Title
Assignment to enumeration variable with a braced-init-list
Status
drwp
Section
7.6.19 [expr.ass]
Submitter
Shafik Yaghmour

Created on 2023-07-06.00:00:00 last changed 1 month ago

Messages

Date: 2023-11-07.07:37:14

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]).
Date: 2023-11-15.00:00:00

[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.

History
Date User Action Args
2024-04-05 21:43:46adminsetstatus: dr -> drwp
2023-12-19 10:15:28adminsetstatus: ready -> dr
2023-11-07 07:37:14adminsetstatus: review -> ready
2023-09-15 21:51:11adminsetmessages: + msg7429
2023-09-15 21:51:11adminsetstatus: open -> review
2023-07-06 00:00:00admincreate