encoding/json: unmarshal accepts duplicate keys

[Dirbaio]

Unmarshaling a JSON object with repeated keys into a struct doesn't return an error, it keeps the value specified last in the JSON string instead.

https://play.golang.org/p/cv6HeD-hERx

More confusingly, it causes maps on repeated keys to be merged: https://play.golang.org/p/47_7kSPFCW2

Other languages' JSON libraries error in this case

[dpinela]

The documentation doesn't specify what happens in this case; it says it matches object keys to fields but doesn't specify in what order it does so (it only matters with duplicate keys). Changing the current behaviour might break backwards compatibility too much, but I think at least it should be documented.

Also, the behaviour is inconsistent between a map and a slice field; the maps get merged, the slice field takes the last value seen, like the other types.

[rsc]

This is working exactly as documented (see 'go doc json.Unmarshal'). I'm sorry if it's surprising behavior, but we can't change it now.

[mvdan]

@rsc thanks for the reply. You're right that the godoc says that keys are matched, so it is implicit that nothing is done about duplicate keys. However, I don't see how that should lead to maps getting merged.

That is, in @Dirbaio's second play link, I would expect the output to be {map[bar:1]}.

I understand that unmarshalling multiple times into one map merges the keys - that is fine. What I don't think is fine is for duplicate keys to get merged. For example, as @dpinela said, when the field is a slice they aren't merged and you get a single element: https://play.golang.org/p/nXLs0hd8Uj3

[mvdan]

@zamicol correctly points out that some people are ending up on this thread, which has been rejected and frozen/locked for a few years now. The discussion continues over at #48298, which is a proposal to add an option, and it was accepted.