Swift 4, Generics why do I need to cast to T here? - swift

Question

When running this code in Swift 4, The compiler throws the following error:
"Cannot convert return expression of type 'Comment?' to return type '_?', when running this code in a playground:
import UIKit
class Comment {}
class Other {}
func itemForSelectedTabIndex<T>(index: Int, type: T.Type) -> T? {
return (type is Comment.Type) ? getComment() : getOther()
}
func getComment() -> Comment {
return Comment()
}
func getOther() -> Other {
return Other()
}
let thing = itemForSelectedTabIndex(index: 0, type: Other.self)
In order to make this work, I need to cast the return value as generic, like this:
return (type is Comment.Type) ? getComment() as! T : getOther() as! T
Could someone explain the logic behind this?
If the expected return value is a 'Generic', and basically it won't matter what type I return, why the compiler complains about it? Shouldn't this work without casting?

Answer 1

Generics aren't some magical wildcards that can just have any value at any time.
When you callitemForSelectedTabIndex(index: 0, type: Comment.self), T is inferred to Comment. Likewise, for Other.
When T has been inferred to Comment, the same value of T is consistent throughout everywhere it's used. Thus, the return value must be of type Comment (or a sub-type).
Another is with your expression (type is Comment.Type) ? getComment() : getOther(). There are 2 cases, and neither of them are valid:
type is Comment.Type: getComment() returns a Comment, a type that is compatible with the value of T, which is Comment. But, the two operands of the conditional operator have no common supertype. That's not valid.
type is not Comment.Type: getOther() returns an Other, which may or may not be compatible with T. All we know about T is that it is not comment. This doesn't mean it's necessarily Other. It can be any other type, like Int. Thus, this return expression fails.
What you need is a common supertype of both types you wish to return. Most probably, a protocol is the correct choice (rather than a shared superclass):
protocol TabItem {}
class Comment {}
class Other {}
func itemForSelectedTabIndex<T: TabItem>(index: Int, type: T.Type) -> TabItem {
return getCommentOrOtherOrSomethingElse()
}