Swift 2.0 Generics and Type Safety Issues - swift

Question

I was doing this tutorial http://blog.teamtreehouse.com/introduction-learn-power-swift-generics and I came upon this code;
func someFunction<T, U>(a: T, b: U) {}
The problem is when I call the function using
someFunction<String, Int>(1, "Test")
I get an error saying "cannot explicitly specialize a generic function".
I then change it to
someFunction(1,b: "Test")
and now there is no error. The problem is that there is now no type safety. (Is there something wrong with the code, as it was written before swift 2.0?) What is the best way to re-introduce type safety?

Answer 1

The declaration is completely generic and is specifying that any two types can be used.
func someFunction<T, U>(a: T, b: U) {}
It is not that there is no type safety in Swift, this is how you express a generic without any type constraints.
You get what you ask for.
If you wanted to constrain the function to String and Int, you would have written it as
func someFunction(a:String, b:Int)
Generics are more often used with collections, protocols and classes. Basic types rarely need them :
func someFunction<T:Comparable, U:Comparable>(a:T, b:U) -> Bool
{ return (a < b) || (a > b) }

Answer 2

ok, see this 'self explanatory' example. try it in playground and play with it a little bit.
func foo<T>(t: T)->T {
return t
}
// here the resulting type is infered by compiler
let i = foo(1) // 1
let j: Int = foo(1) // 1
let t = foo("alpha") // "alpha"
// if you declare it self ..
//let s: String = foo(1) // error: cannot convert value of type 'Int' to expected argument type 'String'
/* this fails to compile!
func bar<T>(t:T)->Int {
return t.count
}
*/
/* this fails to compile too !!
func bar0<T:CollectionType>(t:T)->Int {
return t.count
}
*/
func bar<T:CollectionType>(t:T)->T.Index.Distance {
return t.count
}
let arr = [1,2,3]
let k:Int = bar(arr) // 3
print(k.dynamicType) // Int
// and even more interesting
let l = bar(arr)
print(l.dynamicType) // Int