How do I restrict fold expressions with C++ 20 requirements/concepts?

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How do you restrict the allowed types in variadic templates and fold expression using C++20 concepts?

For example suppose I'd like to restrict the following fold expression to only support integral types, how would I do that?

#include <string>
#include <iostream>
#include <concepts>
using namespace std;

template<typename... Args> // requires (is_integral<Args>::value )
    int sum(Args... args) { return (... + args); }
int main()
{
    cout << sum(1,2,3);
}
fold-expression c++-concepts variadic-templates c++20 c++
2 Answers

The minimal change to make it work is:

requires (std::is_integral<Args>::value && ...)

I'd also suggest using the less verbose is_integral_v<...> instead of is_integral<...>::value.

Or, even better, a concept:

template <std::integral ...Args> // no requires needed
user_27520750

You can use type-constraint auto syntax:

int sum(std::integral auto... args) { return (... + args); }
user_116387180
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