`<complex>`: Structured binding for `complex` is accidentally enabled before C++26, in a C++26-incompatible way

When attempting to implement WG21-P2819R2 "Add The Tuple Protocol To complex", I found that the following thing is accepted by MSVC STL (Godbolt link).

#include <complex>

int main() {
    auto [xy] = std::complex<double>{};
}

Currently, MSVC STL's complex satisfies the conditions in [dcl.struct.bind]/8 - it has only one data member of type value_type[2] which is public and inherited from its base class.

Since C++26, the aforementioned example will certainly stop working. Do we want to reject it in old modes by making something private? Given complex doesn't seem able to be passed or returned in registers, perhaps such change is ABI-compatible.

Start with the structured-binding example using and reproduce its acceptance in pre-C++26 modes via the linked Godbolt case. Read [dcl.struct.bind]/8 alongside WG21-P2819R2, then determine whether the compatibility issue requires rejecting the existing form and what ABI implications that has. Done means the expected behavior and an appropriate change or decision are established.