gistfile1.txt

$ go run main.go
fatal error: concurrent map writes

goroutine 34 [running]:
runtime.throw(0x89200, 0x15)
	/Users/jbd/go/src/runtime/panic.go:530 +0x90 fp=0xc8200806d8 sp=0xc8200806c0
runtime.mapassign1(0x5e940, 0xc8200161e0, 0xc8200807a0, 0xc8200807a0)
	/Users/jbd/go/src/runtime/hashmap.go:445 +0xb1 fp=0xc820080780 sp=0xc8200806d8
main.main.func1(0xc8200161e0, 0xc82000a170)
	/Users/jbd/src/github.com/rakyll/experimental/maprace/main.go:11 +0x50 fp=0xc8200807b0 sp=0xc820080780
runtime.goexit()
	/Users/jbd/go/src/runtime/asm_amd64.s:1998 +0x1 fp=0xc8200807b8 sp=0xc8200807b0
created by main.main
	/Users/jbd/src/github.com/rakyll/experimental/maprace/main.go:12 +0x9d

goroutine 1 [sleep]:
time.Sleep(0x3b9aca00)
	/Users/jbd/go/src/runtime/time.go:59 +0xf9
main.main()
	/Users/jbd/src/github.com/rakyll/experimental/maprace/main.go:14 +0xc1
exit status 2

main.go

package main

import "time"

const n = 50

func main() {
	m := make(map[int]struct{}, n)
	for i := 0; i < n; i++ {
		go func() {
			m[i] = struct{}{}
		}()
	}
	time.Sleep(time.Second)
}

how is the race condition caught if you're not passing the -race flag ?