Go 1.18: reflect implicit type conversion

[Dave Keck]

Hi all,

In this code, the SetMapIndex line panics in Go <1.18, but it no longer panics in Go 1.18:

    type CustomString string
    m := reflect.ValueOf(map[CustomString]int{})
    m.SetMapIndex(reflect.ValueOf("key"), reflect.ValueOf(0))

The panic ("value of type string is not assignable to type CustomString") appears to be prevented in Go 1.18 because reflect implicitly converts string -> CustomString, which Go <1.18 doesn't do.

I had 2 questions:

    - Is it possible to get this implicit conversion behavior in earlier versions of Go? Specifically, I'm using Go 1.16 + the firestore package, and I'd like to use custom string types as map keys.
   
    - Is this change discussed anywhere?
   
Thanks!

David

[Dan Kortschak]

This is not mentioned in the release notes. I think it is a bug in
1.18; he docs still say that "As in Go, key's elem must be assignable
to the map's key type". This is not the case here.

Bisected to 23832ba2e2fb396cda1dacf3e8afcb38ec36dcba which touches
this.

Dan

[Ian Lance Taylor]

On Sat, Apr 16, 2022 at 2:09 AM 'Dan Kortschak' via golang-nuts
<golan...@googlegroups.com> wrote:
>
> This is not mentioned in the release notes. I think it is a bug in
> 1.18; he docs still say that "As in Go, key's elem must be assignable
> to the map's key type". This is not the case here.
>
> Bisected to 23832ba2e2fb396cda1dacf3e8afcb38ec36dcba which touches
> this.

For the record, Dan filed https://go.dev/issue/52379 to get this fixed
in 1.19 and 1.18.2 Thanks!

Ian