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

Section: 23.2.4 [sequence.reqmts], 23.3.16.2 [inplace.vector.cons] Status: New Submitter: Hewill Kang Opened: 2025-10-01 Last modified: 2025-10-04

Priority: Not Prioritized

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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.

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).