Proposed resolution:

This wording is relative to N4640.

1. Modify [meta.type.synop], <type_traits> header synopsis, as indicated:

   // 20.15.4.3, type properties
   […]
   template <class T> struct is_final;
   template <class T> struct is_aggregate;
   
   […]
   // 20.15.4.3, type properties
   […]
   template <class T> constexpr bool is_final_v
     = is_final<T>::value;
   template <class T> inline constexpr bool is_aggregate_v
     = is_aggregate<T>::value;
   […]
   

2. Add a new row to Table 42 — "Type property predicates":

   template <class T> struct is_aggregate;
   

   T is an aggregate type ([dcl.init.aggr])

   remove_all_extents_t<T> shall be a complete type or cv void.

[ Kona 2017-03-02 ]

Accepted as Immediate to resolve NB comment.

This will require a new complier intrinsic; Alisdair checked with Core and they're OK with that

[ 2017-03-03, Daniel updates wording ]

In sync with existing writing, this was changed to "remove_all_extents_t<T> shall be a complete type or cv void"

[ 2017-03-03, Kona, LEWG ]

No interest in doing the is-list-initializable instead. No objection to getting that proposal also.

Want this for C++17?

SF | F | N | A | SA

2 | 8 | 1 | 3 | 0

Want this for C++20?

SF | F | N | A | SA

6 | 5 | 2 | 0 | 0

Remember to correct the wording to: "remove_all_extents_t<T> shall be a complete type or cv-void."

[ 2017-03-01, Daniel comments and adjusts wording ]

There is only one minor change performed, namely to mark the constant in the is_aggregate_v variable template as inline following the LWG preferences for "full inline" bullet list (B2) from P0607R0.

[ 2017-02-26, Daniel comments and completes wording ]

The proposed wording is incomplete, since it doesn't update the <type_traits> header synopsis, also ensuring that the corresponding is_aggregate_v variable template is defined.

An is_aggregate type trait is needed. The emplace idiom is now common throughout the library, but typically relies on direct non-list initalization, which does not work for aggregates. With a suitable type trait, we could extend direct non-list-initlaization to perform aggregate-initalization on aggregate types.

Proposed change:

Add a new row to Table 38:

template <class T>
struct is_aggregate;

T is an aggregate type ([dcl.init.aggr])

remove_all_extents_t<T> shall be a complete type, an array type, or (possibly cv-qualified) void.