Concept for a type that checks if a class is one specified template class, which is parametrized by types, that satisfy another concept

Lets say I have a function in C++17:

template<typename Range> void calculate(const Range& range) { /* do something */ }

And I want to specialize it for all pairs of integral types (std::pair<std::integral, std::integral>), and I want to create a separate concept IntegralRange for it: how can I achieve it?

>Solution :

Specializing the function for this purpose isn’t possible; I assume you meant overloading. You can solve this problem by providing as second overload:

template<std::integral First, std::integral Second>
void calculate(std::pair<First, Second> range) { /* do something */ }

Alternatively, if you insist on using a concept:

template <typename T>
concept integral_pair = requires (T pair) {
    []<std::integral First, std::integral Second>(std::pair<First, Second>){}(pair);
};

void calculate(integral_pair auto range) { /* do something */ }

The requires expression uses an immediately invoked lambda expression (IILE) to ensure that First and Second can be deduced from pair.