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

$ go version

gollvm

Does this issue reproduce with the latest release?

yes

What operating system and processor architecture are you using (go env)?

$ go env

What did you do?

error case:

package main
import "fmt"
func mapscase() int {
    one := map[int]int{0: 1299}
    two := map[int]int{0: 2388}

    x := one
    x[0] += func() int {
        x = two
        return 1113
    }()

    return two[0]
}

func main() {
    a := mapscase()
    fmt.Println(a)
}

What did you expect to see?

3501

What did you see instead?

2412
cmd/compile: "x[i] op= y" evaluates x[i] more than once can also be referenced.

Error & Solution

An output of 2412 indicates that x[0] was evaluated twice as follows: once indexing into the old map to read the value 1299, and then once into the new to store the value 1299 + 1113.

gofrontend doesn't consider such case: when evaluates x twice in "x op= y", evaluating y may affect x. In such case, y on the right should be evaluated first and x on the left should be read latter.