But if all of these things are correctly typed, then it would be visually more likely for someone to notice that the algorithm doesn't return the type it says.

But if all of these things are correctly typed, then it would be visually more likely for someone to notice that the algorithm doesn't return the type it says. Also, C++ static types are rather weak. Its optional types are useful, but its not like in SML where the compiler ensures that you handled all possible cases. A good type system can't catch all kinds of errors though.