I have code that allows you to make a std::string format() function on any class and allow that class to be used in calls to std::format(). To do this I say that it requires a method that returns a std::string – can I change this somehow to say format can return “any type that’s std::format()able”?

Example code:

#include <format>
#include <iostream>

template<typename T>
requires requires (T v) {
  { v.format() } -> std::convertible_to<std::string>;
}
struct std::formatter<T> : formatter<std::string>
{
  auto format(T t, format_context& ctx) const
  {
    return formatter<std::string>::format(t.format(), ctx);
  }
};

struct MyStruct
{
  std::string format() const { return "I'm a struct"; }
};

int main()
{
  MyStruct my_struct;

  std::cout << std::format("outside: {}", my_struct) << std::endl;
}