How to (force) overloading of plus for integers in Kotlin?

Question

I would like to make plus mean something else, than addition. For example, creation of lazy expressions for computational graph. Unfortunately, class extensions cant override member functions. The following code will print 3:

operator fun Int.plus(other: Int) = listOf(this, other)

fun main() {
    println( 1 + 2 )
}

Is is possible to force overriding?

Answer 1

No it is not possible. 1 + 2 is lowered into 1.plus(2), and there is a well defined order in how the compiler finds an appropriate plus method. Specification:

If a call is correct, for a callable f with an explicit receiver e of type T the following sets are analyzed (in the given order):

1. Non-extension member callables named f of type T;
2. Extension callables named f, whose receiver type U conforms to type T, in the current scope and its upwards-linked scopes, ordered by the size of the scope (smallest first), excluding the package scope;
3. [...]

[...]

When analyzing these sets, the first set which contains any applicable callable is picked for c-level partition, which gives us the resulting overload candidate set.

So the plus method that is declared in Int is always found first, and the search stops there. Any extension you define will be ignored.

Hypothetically, if the built-in Int.plus is an implicitly imported extension function, then your code would have worked! Implicitly imported extensions are #6 on that list :)

My workaround for this situation is to use the "declare functions with almost any name by adding backticks" feature:

infix fun Int.`+`(other: Int) = listOf(this, other)

fun main() {
    println( 1 `+` 2 )
}

This wouldn't work for some names that have reserved characters like square brackets, angle brackets, slashes, and dot (not an exhaustive list).