Confusion about constant initialization

In cppref it gives the syntax of constant initialization:

static T & ref = constexpr; 
static T object = constexpr;    

But wait,how could a lvalue reference T & without const be bound to a consexptr,which is constant and unmodifiable ?

I try to provide some example but failed:

 static int& ref = 6; //error, need a `const`
 constexpr int a = 6; static int& ref = a; //error, need a `const`