https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104418
--- Comment #4 from Fedor Chelnokov <fchelnokov at gmail dot com> ---
I think `using B::B;` is not the same as redefining each constructor with the
explicit call of base class constructor `C(int a) : B{(int)a}{}`.
Please consider this example proving it:
```
struct A {
A() {}
A(const A&) = delete;
};
struct B {
B(A) {}
};
// ok everywhere
struct C : B {
using B::B;
};
//error everywhere
struct D : B {
D(A a) : B(a) {}
};
```
Here struct C definition is fine, but struct D is ill-formed because it
requires A to be copyable. Demo: https://gcc.godbolt.org/z/T7bhv9jeP
