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

Proposed resolution (May, 2015):

1. 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;

• ...

2. 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;

• ...

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.