is_aggregateSection: 21.3.5.4 [meta.unary.prop] Status: C++23 Submitter: Tomasz Kamiński Opened: 2022-11-09 Last modified: 2023-11-22
Priority: Not Prioritized
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Discussion:
This is resolution of GB-090
(21.3.5.4 [meta.unary.prop] Unnecessary precondition for is_aggregate).
The precondition for is_aggregate is "remove_all_extents_t<T> shall be a complete type
or cv void." This means that is_aggregate_v<Incomplete[2]> is undefined, but
an array is always an aggregate, we don't need a complete element type to know that.
Historically the is_aggregate was introduced by LWG 2911 as part of the resolution of the NB comments.
The comment proposed to introduce this trait with requirement "remove_all_extents_t<T> shall
be a complete type, an array type, or (possibly cv-qualified) void.",
that is close to resolution proposed in this issue.
According to notes this was simplified during review, after realizing that remove_all_extents_t<T>
is never an array type.
[Kona 2022-11-09; Move to Immediate]
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Immediate → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 21.3.3 [meta.type.synop], Table 46 ([tab:meta.unary.prop]) — "Type property predicates" — as indicated:
Table 48: Type property predicates [tab:meta.unary.prop] Template Condition Preconditions …template<class T, class U>
struct is_aggregate;Tis an aggregate type (9.4.2 [dcl.init.aggr])shall be an array type, a complete type, or cvremove_all_extents_t<T>void.