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 (6.9.1 [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;

• ...

2. 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;

3. 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;

[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;
  }