Unless your type system is very weak -- and in effect, an untyped language can be considered to already be doing what you said only in a type system that has a

Unless your type system is very weak -- and in effect, an untyped language can be considered to already be doing what you said only in a type system that has a single type -- this is generally impossible (halting problem and its extensions). So you have the choice of something that's not very useful or something that's impossible -- take your pick. Type inference, as others have suggested, does not do what you want. If your program is well-typed according to a particular type system which you must have in mind while writing the program, they'll save you from writing explicit type annotations, but they cannot come up with "correct" type for a correct program that's not written for that particular type system.