OverloadTest.swift

import Foundation

struct Box<Value> {
    let value: Value
}

extension Box {
    // 1.
    func map<T>(_ f: @escaping (Value) -> T) -> Box<T> {
        return Box<T>(value: f(value))
    }
    
    // 2.
    func map<T>(_ value: T) -> Box<T> {
        return map { _ in value }
    }
    
    // 3.
    func map<T>(_ keyPath: KeyPath<Value, T>) -> Box<T> {
        return map { $0[keyPath: keyPath] }
    }
}

func test(input: Box<Void>) -> Box<String> {
    // Xcode 12b4: output is Box<String> (overload #1)
    // Xcode 12b5: output is Box<() -> String> (overload #2, but only when both #2 AND #3 exist!!)
    let output = input.map { "foo" }
    return output
}