encoding/json: existing entries in map are ignored when unmarshaling

[thatguystone]

1. What version of Go are you using (go version)?
   1.6.2

2. What operating system and processor architecture are you using (go env)?
   Linux 4.6 // amd64

3. What did you do?
   https://play.golang.org/p/5EmXhFLhOj

4. What did you expect to see?
   According to the docs: "Unmarshal reuses the existing map, keeping existing entries". This sounds like it should attempt to unmarshal into existing values, if there are any.

5. What did you see instead?
   New values were created and replaced existing ones in the map.

   The documentation is clear that specific types are stored when unmarshaling into an interface value (eg. "map[string]interface{}, for JSON objects"), but this only happened when unmarshaling into a map. With an equivalent struct, existing interface values were preserved and used.

   Concretely, in the output from the example, the map is being transformed from map[a: *Val{I: 1}, ...] to map[a: map[i: 2], ...], whereas the struct goes from {A: *Val{I: 1}} to {A: *Val{I: 2}}.

[bradfitz]

I imagine we won't be able to do anything more than document this, for compatibility reasons.

[bradfitz]

Somebody left a comment saying that this was a dup of #12972, but I don't see the comment here. Deleted on purpose?

[rsc]

This seems pretty clear to me:

To unmarshal a JSON object into a map, Unmarshal first establishes a map to
use. If the map is nil, Unmarshal allocates a new map. Otherwise Unmarshal
reuses the existing map, keeping existing entries. Unmarshal then stores
key-value pairs from the JSON object into the map. The map's key type must
either be a string, an integer, or implement encoding.TextUnmarshaler.

Step 1: pick the map to use.
Step 2: replace entries.