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4432. Clarify element initialization for meta::reflect_constant_array

Section: 21.4.3 [meta.define.static] Status: WP Submitter: Tomasz Kamiński Opened: 2025-10-27 Last modified: 2025-11-11

Priority: Not Prioritized

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Discussion:

21.4.15 [meta.reflection.array] p10 Clarify e_i type. It is not clear what e_i is when proxy references are involved.

21.4.15 [meta.reflection.array] Clarify copy-initialization vs. direct-initialization use The initialization of P uses copy-initialization but the Mandates clause uses direct-initialization.

Previous resolution [SUPERSEDED]:

This wording is relative to N5014.

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

   template<ranges::input_range R>
     consteval info reflect_constant_array(R&& r);
   

   -8- Let T be ranges::range_value_t<R>.

   -9- Mandates: T is a structural type (13.2 [temp.param]), is_constructible_v<T, ranges::range_reference_t<R>> is true, and is_copy_constructible_v<T> is true T satisfies copy_constructible.

   -10- Let V be the pack of values of type info of the same size as r, where the i^th element is reflect_constant(eistatic_cast<T>(*iti)), where e_iit_i is an iterator to the i^th element of r.

   […]

[Kona 2025-11-04; 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 21.4.3 [meta.define.static] as indicated:

   template<ranges::input_range R>
     consteval info reflect_constant_array(R&& r);
   

   -8- Let T be ranges::range_value_t<R> and e_i be static_cast<T>(*iti), where it_i is an iterator to the i^th element of r.

   -9- Mandates: T is a structural type (13.2 [temp.param]), is_constructible_v<T, ranges::range_reference_t<R>> is true, and is_copy_constructible_v<T> is true T satisfies copy_constructible.

   -10- Let V be the pack of values of type info of the same size as r, where the i^th element is reflect_constant(ei), where e_i is an iterator to the i^th element of r.

   […]

   -13- Throws: Any exception thrown by the evaluation of any e_i, or meta::exception unlessif evaluation of any reflect_constant(eei) would exit via an exception is a constant subexpression for every element e of r.