When can you use a const int as the size of an array?

Question

Why am I able to use a locally declared const int as the size of an array declaration but am not allowed to do the same with a const int passed as an argument?

For example, in the below code why do I get compiler errors only on line 2?

void f1(const int dim){
  int nums[dim];  // line 2: errors
}

void f2(){
  const int dim = 5;
  int nums[dim];  // ok
}

Answer 1

Array size should be known at compile time.

const int with local variables may do not work neither if the value is not known at compile time as:

void f2(){
  const int dim = bar();
  int nums[dim];  // error
}

In Both case, const int tells that the value doesn't change, not that is it known at compile time.