CWG2885 [class.default.ctor] Trivial default constructors should also be required to be eligible

[royjacobson]

Full name of submitter: Roy Jacobson

Reference (section label): [class.default.ctor]

Came up while thinking about this bug report - llvm/llvm-project#59206

Issue description:

The wording in [class.default.ctor]p3 says a base class or a member of a class need to have a trivial default constructors for a default constructor to be trivial.

Under the current wording, class B in the following example has a trivial default constructor, which is undesirable:

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

struct B : A {};

Suggested resolution:

 \item
-all the direct base classes of its class have trivial default constructors, and
+all the direct base classes of its class have eligible trivial default constructors, and

 \item
 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.
+type (or array thereof), each such class has an eligible trivial default constructor.
 \end{itemize}

[jensmaurer]

I think we just want to defer to overload resolution, similar to what we do for (trivial) copy constructors.

[jensmaurer]

CWG2885