struct W(T) {

    const T value;

    this(U : T)(auto ref const U value) {

        this.value = value;

    }

}

​

auto defined(T)(auto ref const T value) {

    return W!T(value);

}

​

auto undefined(T)() {

    return W!T();

}

​

void main() {

    auto a = defined("hello");

    auto b = undefined!string;

    static assert(is(typeof(a) == W!string));

    static assert(is(typeof(b) == W!string));

​

    static class C {}

    auto c = defined(cast(immutable C)(new C));

    auto d = [undefined!string, defined("hello")];

}