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

2768. Assignment to enumeration variable with a braced-init-list

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

Consider:

   enum class E {E1};

   void f() {
     E e;
     e = E{0}; // #1
     e = {0};  // #2
   }

#1 first initializes a temporary of type E and then assigns that to e. For #2, 7.6.19 [expr.ass] bullet 8.1 specifies that #2 is equivalent to #1:

A braced-init-list may appear on the right-hand side of

• an assignment to a scalar, in which case the initializer list shall have at most a single element. The meaning of x = {v}, where T is the scalar type of the expression x, is that of x = T{v}. The meaning of x = {} is x = T{}.
• ...

However, there is no syntactic hint that #2 would invoke direct-initialization, and in fact gcc, icc, and MSVC reject #2, but clang accepts.

Proposed resolution (approved by CWG 2023-11-06):

Change in 7.6.19 [expr.ass] paragraph 8 as follows:

A braced-init-list B may appear on the right-hand side of

• an assignment to a scalar of type T, in which case the initializer list B shall have at most a single element. The meaning of x = {v}B is x = t, where t is an invented temporary variable declared and initialized as T t = B, where T is the scalar type of the expression x, is that of x = T{v}. The meaning of x = {} is x = T{}.
• an assignment to an object of class type, in which case the initializer list B is passed as the argument to the assignment operator function selected by overload resolution (12.4.3.2 [over.ass], 12.2 [over.match]).