lst-0069-book.cpp
if constexpr std::integral<T> {
// T is an integral type
} else if constexpr std::floating_point<T> {
// T is a floating-point type
} else {
// T is neither
}
if constexpr std::integral<T> {
// T is an integral type
} else if constexpr std::floating_point<T> {
// T is a floating-point type
} else {
// T is neither
}