This page is a snapshot from the LWG issues list, see the Library Active Issues List for more information and the meaning of WP status.

4424. meta::define_aggregate should require a class type

Section: 21.4.16 [meta.reflection.define.aggregate] Status: WP Submitter: Jakub Jelinek Opened: 2025-10-20 Last modified: 2025-11-11

Priority: 1

View all other issues in [meta.reflection.define.aggregate].

View all issues with WP status.

Discussion:

The meta::define_aggregate function doesn't say what happens if C does not represent a class type.

It's also unclear whether it should work with aliases to class types, e.g.

struct S; using A = S; ... meta::define_aggregate(^^A, {});

And what happens if you try to define a cv-qualified type:

struct S; meta::define_aggregate(^^const S, {});

[2025-10-23; Reflector poll.]

Set priority to 1 after reflector poll.

[Kona 2025-11-03; approved by LWG. Status changed: New → Immediate.]

Previous resolution [SUPERSEDED]:

This wording is relative to N5014.

1. Modify 21.4.16 [meta.reflection.define.aggregate] as indicated:

   -7- Let C be the class represented by dealias(class_type) and r_K be the K^th reflection value in mdescrs. For every r_K in mdescrs, let (T_K, N_K, A_K, W_K, NUA_K) be the corresponding data member description represented by r_K.

   -8- Constant when:

   • (8.?) — dealias(class_type) represents a class type;
   • (8.1) — C is incomplete from every point in the evaluation context;

[2025-10-24; LWG telecon. Jonathan updates wording]

Make a minimal change for now, can add support for aliases later.

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

Proposed resolution:

This wording is relative to N5014.

1. Modify 21.4.16 [meta.reflection.define.aggregate] as indicated:

   -7- Let C be the class type represented by class_type and r_K be the K^th reflection value in mdescrs. For every r_K in mdescrs, let (T_K, N_K, A_K, W_K, NUA_K) be the corresponding data member description represented by r_K.

   -8- Constant when:

   • (8.?) — class_type represents a cv-unqualified class type;
   • (8.1) — C is incomplete from every point in the evaluation context;