Defining constexpr static data members

So, I'm aware that in C++ static members can be initialized inside the class if they are a const literal type like the following

class test{
        static constexpr int stc = 1;
private:
        int a = 0;
        int b = 0;
        int c = 0;
};

and the static constexpr variable stc can be used where the compiler can directly substitute the value of the member i.e

int array[stc]; 

However, if used in a context where the value cannot be directly substituted by the compiler then the compiler generates an error unless the static member is defined outside the class like so:

constexpr int test::stc;

Why is this the case?