Cannot convert value of type '(T) -> Void' - swift

Question

Example:
struct Wrapper<T> {
var key: Int = 0
var listeners: [Int: (T) -> Void] = Dictionary()
mutating func add(_ handler:#escaping (T) -> Void) {
self.key += 1
self.listeners[self.key] = handler
}
func get(key: Int) -> (T) -> Void {
return self.listeners[key]!
}
}
Test protocol:
protocol CommonProtocol {
}
Class that create Wrapper of test class
class C {
var wrapper: Wrapper = Wrapper<CommonProtocol>()
func add<T: CommonProtocol>(_ handler: #escaping (T) -> Void) {
self.wrapper.add(handler) //Cannot convert value of type '(T) -> Void' to expected argument type '(CommonProtocol) -> Void'
}
}
Image with error
I get error:
Cannot convert value of type '(T) -> Void' to expected argument type '(CommonProtocol) -> Void'
Question:
Why (T) -> Void can't be casted to (CommonProtocol) -> Void ? The T
is explicitly declared as <T: CommonProtocol>
This is my first question, if you have some suggestions please don't hesitate to contact me

Answer 1

You don't need to make func add generic.
When you specify in func add<T: CommonProtocol>... you explicitly telling the compiler that your function accepts all Types that inherit CommonProtocol but your Wrapper specifies that accepts CommonProtocol not inherited types.
Solution
Either type-erase class C:
Class C<T: CommonProtocol> {
var wrapper: Wrapper<T>
....
}
or if type T doesn't actually matter to you then:
func add(_ handler: #escaping (CommonProtocol) -> Void)
but second one doesn't make sense at all. You have to downcast it every-time you'll use this method (and downcasts are very bad :D)
Note: It's actually not related to this question, but one of your options is to type-erase the CommonProtocol too.