[account42]

> The above definition of add is equivalent to the one using static_assert.

They are not though - the equivalent C++17 would be

  template<typename T>
  std::enable_if_t<std::is_intregral_v<T>, T> add(T a, T b)
  {
    return a + b;
  }

The difference is that these make the function unavailable for overload resolution for non-integral T but another implementation might still cover them while the static_assert version leaves the function available for overload resolution but then errors if it is selected.