C++ Separate different instances of a class at compile time

Question

What are available options to separate different instances of a class at compile time?

I basically want the following code snippet to compile without errors:

struct equal_ret_t {};
struct different_ret_t {};

struct Foo
{
    Foo() = default;
    // ... add relevant code here
};

constexpr auto eval(Foo const& a, Foo const& b)
{
    // return equal_ret_t for same instances,
    // different_ret_t for different instances
}

int main()
{
    Foo a;
    Foo b;
    static_assert(std::is_same_v<decltype(eval(a,a)),equal_ret_t>);
    static_assert(std::is_same_v<decltype(eval(b,b)),equal_ret_t>);
    static_assert(std::is_same_v<decltype(eval(a,b)),different_ret_t>);
}

What is the required code to add to the Foo class and/or the eval function?

Are there any solutions approved by the current C++20/23 standard? If not, can the solution made portable across major compilers?

Answer 1

You will not be able to achieve the exact syntax you expect to use, but here's something that may be close enough for your purposes:

#include <type_traits>

struct Foo {
  constexpr bool operator==(const Foo &other) const {
    return this == &other;
  }
};

struct equal_ret_t {};
struct different_ret_t {};

template <bool Equal>
using ret_t = std::conditional_t<Equal, equal_ret_t, different_ret_t>;

int main() {
  Foo a;
  Foo b;
  static_assert(std::is_same_v<ret_t<a == a>, equal_ret_t>);
  static_assert(std::is_same_v<ret_t<b == b>, equal_ret_t>);
  static_assert(std::is_same_v<ret_t<a == b>, different_ret_t>);
}

Try it on Compiler Explorer