C++ alias template (typedef) with concepts?

Question

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4553.pdf

gcc6: -fconcepts

template<typename T>
concept bool String = requires(T s)
{
    { s.clear() } -> void;
    // etc.
};

void print(const String& message);
//void print(Str message); // I want Str = const String&

void test()
{
  std::string str;
  print(str);
}

Is there any way to declare Str as const String& ?

Answer 1

Is there any way to declare Str as const String& ?

No. And moreover, you don't want that anyway. Concepts are about adding constraints to types. So if you want to constraint print to take something that models String , you can do that:

template <typename T> requires String<T> void print(T const&); // requires-clause
template <String T> void print(T const&); // partial-concept-id
void print(String auto const&); // probably what C++20 will allow

But the constraint and the value category are orthogonal. You could take String by value:

void print(String auto);

you could take a String by forwarding reference:

void print(String auto&&);

These are all separate options from the "I want a String " aspect. You can't really group those together.

---

The best you could do is:

template <String T> using Str = T const&;
template <typename T> void print(Str<T>); // silently a const&

or

template <typename T> using CR = T const&;
template <String T> void print(CR<T>); // silently a const&

That works. For some definition of works. But like... don't do that. The fact that a function takes a const& as opposed to a value is very important visual information, don't just hide it.