What is the goroutine loop-variable capture bug and how do you fix it?

// BUG (Go 1.21 and earlier): all goroutines may print 5
for i := 0; i < 5; i++ {
    go func() {
        fmt.Println(i) // captures &i — reads whatever i is RIGHT NOW
    }()
}
// By the time goroutines execute, the loop finished and i == 5
// Typical output: 5 5 5 5 5

// FIX 1: pass as argument — creates an independent copy per iteration
for i := 0; i < 5; i++ {
    go func(n int) { // n is a local copy of i at this point
        fmt.Println(n) // 0 1 2 3 4 (in any order)
    }(i)
}

// FIX 2: shadow the loop variable inside the loop body
for i := 0; i < 5; i++ {
    i := i // new variable, shadows the outer i
    go func() { fmt.Println(i) }()
}

// GO 1.22+: loop variables are per-iteration by default
// for i := range 5 { go func() { fmt.Println(i) }() } // safe in 1.22!

// Same bug with range over slice
names := []string{"Alice", "Bob", "Carol"}
for _, name := range names {
    go func() { fmt.Println(name) }() // BUG: all may print "Carol"
}

// Fix
for _, name := range names {
    go func(n string) { fmt.Println(n) }(name)
}