Constructor called instead of opAssign()

[frame via Digitalmars-d-learn]

I have generic types that are initialized by a base class for 
each derived object:

struct S(T) {
    this(T val) {
        value = 100;
    }
    void opAssign(T val) {
        value = val;
    }

    T value;
}

class A {
    this(this T)() {
        foreach (property; __traits(allMembers, T)) {
            static if (property == "x") {
                __traits(getMember, cast(T) this, property) = 50;
            }
        }
    }
}

class B : A {
    S!int x;

    this(int val) {
       assert(x.value == 50);
       x = val;
    }
}

auto b = new B(200);
assert(b.x.value == 200); // 100


Although x is well initialized by A by calling opAssign(), the 
compiler doesn't care and calls the S!T ctor() on B again and 
opAssign() is ignored. Is this a bug or a rule?