This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 110b. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2023-01-15


2004. Unions with mutable members in constant expressions

Section: 7.7  [expr.const]     Status: CD4     Submitter: Richard Smith     Date: 2014-09-16

[Moved to DR at the October, 2015 meeting.]

In an example like

  union U { int a; mutable int b; };
  constexpr U u1 = {1};
  int k = (u1.b = 2);
  constexpr U u2 = u1; // ok!!

The initialization of u2 is not disqualified by the current wording of the Standard because the copy is done via the object representation, not formally involving an lvalue-to-rvalue conversion. A restriction should be added to 7.7 [expr.const] forbidding the evaluation of a defaulted copy/move construction/assignment on a class type that has any variant mutable subobjects.

Proposed resolution (May, 2015):

  1. Add the following bullet after bullet 3.1 of 9.2.6 [dcl.constexpr]:

  2. The definition of a constexpr function shall satisfy the following constraints:

  3. Add the following bullet after bullet 4.1 of 9.2.6 [dcl.constexpr]

  4. The definition of a constexpr constructor shall satisfy the following constraints: