This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 114a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-04-18

2685. Aggregate CTAD, string, and brace elision

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

Consider:

  template <class T>
  struct A {
    T ar[4];
  };
  A a = { "foo" }; 

Subclause 12.2.2.9 [over.match.class.deduct] bullet 1.5 specifies:

For each xi, let ei be the corresponding aggregate element of C or of one of its (possibly recursive) subaggregates that would be initialized by xi (9.4.2 [dcl.init.aggr]) if

• brace elision is not considered for any aggregate element that has a dependent non-array type or an array type with a value-dependent bound, and
• ...

The normative rule does not properly consider arrays with dependent element type, initialized by a string literal. MSVC accepts, gcc and clang reject the example.

Suggested resolution [SUPERSEDED]:

Change in 12.2.2.9 [over.match.class.deduct] bullet 1.5 as follows:

For each xi, let ei be the corresponding aggregate element of C or of one of its (possibly recursive) subaggregates that would be initialized by xi (9.4.2 [dcl.init.aggr]) if

• brace elision is not considered for any aggregate element that has
  • a dependent non-array type or ,
  • an array type with a value-dependent bound, or
  • a string literal as the corresponding initializer and that has an array type whose element type is a (possibly cv-qualified) template parameter or is a dependent type specified with a qualified-id whose nested-name-specifier is dependent or with a decltype-specifier; and
• ...

Proposed resolution (approved by CWG 2023-02-09):

Change in 12.2.2.9 [over.match.class.deduct] bullet 1.5 as follows:

For each xi, let ei be the corresponding aggregate element of C or of one of its (possibly recursive) subaggregates that would be initialized by xi (9.4.2 [dcl.init.aggr]) if

• brace elision is not considered for any aggregate element that has
  • a dependent non-array type or ,
  • an array type with a value-dependent bound, or
  • an array type with a dependent array element type and x_i is a string literal; and
• ...