reflect: Calling reflect.SetMapIndex(k, v) causes mutations to k if k is removed in the map

[gopherbot]

by gmadrid:

See code at http://play.golang.org/p/MIkFP_Zrxb

What does 'go version' print?
go version go1.2.1 darwin/amd64

What steps reproduce the problem?
Calling reflect.SetMapIndex() causes changes to the key that prevent future calls to
SetMapIndex from working.

If possible, include a link to a program on play.golang.org.
http://play.golang.org/p/MIkFP_Zrxb

What happened?
1. Run the linked code.
2. Notice that the key after the call to SetMapIndex is different
3. Notice that the second call to SetMapIndex doesn't work.

What should have happened instead?
The key should be unchanged and both calls to SetMapIndex should work.


Code copied below: 

package main

import (
    "fmt"
    "reflect"
)

func main() {
    m1 := map[string]bool{"a": true, "b": true}
    m2 := map[string]bool{"a": true, "b": true}

    fmt.Println(m1)

    v1 := reflect.ValueOf(m1)
    k := v1.MapKeys()[0]

    fmt.Println("KEY BEFORE", k)
    v1.SetMapIndex(k, reflect.Value{})  // COMMENT THIS OUT
    fmt.Println("m1:", m1)
    fmt.Println("KEY AFTER", k)

    v2 := reflect.ValueOf(m2)
    v2.SetMapIndex(k, reflect.Value{})
    fmt.Println("KEY AFTER SECOND CALL", k)
    fmt.Println("m2:", m2)
}

[minux]

Comment 1:

http://play.golang.org/p/EtZmoIyYPz
Because you removed key "a" from the map, it makes sense that the prior key value isn't
valid anymore.
I think this is a documentation issue.

Labels changed: added release-go1.3maybe, repo-main, documentation.

Status changed to Accepted.

[ianlancetaylor]

Comment 2:

I do think this is a bug, but it seems to be fixed on tip.

Status changed to Fixed.

[gopherbot]

Comment 3 by gmadrid:

If this is WAI, it's a pretty awkward API. The behavior of SetMapIndex depends on 1) the
origin of the key value, and 2) the order of the SetMapIndex calls. Both of these could
lead to some very difficult to find bugs.
http://play.golang.org/p/8HY4i0N8_z

[gopherbot]

Comment 4 by gmadrid:

Sure enough. Looks like it was fixed here:
https://code.google.com/p/go/source/detail?r=ecccf07e7f9d341ca577433b794ab8061fbdc092&path=/src/pkg/reflect/value.go
I can work around this by deleting from the maps in a different order. 
Thanks, y'all.