C++20: Concepts of multiple types and its constraint, correct syntax?

Question

It is confirmed that in the upcoming c++20 standard, according to this reddit report from the recent Cologne ISO C++ Meeting, we will be able to specify a template's concept and for each class/function template, we will be able to set the constraints on its types. However, in documentations and tutorials (e.g. here), I could not find the correct syntax for the multi-type use-case.

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Suppose we have a multi-type concept:

template<typename T1, typename T2> concept AreEqComparable = requires(T1 a, T2 b) {     { a == b } -> bool; }; 

Let's say, I want to define a simple comparison function between two different types. How can I do that? More specifically, what should I write in the ??? part of the code below:

??? bool are_equal(T1 a, T2 b) { return a == b; } 

I couldn't find any reference to this case in here, here, and even here. I have randomly tried something like:

/* 1 */ template<AreEqComparable T1, T2> /* 2 */ AreEqComparable<T1, T2> /* 3 */ template<AreEqComparable<T1, T2>> 

But all of them throw syntax errors. I think the answer should lie somewhere in the specification P0557 by Bjarne Stroustrup, but I wasn't able to find it after a quick look.

Answer 1

You can write it like this:

template <typename T1, typename T2>     requires AreEqComparable<T1, T2> bool are_equal(T1 a, T2 b) {     // ... } 

Here, we use a requires-clause to impose a requirement on the type template parameters.