why @property cannot be pass as ref ?

[ChangLong via Digitalmars-d-learn]

=======
struct A {
                alias This      = typeof(this) ;
                
                void opAssign(ref This ){
                }
                
                ref auto left(){
                        return This() ;
                }
}
void main(){
        A       root ;
        ref find() {
            auto p = root ;
            p = p.left;
        }
}
===========

test.d(16): Error: function tree.A.opAssign (ref A _param_0) is 
not callable using argument types (A)


Is this by design ?