`reflect.Value.SetString using unaddressable value` on concrete values contained within any structures

[element-of-surprise]

This is my second bug report around any, so I'm guessing that any support in version 5 has some challenges. This bug isn't as serious as the first one, as this can be worked around and my guess would require substantial work. But I wanted to note it for the authors.

Given a structure to encode and decode:

[]any{
	"name",
	[]any{
		[]any{
			0, // timestamp in seconds
			Record{},
		},
	},
	struct{TimeFormat string }{TimeFormat: "DateTime"},
}

(Yeah, I know....)

You cannot decode back into the same structure, it gives an error. The error is different in version 4 and 5 but are likely related to the same problem with handling the reflection.

Expected Behavior

It should decode back into the structure. There are two ways that it could do this:

• encoding/json way
• json-experiment way (better)

The original encoding/json would decode it, but replace Record with map[string]any. That is not ideal and unexpected. But it does kinda work.

The better approach is the json-experiment way. This package is likely to be incorporated as encoding/json/v2. It decodes it into the structure.

Current Behavior

v5: panic: reflect: reflect.Value.SetString using unaddressable value
v4: panic: Decode(nonsettable string)

Possible Solution

Adopt the decoding practices of json-experiment around any types.

Steps to Reproduce

https://go.dev/play/p/5vpaqDRrD5d

Here is JSON experiment doing the same type of encode/decode:

https://go.dev/play/p/Slea4ZbvjR7

Context (Environment)

Well, you can still decode into plainany with v4. Because of the other bug I left, you can't use version 5.
But this does mean you have to then manually convert from map[string]any to the concrete type. Not the end of the world.