Is a constexpr size needed for initialising an array with an explicit size? [duplicate]

C++ Primer (5th edition) states that when an array is initialised with a size, the size provided has to be a constexpr, like this:

int arr[10];

// or

constexpr size_t sz = 10;
int arr[sz];

However, this code compiles just fine for me:

size_t sz = 10;
int arr[sz];

So, is the constexpr really needed?