Doesn't that mean you're asking the compiler to have a full unification engine? Your example is simple, but the general case is more complicated:

 

void foo(T,U,V)(Tuple!(T,U) tup, V function(U) fun)

{ ... }

 

void main()

{

    foo(tuple(1,"abc"), (a) => 1.34); // so, I can deduce V to be double

 

    foo(tuple(1,"abc"), (int i) => i+); // error, U unified to string and int!

}