encoding/json: clarify merge semantics of Unmarshal

[gpaul]

Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

Go playground

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

Go playground

What did you do?

https://play.golang.org/p/QM3FKwmR9h1

What did you expect to see?

{john}
{smith}

What did you see instead?

{john }
{john smith}

In short...

The Unmarshal documentation says:

Unmarshal parses the JSON-encoded data and stores the result in the value pointed to by v.

~ https://golang.org/pkg/encoding/json/#Unmarshal

I assumed that after Unmarshal returns, the struct would exactly match the provided JSON. I found this quite surprising and I think the documentation should mention that "Fields that are not present in the JSON document will retain their original values."

The existing functionality does give one an elegant way to set default values by having Unmarshal only update those fields which are present in the JSON document. I do not dispute that it is useful. Still, I would like to see the documentation mention this behaviour explicitly.

[gopherbot]

Change https://golang.org/cl/148834 mentions this issue: encoding/json: document the unmarshalling of missing json fields

[gopherbot]

Change https://golang.org/cl/221117 mentions this issue: encoding/json: clarify how we decode into structs

[mvdan]

The first CL hasn't been updated after the last round of reviews, and I'm tackling a related documentation issue as well, so I've sent a CL of my own.