Cf. https://wg21.link/cwg2120 Test: struct A {}; struct B : A {}; struct C { A a; }; struct D { C c[5]; }; struct E : B { D d; }; static_assert(__is_standard_layout(B), ""); static_assert(__is_standard_layout(D), ""); static_assert(!__is_standard_layout(E), "");