How do I use concepts in if-constexpr?

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> )

It is sufficient to do:

if constexpr ( integral<T> )

since integral<T> is already testable as bool