`x !|| y` operator to provide the equivalent of `x ? x! : y`

!||.swift

operator infix !|| {
    associativity right
    precedence 100
}

func !||<T>(left: T?, right: @auto_closure () -> T) -> T {
    if let val = left {
        return val
    }
    return right()
}

let x = foo() !|| "default value"