This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 116a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-12-19

2713. Initialization of reference-to-aggregate from designated initializer list

[Accepted as a DR at the June, 2023 meeting.]

Aggregates can be initialized by a designated initializer list, but references to aggregates cannot, although list-initialization of such with a regular braced-init-list is fine.

Subclause 9.4.5 [dcl.init.list] paragraph 3 specifies:

List-initialization of an object or reference of type T is defined as follows:

• If the braced-init-list contains a designated-initializer-list, T shall be an aggregate class. ...
• ...
• Otherwise, if T is a reference type, a prvalue is generated. The prvalue initializes its result object by copy-list-initialization from the initializer list. The prvalue is then used to direct-initialize the reference. The type of the prvalue is the type referenced by T, unless ...
• ...

Subclause 12.2.4.2.6 [over.ics.list] paragraph 2 specifies:

If the initializer list is a designated-initializer-list, a conversion is only possible if the parameter has an aggregate type that can be initialized from the initializer list according to the rules for aggregate initialization (9.4.2 [dcl.init.aggr]), in which case the implicit conversion sequence is a user-defined conversion sequence whose second standard conversion sequence is an identity conversion.

Proposed resolution (approved by CWG 2023-04-28):

1. Change in 9.4.5 [dcl.init.list] bullet 3.1 as follows:

   • If the braced-init-list contains a designated-initializer-list and T is not a reference type, T shall be an aggregate class. ...
   • ...

2. Change in 9.4.5 [dcl.init.list] bullet 3.9 as follows:

   Otherwise, if the initializer list is not a designated-initializer-list and has a single element of type E and ...

3. Change in 9.4.5 [dcl.init.list] bullet 3.10 as follows:

   [ Example:

     ...
     const B& b2{a};  // error: cannot copy-list-initialize B temporary from A
     struct C { int x; };
     C&& c = { .x = 1 };  // OK
   

   -- end example ]

4. Change in 12.2.4.2.6 [over.ics.list] paragraph 2 as follows:

   If the initializer list is a designated-initializer-list and the parameter is not a reference, a conversion is only possible if the parameter has an aggregate type that can be initialized from the initializer list according to the rules for aggregate initialization (9.4.2 [dcl.init.aggr]), in which case the implicit conversion sequence is a user-defined conversion sequence whose second standard conversion sequence is an identity conversion.