Static duck typing in C++

duck_typing_static.cpp

#include <iostream>
#include <source_location>
#include <string>

struct Quack;
template <typename T>
concept Duck = requires(T t) {
  { t.quack() } -> std::convertible_to<Quack>;
  //   could be -> std::same_as<Quack>;
};

struct Quack {
  std::string data;
};

template <typename T>
void print(const T &inp, const std::source_location &location =
                             std::source_location::current()) noexcept {
  std::cout << location.function_name() << ", " << inp << '\n';
}

class Foo {
public:
  Quack quack(void) {
    Quack res;
    res.data = "hey!";
    return res;
  }
};

class Bar {
  static constexpr Quack _quack{.data = "static hey!"};

public:
  static Quack quack(void) {
    print(&_quack); // is it really constexpr if there is a pointer?
                    // a std::string has to be in memory
    return _quack;
  }
};

class Baz {
public:
  static Quack quack(void) {
    Quack res;
    res.data = "Baz quack";
    print(&res);
    return res;
  }
};

void checkDucks(Duck auto d) {
  auto res = d.quack();
  print(res.data);
}

int main() {
  Foo f;
  Bar b;
  Baz z;
  checkDucks(f);
  b.quack();
  checkDucks(b);
  z.quack();
  checkDucks(z);
  return 0;
}

$ clang++ -std=c++23 -Wall -Wconversion duck_typing_static.cpp 
$ ./a.out 
void checkDucks(Duck auto) [d:auto = Foo], hey!
static Quack Bar::quack(), 0x5ca096e31d88
static Quack Bar::quack(), 0x5ca096e31d88
void checkDucks(Duck auto) [d:auto = Bar], static hey!
static Quack Baz::quack(), 0x7ffcaafe2d30
static Quack Baz::quack(), 0x7ffcaafe2ce8
void checkDucks(Duck auto) [d:auto = Baz], Baz quack

Blogs about requires and checking for the methods with { t.member() } -> std::convertible_to<...>:
https://akrzemi1.wordpress.com/2020/01/29/requires-expression/
https://www.sandordargo.com/blog/2021/03/17/write-your-own-cpp-concepts-part-ii