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What does this goroutine loop print on Go 1.22+ vs Go ≤1.21?

Predict what this program prints, separately for Go 1.22+ and Go ≤1.21, and explain why the two differ. Note: the goroutines run concurrently, so account for the order of the printed values, not just their values.

func main() {
    var wg sync.WaitGroup
    for _, v := range []int{1, 2, 3} {
        wg.Add(1)
        go func() {
            defer wg.Done()
            fmt.Print(v, " ")
        }()
    }
    wg.Wait()
}

Predict the output.

On Go 1.22+ each iteration gets its own v, so the goroutines print 1 2 3 in some order. On Go ≤1.21 they all shared one v mutated by the loop and typically printed 3 3 3. The portable fix is v := v inside the loop or passing v as an argument.

✗Believing go func(){...}() copies the loop variable at call time — it captures the variable, not its value
✗Assuming the 3 3 3 behavior is a data race rather than a deterministic capture of one shared variable
✗Thinking Go 1.22 scoping also reorders the goroutines into 1 2 3 — the order is still unspecified

→How does the Go 1.22 change scope the loop variable, and does it cost an allocation per iteration?
→Why does passing v as a function argument fix the bug on every Go version?

Contents

What does this code print?

package main

import (
	"fmt"
	"sync"
)

func main() {
	var wg sync.WaitGroup
	for _, v := range []int{1, 2, 3} {
		wg.Add(1)
		go func() {
			defer wg.Done()
			fmt.Print(v, " ")
		}()
	}
	wg.Wait()
}

Output

Go 1.22+:  1 2 3   (in some order)
Go ≤1.21:  3 3 3   (typically)

Why

The closure go func(){ ... }() captures the variable v, not its value at launch time.

Go ≤1.21: there is one loop variable v for the whole loop. By the time the scheduler runs the goroutines the loop has finished and v holds its last value 3, so all three goroutines print 3.
Go 1.22+: the language creates a fresh v each iteration, so every goroutine captures its own copy and prints 1, 2, 3. The order is still unspecified — goroutines are scheduled independently.

⚠️ This is the single most common Go interview gotcha. The portable fix that works on every version: