sync.WaitGroup panic trouble me

[littlejiancc]

What version of Go are you using (go version)?

$ go version
go version go1.14.1 windows/amd64

What did you do?

When I study sync.WaitGroup. And this problem has troubled me for a long time!
This is my code:

package main

import "sync"

func main() {
	for i := 0; i < 1e6; i++ {
		var wg sync.WaitGroup
		wg.Add(1)
		go func() {
			wg.Add(1)
			wg.Done()
		}()
		go func() {
			wg.Wait()
		}()
		wg.Done()
	}
}

As we know,Add called concurrently with Wait will panic:

GOROOT=C:\Go #gosetup
GOPATH=D:\go #gosetup
C:\Go\bin\go.exe build -o C:\Users\AH\AppData\Local\Temp\___go_build_awesomeProject.exe awesomeProject #gosetup
C:\Users\AH\AppData\Local\Temp\___go_build_awesomeProject.exe #gosetup
panic: sync: WaitGroup misuse: Add called concurrently with Wait

goroutine 150022 [running]:
sync.(*WaitGroup).Add(0xc00053bda0, 0x1)
        C:/Go/src/sync/waitgroup.go:77 +0x124
main.main.func1(0xc00053bda0)
        D:/go/src/awesomeProject/main.go:10 +0x3b
created by main.main
        D:/go/src/awesomeProject/main.go:9 +0x82

However!
When the code like this,it's well done!

package main

import "sync"

func main() {
	for i := 0; i < 1e6; i++ {
		var wg sync.WaitGroup
		//wg.Add(1)
		go func() {
			wg.Add(1)
			wg.Done()
		}()
		go func() {
			wg.Wait()
		}()
	//	wg.Done()
	}
}

I think it also has chance Add called concurrently with Wait .
So why it always well done?

[ALTree]

Both programs are racy and thus invalid; but I think that the first one triggers more easily the check that prints the WaitGroup is reused before previous Wait has returned error.

Run them with the race detector enabled using go run -race program.go and you'll see this data race reported in both cases:

==================
WARNING: DATA RACE
Write at 0x00c000094298 by goroutine 72:
  internal/race.Write()
      /usr/local/go/src/internal/race/race.go:41 +0x114
  sync.(*WaitGroup).Wait()
      /usr/local/go/src/sync/waitgroup.go:128 +0x115
  main.main.func2()
      /home/alberto/test.go:14 +0x38

Previous read at 0x00c000094298 by goroutine 7:
  internal/race.Read()
      /usr/local/go/src/internal/race/race.go:37 +0x1e8
  sync.(*WaitGroup).Add()
      /usr/local/go/src/sync/waitgroup.go:71 +0x1fb
  main.main.func1()
      /home/alberto/test.go:10 +0x41

Goroutine 72 (running) created at:
  main.main()
      /home/alberto/test.go:13 +0xd0

Goroutine 7 (finished) created at:
  main.main()
      /home/alberto/test.go:9 +0xae
==================

So the second program is invalid too. Closing here, since we only use the issue tracker for Go bugs, not for asking questions.