[swift-users] Type "T?" does not conform to protocol 'Equatable'

[davesweeris at mac.com]

To the best of my understanding, Int? is not equatable. Your let c = a == b  code works because this function (or something like it) is defined:
func == <T: Equatable> (lhs: Optional<T>, rhs: Optional<T>) -> Bool {
    switch (lhs, rhs) {
    case (.None, .None): return true
    case (.Some(let l), .Some(let r)): return l == r
    default: return false
    }
}

The reason it’s done this way is that currently there’s no mechanism for conditional protocol conformance. Ideally, you’d be able to write something like this:
extension Optional : Equatable where T: Equatable {}
… which, combined with that existing == function, would make Int? be equatable.

IIRC, Apple is intending to remove this limitation, but I can’t remember if it’s made the cut for Swift 3 or if it’ll be later on.

In the meantime, if Keith’s solution of making the variable be a T? instead of T doesn’t work for you, I think this will let you continue on with other parts of your code (although I have NOT thoroughly tested it… use at your own risk):
public struct OptionalWrapper<T> : Equatable {
    typealias Wrapped = T
    var unwrap: Wrapped?
    init(_ value: Wrapped?) { unwrap = value }
}
public func == <T: Equatable> (lhs: OptionalWrapper<T>, rhs: OptionalWrapper<T>) -> Bool {
    switch (lhs.unwrap, rhs.unwrap) {
    case (.None, .None): return true
    case (.Some(let l), .Some(let r)): return l == r
    default: return false
    }
}
public func == <T: Equatable> (lhs: OptionalWrapper<T?>, rhs: OptionalWrapper<T?>) -> Bool {
    switch (lhs.unwrap, rhs.unwrap) {
    case (.None, .None): return true
    case (.Some(let l), .Some(let r)): return l == r
    default: return false
    }
}
public func == <T> (lhs: OptionalWrapper<T>, rhs: OptionalWrapper<T>) -> Bool {
    switch (lhs.unwrap, rhs.unwrap) {
    case (.None, .None): return true
    case (.Some, .None): return false
    case (.None, .Some): return false
    case (.Some, .Some): fatalError("\(__FILE__):\(__LINE__):\(__COLUMN__): Binary operator '==' cannot be applied to two '\(T.self)' operands")
    }
}

The syntax for actually using it is a bit long, but it should work: 
let a = C(t: OptionalWrapper(4)) // infers that Wrapped == Int
let b = C(t: OptionalWrapper<Int>(nil)) // must be explicit since nil is just Optional.None, without any other type info
let c = a.t == b.t // false

Hope that helps

- Dave Sweeris

> On Feb 29, 2016, at 8:24 AM, Rudolf Adamkovič via swift-users <swift-users at swift.org> wrote:
> 
> Hi everyone!
> 
> I have a generic class similar to this one:
> 
> class C<T: Equatable> {
>     let t: T
>     init(t: T) { self.t = t }
> }
> 
> When try to wrap ‘Int?' inside, I get the following error:
> 
> let a = C<Int?>(t: nil) // ERROR: Type "Int?" does not conform to protocol 'Equatable'
> 
> Yet when I try to compare two ‘Int?’ values, everything works:
> 
> let a: Int? = 5
> let b: Int? = 6
> 
> let c = a == b // NO ERROR
> 
> So, is ‘Int?' equatable or not?
> 
> Thanks!
> 
> R+
> _______________________________________________
> swift-users mailing list
> swift-users at swift.org
> https://lists.swift.org/mailman/listinfo/swift-users

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