CWG Issue 2004 (original) (raw)
This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 118e. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.
2025-11-05
2004. Unions with mutable members in constant expressions
Section: 7.7 [expr.const]Status: CD4Submitter: Richard SmithDate: 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):
- Add the following bullet after bullet 3.1 of 9.2.6 [dcl.constexpr]:
The definition of a constexpr function shall satisfy the following constraints:
- it shall not be virtual (11.7.3 [class.virtual]);
- for a defaulted copy/move assignment, the class of which it is a member shall not have a mutable subobject that is a variant member;
- ...
- Add the following bullet after bullet 4.1 of 9.2.6 [dcl.constexpr]
The definition of a constexpr constructor shall satisfy the following constraints:
- the class shall not have any virtual base classes;
- for a defaulted copy/move constructor, the class shall not have a mutable subobject that is a variant member;
- ...