Proposed resolution (approved 2025-11-05):

1. Change in 11.4.5.3 [class.copy.ctor] paragraph 11 as follows:

   A copy/move constructor for class X is trivial if it is not user-provided and if

   • class X has no virtual functions (11.7.3 [class.virtual]) and no virtual base classes (11.7.2 [class.mi]), and
   • the constructor selected to copy/move each direct base class subobject is a direct member of that base class and is trivial, and
   • for each non-static data member of X that is of class type (or array thereof), the constructor selected to copy/move that member is trivial;

   otherwise the copy/move constructor is non-trivial.

2. Change in 11.4.6 [class.copy.assign] paragraph 9 as follows:

   A copy/move assignment operator for class X is trivial if it is not user-provided and if

   • class X has no virtual functions (11.7.3 [class.virtual]) and no virtual base classes (11.7.2 [class.mi]), and
   • the assignment operator selected to copy/move each direct base class subobject is a direct member of that base class and is trivial, and
   • for each non-static data member of X that is of class type (or array thereof), the assignment operator selected to copy/move that member is trivial;

   otherwise the copy/move assignment operator is non-trivial.

(From submission #759.)

Consider:

  struct B0 { int b0; };

  struct B {
    B &operator=(const B &) = default;
    int x;
  };

  struct D : B0, B {
    using B::operator=;
  private:
    D &operator=(const D &) && = default;
  };

  struct Q {
    Q &operator=(const Q &) = default;
    D d;
  };

According to the rules, Q::operator= is trivial, but it does not copy the d.B0::b0 member of Q. Implementations disagree and copy that member regardless.

We can make Q::operator= non-trivial or deleted. We can also make that assignment operator copy all subobjects, even though overload resolution on the base classes clearly is at odds with that outcome.