Slightly simpler version, as a playground link: https://play.golang.org/p/5hasX7yOX1

Looks like a bug to me. The spec says that the right-hand side is first fully evaluated before any assignments take place. A nil-pointer deref causes a runtime panic similar to calling panic() which (immediately) terminates the surrounding function.

Yeah, this is probably an order.go issue. We can't call mapassign until we know the RHS will evaluate without panicking.

At least OIND, OINDEX, ODIV, and OMOD can panic, but are currently allowed after the mapassign call.

Change https://golang.org/cl/81817 mentions this issue: cmd/compile: fix map assignment with panicking right-hand side

Reopening for backport consideration (both 1.9.3 and 1.8.6).

This is not only for map

package main

var x int
func f2() *int {
  println("bye") // this line shouldn't be output.
  return &x
}

func main() {
        var p *int
        *f2() = *p
}

[update] Aha, this may be some different.

however, if we view left m[key]as *GetMapValueAddr(key), then the example in the first comment may be also viewed as not a bug.

Similar to this problem: https://groups.google.com/forum/#!topic/golang-nuts/J3kXXZivlHA

@go101 The order of evaluation rules are at https://golang.org/ref/spec#Order_of_evaluation. There is no specified order between the function call on the left hand side and the indirection on the right hand side. It's clear that the indirection must happen before the assignment, but there is no requirement that the indirection happen before the function call.

Then this

package main

import "fmt"

func main() {
  var m = map[int]int{}
  
  defer func() {
    fmt.Println(recover())
    fmt.Println(len(m)) // 0
  }()
  var p *int
  m[2], *p = 1, 2
}

The spec says m[2]=1 happens before *p=2, so the print result should be 1 instead.

Let's take detailed discussion of order of evaluation away from this issue, which is about a specific bug. Thanks.

Closing proposed backport per discussion in #23005. (This remains fixed in the upcoming 1.10 release.)