Why do we need type annotation in a Swift propertyWrapper?

Consider this @propertyWrapper:

@propertyWrapper struct Capitalized {
    var wrappedValue: String {
        didSet { wrappedValue = wrappedValue.capitalized }
    }
    init(wrappedValue: String) {
        self.wrappedValue = wrappedValue.capitalized
    }
}

Using it this way:

struct User {
    @Capitalized var userName: String
}

My question is:

Why do I need to specify a type annotation with the property userName. Isn't the compiler smart enough to deduce that since the wrappedValue of @Capitalized type is String, the property userName can only be of type String.

If I omit the type annotation, I get a compiler error:

Type annotation missing in pattern

The diagnostic here is poor and doesn't point you to the actual issue. The problem is that @Capitalized with no assignment translates into Capitalized(), and there is no such initializer. You can fix this by providing a default initializer:

@propertyWrapper struct Capitalized {
    ...

    init() { self.init(wrappedValue: "") }
}

With this, your struct is fine:

struct User {
    @Capitalized var userName
}

Without the default init(), the following would also be fine, since it calls Capitalized(wrappedValue:):

struct User {
    @Capitalized var userName = "bob"
}