when a const isn't? (typedef?)

How come this behaves the way it does? val_D() and val_E() cause a
(correct) compiler error. val_F() and val_G() do not, but I don't see
why.

Can anyone enlighten me?

#include <iostream>
using namespace std;

class foo {
public:
        typedef int value_type;
        typedef value_type& reference;
        typedef const reference const_reference;

        explicit foo(int x): m_val(x) {}

        int val() { return m_val; }

        int& val_A() { return m_val; }
        value_type& val_B() { return m_val; }
        reference val_C() { return m_val; }

        const int& val_D() { return m_val; }
        const value_type& val_E() { return m_val; }
        const reference val_F() { return m_val; }
        const_reference val_G() { return m_val; }

private:
        int m_val;
};

int main()
{
        foo x(0);

        cout << x.val() << endl;
        x.val_A() = 1;
        x.val_B() = 2;
        x.val_C() = 3;
        x.val_D() = 4; // compile error
        x.val_E() = 5; // compile error
        x.val_F() = 6;
        x.val_G() = 7;
        cout << x.val() << endl;

        return 0;
}