ImmutableMutableFunc.swift

// `Foo` is an immutable class
final class Foo {
    let value: Int
    init(_ value: Int) {
        self.value = value
    }
}

// Adds `mutating func` to `Foo`
protocol Incremental {
    init(_ value: Int)
    var value: Int { get }
    mutating func increment()
}

extension Incremental {
    mutating func increment() {
        self = Self(value + 1)
    }
}

extension Foo: Incremental {}

// `Foo` is immutable but it can be changed
var foo: Foo = Foo(2)
foo.increment()
print(foo.value) // 3