UnderscoreOpDispatcher

{
    struct Foo
    {
        int _i;
        string _s;
        bool _b;
        string[] _add;
        alias wordList = _add;

        mixin UnderscoreOpDispatcher;
    }

    Foo f;
    f.i = 42;         // f.opDispatch!"i"(42);
    f.s = "hello";    // f.opDispatch!"s"("hello");
    f.b = true;       // f.opDispatch!"b"(true);
    f.add("hello");   // f.opDispatch!"add"("hello");
    f.add("world");   // f.opDispatch!"add"("world");

    assert(f.i == 42);
    assert(f.s == "hello");
    assert(f.b);
    assert(f.wordList == [ "hello", "world" ]);

    /+
        Returns `this` by reference, so we can chain calls.
    +/
    auto f2 = Foo()
        .i(9001)
        .s("world")
        .b(false)
        .add("hello")
        .add("world");

    assert(f2.i == 9001);
    assert(f2.s == "world");
    assert(!f2.b);
    assert(f2.wordList == [ "hello", "world" ]);
}
{
    struct Bar
    {
        string[string] _aa;

        mixin UnderscoreOpDispatcher;
    }

    Bar bar;
    bar.aa = [ "hello" : "world" ];
    bar.aa = [ "foo" : "bar" ];
    assert(bar.aa == [ "hello" : "world", "foo" : "bar"]);
}