Priority: 3

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Discussion:

Consider:

std::array<int, 42> a;
std::inplace_vector<int, 5> v(std::from_range, a);

The above throws std::bad_alloc at runtime because the size of array is larger than capacity of inplace_vector. However, we should reject it at compile time since the array size is a constant expression.

Given that we do a lot of compile-time size checking in <simd>, it's worth applying that here as well. Compile-time errors are better than runtime ones.

[2025-10-22; Reflector poll. Status changed: New → LEWG and P3.]

General support for change, after LEWG approval.

Suggestion was made that this could be extended to all containers, but is unlikely to be triggred in real word, as it requires ranges with static size greater than size_t(-1).

[Kona 2025-11-06; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 23.2.4 [sequence.reqmts] as indicated:

   a.assign_range(rg)
   

   -60- Result: void

   -61- Mandates: assignable_from<T&, ranges::range_reference_t<R>> is modeled. For inplace_vector, if ranges::size(rg) is a constant expression then ranges::size(rg) ≤ a.max_size().

2. Modify 23.3.16.2 [inplace.vector.cons] as indicated:

   template<container-compatible-range<T> R>
     constexpr inplace_vector(from_range_t, R&& rg);
   

   -?- Mandates: If ranges::size(rg) is a constant expression then ranges::size(rg) ≤ N.

   -9- Effects: Constructs an inplace_vector with the elements of the range rg.

   -10- Complexity: Linear in ranges::distance(rg).