When can const members be used in constexpr members?

Question

I came across a situation that I do not understand. Someone will explain so well why the first code compiles correctly and the second gives an error:

error: the value "TestClass :: z" cannot be used in the constant expression
  static constexpr int sum () {return x + y + z;}
  -------------------- ------------------------------ - ^
note: 'int TestClass :: z' is not a constant static int z; "

Work code:

#include <iostream>

using namespace std;

class TestClass
{
    public:
        constexpr int sum() {return x+y+z;}

    private:
        static constexpr int x = 2;
        static const int y = 3;
        int z = 5;

};

int main()
{
    TestClass tc;
    cout << tc.sum() << endl;

    return 0;
}

But when I try to make static TestClass::sum()static, I get the above error:

#include <iostream>

using namespace std;

class TestClass
{
    public:
        static constexpr int sum() {return x+y+z;}

    private:
        static constexpr int x = 2;
        static const int y = 3;
        static int z;

};

int TestClass::z = 5;

int main()
{
    TestClass tc;
    cout << tc.sum() << endl;

    return 0;
}

PS I am using mingw32-g ++ 4.8.1

Answer 1

, this, z. constexpr - , .

( tc is not), , , . , :

constexpr TestClass tc;
array<int, tc.sum()> a;
cout << a.size() << endl;

, . , , z , .