Variable length array error with constexpr

I have a class with a member function which declares an array whose size is based off a formula.

template <int SIZE>
class{
   constexpr int lookup(const int) const
   {
      return n * n + n;
   }
   inline void func()
   {
       double array[lookup(SIZE)];
   }
};

This gives me the vla error. I think it should work because SIZE is resolved at compile time, and lookup is a constexpr. I know the following will work:

template <int SIZE>
class{

   inline void func()
   {
       constexpr int sz = SIZE * SIZE + SIZE;
       double array[sz];
   }
};

I guess I'm just trying to figure out why