Nobody's mentioned dependent typing yet, but languages like Idris and Agda will "narrow types based on control flow" (in fact, since they're pure-functional, control-flow is the same as data-flow). For example, zipping two vectors of the same length:

    zip: Vec n t1 -> Vec n t2 -> Vec n (t1, t2)
    zip Nil Nil = Nil
    zip (Cons x xs) (Cons y ys) = Cons (x, y) (zip xs ys)

Here the 'Vec' type has two constructors, Nil and Cons. Since their types specify the same length, the type-checker knows we can ignore the 'zip Nil Cons' or 'zip Cons Nil' cases.