How do I use concepts in if-constexpr?

Question

How does one use concepts in if constexpr?

Given the example below, what would one give to if constexpr to return 1 in case T meets the requirements of integral and else 0?

template<typename T> concept integral = std::is_integral_v<T>;  struct X{};  template<typename T> constexpr auto a () {     if constexpr (/* T is integral */) {         return 1;     }      else {         return 0;     } }  int main () {     return a<X>(); } 

Answer 1

Concepts are named boolean predicates on template parameters, evaluated at compile time.

In a constexpr if statement, the value of the condition must be a contextually converted constant expression of type bool.

So in this case, usage is simple:

if constexpr ( integral<T> ) 

Answer 2

It is sufficient to do:

if constexpr ( integral<T> ) 

since integral<T> is already testable as bool