struct Person {
    let name: String
}

let persons = [
    Person(name: "Mark"),
    Person(name: "Emma"),
]

let mapped1 = persons.compactMap { $0.name }
print(mapped1)

//let mapped2 = persons.compactMap(\.name)
//print(mapped2)

Here I get the expected print output ["Mark", "Emma"].

But when I uncomment the commented lines I get the error

error: Playground.playground:13:34: error: key path value type 'String' cannot be converted to contextual type 'String?'

let mapped2 = persons.compactMap(.name)

Why can compactMap deal just fine with the non-optional strings in the case of { $0.name } but the KeyPath version doesn't work?

I assume it has to do with the fact that a keypath of type \Root.value can be used where a function like (Root) -> Value is expected, so in this instance \.name is the equivalent of (Person) -> String but compactMap is expecting a function like (Person) -> String?

But I don't know for certain, given that this works:

func getName(_ person: Person) -> String {
    person.name
}

let mapped3 = persons.compactMap(getName)
print(mapped3)
//prints ["Mark", "Emma"]

This might be better asked at the Swift forums. They can usually get into the nitty gritty technical details about this kind of thing.