Proposed resolution (approved by CWG 2024-05-31):

Change in 11.4.5.2 [class.default.ctor] paragraph 3 as follows:

A default constructor for class X is trivial if it is not user-provided and if:

• its class X has no virtual functions (11.7.3 [class.virtual]) and no virtual base classes (11.7.2 [class.mi]), and
• no non-static data member of its class X has a default member initializer (11.4 [class.mem]), and
• all the direct base classes of its class have trivial default constructors, and
• the constructor selected to default-initialize each direct base class subobject of X is trivial,
• for all the non-static data members of its class that are of class type (or array thereof), each such class has a trivial default constructor.
• for each direct non-static data member of X that is of class type (or array thereof), the constructor selected to default-initialize that member (or an array element of that member) is trivial.

Otherwise, the default constructor is non-trivial.

(From submission #521.)

Consider:

  template <class T>
  struct A {
    A() requires (false) = default;
    A() : t(42) {}
    T t;
  };

  struct B : A<int> {};

According to the current wording, class B has a trivial default constructor, which is undesirable.