How to define a concept of a object that is structure bindable?

I want to define a concept that can detect whether or not the type T can be assigned to a structured binding:

template <typename T>
concept two_elements_structured_bindable = requires (T t) {
  auto [x, y] = t;
};

but this cannot be compiled. Is there a proper way to define a concept like that?

No.

There are three cases in structured bindings:

1. Arrays. That's easy enough to detect.

2. Tuple-like. You can easily check if std::tuple_size<E>::value is valid, and then check if std::tuple_element<I, E>::type is valid as a type for all the right types. But the get case is harder since you have to deal with member vs non-member... but otherwise I think doable.

3. Types which have all public (yeah yeah, technically accessible) members as direct members of the same class. This is impossible to detect with current technology. magic_get can, I think, handle struct X { int a, b; }; but neither struct Y : X { }; nor struct Z { X& x; }; You would need to have proper reflection to check this case.

As of C++20, you would need some kind of compiler intrinsic to do this.