Binary operator '==' cannot be applied to two operands

Question

I have a class with the protocol Equatable. The class looks like this:

class Item: Equatable {

    let item: [[Modifications: String]]

    init(item: [[Modifications: String]]) {
        self.item = item
    }
}

func ==(lhs: Item, rhs: Item) -> Bool {
    return lhs.item == rhs.item
}

But this is giving me the error (see title). The property item was [[String: String]] before and there was no problem and I have no idea how to fix this. I tried googling and searching all over SO but with no luck..

The enum is just a simple basic one:

enum Modifications: Int {
    case Add    = 1
    case Remove = 2
    case More   = 3
    case Less   = 4
}

Answer 1

Update: SE-0143 Conditional conformances has been implemented in Swift 4.2.

As a consequence, your code does compile now. And if you define Item as a struct

struct Item: Equatable {
    let item: [[Modifications: String]]

    init(item: [[Modifications: String]]) {
        self.item = item
    }
}

then the compiler synthesizes the == operator automatically, compare SE-0185 Synthesizing Equatable and Hashable conformance

---

(Pre Swift 4.1 answer:)

The problem is that even if == is defined for the dictionary type [Modifications: String], that type does not conform to Equatable. Therefore the array comparison operator

public func ==<Element : Equatable>(lhs: [Element], rhs: [Element]) -> Bool

cannot be applied to [[Modifications: String]].

A possible concise implementation of == for Item would be

func ==(lhs: Item, rhs: Item) -> Bool {
    return lhs.item.count == rhs.item.count 
           && !zip(lhs.item, rhs.item).contains {$0 != $1 }
}

---

Your code compiles for [[String: String]] – if the Foundation framework is imported, as @user3441734 correctly said – because then [String: String] is automatically converted to NSDictionary which conforms to Equatable. Here is a "proof" for that claim:

func foo<T : Equatable>(obj :[T]) {
    print(obj.dynamicType)
}

// This does not compile:
foo( [[Modifications: String]]() )

// This compiles, and the output is "Array<NSDictionary>":
foo( [[String: String]]() )