OrEquals.swift

infix operator ||=

func ||=<T>(lhs: inout T?, rhs: T) {
    guard lhs == nil else { return }
    lhs = rhs
}

OrEqualsTests.swift

import XCTest

final class OrEqualsTests: XCTestCase {
    func test_orEquals() {
        var int: Int?
        int ||= 1
        XCTAssertEqual(int, 1)
        
        int = 2
        int ||= 3
        XCTAssertEqual(int, 2)
        
        var dictionary: [Int : Int] = [1 : 1]
        dictionary[2] ||= 2
        XCTAssertEqual(dictionary[2], 2)
        
        dictionary[1] ||= 5
        XCTAssertEqual(dictionary[1], 1)
        XCTAssertEqual(dictionary, [1 : 1, 2 : 2])
    }
}