Two-Way Dictionary in Swift

DictionaryTwoWay.swift

struct DictionaryTwoWay<S:Hashable,T:Hashable> : DictionaryLiteralConvertible
{
    // Literal convertible
    typealias Key   = S
    typealias Value = T
    
    // Real storage
    private var st : [S : T] = [:]
    private var ts : [T : S] = [:]
    
    init(leftRight st : [S:T])
    {
        var ts : [T:S] = [:]
        
        for (key,value) in st
        {
            ts[value] = key
        }
        
        self.st = st
        self.ts = ts
    }
    
    init(rightLeft ts : [T:S])
    {
        var st : [S:T] = [:]
        
        for (key,value) in ts
        {
            st[value] = key
        }
        
        self.st = st
        self.ts = ts
    }
    
    init(dictionaryLiteral elements: (Key, Value)...)
    {
        for element in elements
        {
            st[element.0] = element.1
            ts[element.1] = element.0
        }
    }
    
    init() { }
    
    subscript(key : S) -> T?
    {
        get
        {
            return st[key]
        }
        
        set(val)
        {
            if let val = val
            {
                st[key] = val
                ts[val] = key
            }
        }
    }
    
    subscript(key : T) -> S?
    {
        get
        {
            return ts[key]
        }
        
        set(val)
        {
            if let val = val
            {
                ts[key] = val
                st[val] = key
            }
        }
    }
}

// Example usage, starting from Dictionary Literal
let dict : DictionaryTwoWay<String, Int> = ["Hello" : 3]
dict[3]       // Prints "Hello"
dict["Hello"] // Prints 3

// Start with initializer
let dict2 : DictionaryTwoWay<String, Int>(leftRight: ["World" : 10])

// Start with backwards initializer
let dict3 : DictionaryTwoWay<Int, String>(rightLeft: [10 : "World"])

Does NOT compile now.

By the way, it's not suitable to suppose S and T are different types.