Polymorphic monostate

final class PolymorphicMonostate(T) if (is(T == class)) {
    static T _instance;
    static void Create(T2 : T = T, U...)(U args) if (is(typeof(new T2(args)))) {
        _instance = new T2(args);
    }
    T get() {
        return _instance;
    }
    alias get this;
} version (unittest) {
    class A {
        this(float f) {}
        int foo() { return 3; }
    }
    class B : A {
        this(string s) {super(2f);}
        override int foo() { return 5; }
    }
    alias MS = PolymorphicMonostate!A;
} unittest {
    MS.Create!B("Arg!");
    auto myms = new MS();
    assert(myms.foo() == 5);
}