Well this still hasn't solved my problem. Because I shouldn't bind from left to right, but arbitrarily.

So ideally I would want this:

void foo(string str, int x, int y, string str2) { }

alias bind!(foo, null, 1, 2, null) twoStrings;
twoStrings("abc", "def");   ->   foo("abc", 1, 2, "def");

I'm pretty sure I could do this with D's templates.