[Bug c++/88165] error: default member initializer for 'A::B::m' required before the end of its enclosing class

[fchelnokov at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88165

--- Comment #7 from Fedor Chelnokov <fchelnokov at gmail dot com> ---
This struct definition:
```
struct A {
    struct B {
        int i = 0;
        B() {}
    };
    A(B = {});
};
```
is accepted by GCC, but another one ({} replaced with = default) is not:
```
struct C {
    struct D {
        int i = 0;
        D() = default;
    };
    C(D = {});
};
```
Demo: https://gcc.godbolt.org/z/WTPdTn1Yf

Could you please explain why? I though that both must be same accepted or same
rejected.