CWG Issue 2268 (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

2268. Unions with mutable members in constant expressions revisited

Section: 9.2.6 [dcl.constexpr]Status: C++17Submitter: Richard SmithDate: 2016-05-26

[Adopted at the February/March, 2017 meeting.]

Issue 2004 concerns this example:

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

Clearly this must be ill-formed. But issue 2004 goes too far by making the copy and move operations of U non-constexpr. This breaks reasonable code such as:

constexpr int f() { U u = {1}; U v = u; return v.a; }

Proposed resolution (February, 2017):

1. Add the following as a new bullet following 7.7 [expr.const] bullet 2.8

A conditional-expression e is a core constant expression unless the evaluation of e , following the rules of the abstract machine (6.10.1 [intro.execution]), would evaluate one of the following expressions:

• ...
• an lvalue-to-rvalue conversion (7.3.2 [conv.lval]) that is applied to a glvalue that refers to a non-active member of a union or a subobject thereof;
• an invocation of an implicitly-defined copy/move constructor or copy/move assignment operator for a union whose active member (if any) is mutable, unless the lifetime of the union object began within the evaluation of e;
• ...

1. Delete bullet 3.2 in 9.2.6 [dcl.constexpr]:
   • 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;
2. Delete bullet 4.2 in 9.2.6 [dcl.constexpr]:
   • for a defaulted copy/move constructor, the class shall not have a mutable subobject that is a variant member;