import std.stdio;

​

struct Field {

    int n = -1;

    void opAssign(int a) {

        n = a;

        writefln("Field.opAssign(%s)", a);

    }

}

​

struct Register {

    Field[] clocks;

    ref Field clock(int a) {

        writefln("Register.clock(%s)", a);

        clocks ~= Field();

        return clocks[$-1];

    }

    void good() {

        foreach (clock; clocks)

            assert(clock != Field.init);

    }

}

​

​

void main() {

    Register register;

    register.clock(1) = 10; // works, good

    register.clock = 10;    // works too, but adds Field.init to clocks

    writeln(register.clocks);

    register.good();        // runtime error

}