Custom Operation and Inline Attribute.

Dictionary+Extension.swift

@inline(__always)
func + <K,V>(left: Dictionary<K,V>, right: Dictionary<K,V>) -> Dictionary<K,V> {
    return merge(left: left, right: right)
}

func merge<K,V>(left: Dictionary<K,V>, right: Dictionary<K,V>) -> Dictionary<K,V> {
    var map = left
    for (k, v) in right {
        map[k] = v
    }
    return map
}

extension Dictionary where Value == Any {
    func value(forKey key: Key, defaultValue: @autoclosure () -> T) -> T {
        guard let value = self[key] as? T else {
            return defaultValue()
        }
        return value
    }
}

IMHO, The expression of custom operations can use the inline attribute functions. which is the custom operator to easy use and aliased to real function having the contexts.