Issue 4537: Improve define_static

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4537. Improve `define_static_array`

Section: 21.4.3 [meta.define.static] Status: Immediate Submitter: Hewill Kang Opened: 2026-03-09 Last modified: 2026-03-27

Priority: 2

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Discussion:

define_static_array can transform any input range, even a non-contiguous one, into a span.

However, the returned span always has a dynamic extent. If the size of the input range is known at compile time, returning a static span seems reasonable and may offer slight runtime efficiency.

[2026-03-25; Reflector poll.]

Set priority to 2 after reflector poll.

[Croydon 2026-03-25; move to Immediate.]

Previous resolution [SUPERSEDED]:

This wording is relative to N5032.

Modify 21.4.1 [meta.syn], header <meta> synopsis, as indicated:

#include <compare>              // see 17.12.1 [compare.syn]
#include <initializer_list>     // see 17.11.2 [initializer.list.syn]

namespace std {
  […]
  // 21.4.3 [meta.define.static], promoting to static storage
  […]
  template<ranges::input_range R>
    consteval span<const ranges::range_value_t<R>, see below> define_static_array(R&& r);
  […]
}

Modify 21.4.3 [meta.define.static] as indicated:

template<ranges::input_range R>
  consteval span<const ranges::range_value_t<R>, see below> define_static_array(R&& r);
-16- Effects: Equivalent to:
using T = ranges::range_value_t<R>;
meta::info array = meta::reflect_constant_array(r);
if (meta::is_array_type(meta::type_of(array))) {
  return span<const T, see below>(meta::extract<const T*>(array), meta::extent(meta::type_of(array)));
} else {
  return span<const T, see below>();
}
-?- Remarks: The second template argument of the returned span type is static_cast<size_t>(ranges::size(r)) if ranges::size(r) is a constant expression, and dynamic_extent otherwise.

[2026-03-26; Tim reopens and provides revised wording]

The else branch is ill-formed when the returned has non-zero static extent.

[Croydon 2026-03-27; move to Immediate.]

Proposed resolution:

This wording is relative to N5032.

Modify 21.4.1 [meta.syn], header <meta> synopsis, as indicated:

#include <compare>              // see 17.12.1 [compare.syn]
#include <initializer_list>     // see 17.11.2 [initializer.list.syn]

namespace std {
  […]
  // 21.4.3 [meta.define.static], promoting to static storage
  […]
  template<ranges::input_range R>
    consteval span<const ranges::range_value_t<R>, see below> define_static_array(R&& r);
  […]
}

Modify 21.4.3 [meta.define.static] as indicated:

template<ranges::input_range R>
  consteval span<const ranges::range_value_t<R>, see below> define_static_array(R&& r);
-16- Effects: Equivalent to:
using T = ranges::range_value_t<R>;
meta::info array = meta::reflect_constant_array(r);
if (meta::is_array_type(meta::type_of(array))) {
  return span<const T, see below>(meta::extract<const T*>(array), meta::extent(meta::type_of(array)));
} else {
  return span<const T, see below>(static_cast<const T*>(nullptr), 0);
}
-?- Remarks: The second template argument of the returned span type is static_cast<size_t>(ranges::size(r)) if ranges::size(r) is a constant expression, and dynamic_extent otherwise.

4537. Improve define_static_array

4537. Improve `define_static_array`