When does a constexpr function get evaluated at compile time?

constexpr functions will be evaluated at compile time when all its arguments are constant expressions and the result is used in a constant expression as well. A constant expression could be a literal (like 42), a non-type template argument (like N in template<class T, size_t N> class array;), an enum element declaration (like Blue in enum Color { Red, Blue, Green };, another variable declared constexpr, and so on.

They might be evaluated when all its arguments are constant expressions and the result is not used in a constant expression, but that is up to the implementation.

The function has to be evaluated at compile-time when a constant expression is needed.

The simplest method to guarantee this is to use a constexpr value, or std::integral_constant:

constexpr auto result = POW(i, 2); // this should not compile since i is not a constant expression
std::cout << result << std::endl;

or:

std::cout << std::integral_constant<int, POW(i, 2)>::value << std::endl;

or

#define POW_C(base, power) (std::integral_constant<decltype(POW((base), (power)), POW((base), (power))>::value)

std::cout << POW_C(63, 2) << std::endl;

or

template<int base, int power>
struct POW_C {
  static constexpr int value = POW(base, power);
};

std::cout << POW_C<2, 63>::value << std::endl;