What does this print? (select with a closed channel and a nil channel)

package main

import "fmt"

func main() {
	var nilCh chan int
	closed := make(chan int)
	close(closed)

	counts := map[string]int{}
	for range 1000 {
		select {
		case <-nilCh:
			counts["nil"]++
		case <-closed:
			counts["closed"]++
		default:
			counts["default"]++
		}
	}
	fmt.Println(counts)
}

It prints map[closed:1000]. The nil-channel case is never ready. The closed channel is always ready (returning the zero value), and default runs only when no case is ready. So the closed case wins all 1000 times. The random choice only applies among cases that are ready, and here only one is.

A follow-up interviewers often ask: "What if you delete default and closed?" Then the select waits on a nil channel only, which is equivalent to select {}. The main goroutine blocks forever, and the program crashes with fatal error: all goroutines are asleep - deadlock!