Why isn't the Swift programming language type inference more aggressive?

Couldn't the type inference in Apple's new programming language Swift had been done more aggressive? For instance why can't the return type of a function be deduced?

func sayHello(personName: String) -> String {
   let greeting = "Hello, " + personName + "!"
   return greeting
}

I don't understand Swift's typing system yet, so I can only speculate, but I imagine that it's for the same reason that Scala doesn't have full type inference: nobody knows how to do it in a pragmatically viable way. The combination of parametric polymorphism and subtyping is deadly from the point of view of algorithmic complexity.

StackExchange Computer Science Q#26389, answer score: 10