encoding/json: Decoder does not keep context for nested UnmarshalJSON errors

[LeGEC]

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

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

playground : go1.7 amd64p32 - nacl

What did you do?

I triggered a json error :
the error is raised by a specific UnmarshalJSON() function on one of the fields of the go struct value

In this specific example : a time.Time field tries to parse the string "null"

https://play.golang.org/p/SH1R641VzV

What did you expect to see?

some error wrapped into a json.SomeErrorType value.
maybe a new json.UnmarshalValueError :

json.UnmarshalValueError{Offset: 24, Fieldname: "Clock", Suberror: time.ParseError{ ... } ...

(some way to get the context of where the error occurred)

What did you see instead?

the raw error value from the time package :

&time.ParseError{Layout:"\"2006-01-02T15:04:05Z07:00\"", Value:"null", LayoutElem:"\"", ValueElem:"null", Message:""}

additional notes

Since using this as the default behaviour would break the backward compatibility promise, maybe add a switch on the json.Decoder() type :

 func (dec *Decoder) UseDetailedUnmarshalErrors(v bool) {
     ...
}

This would not fix the direct use of json.Unmarshal() function, but would still give a way to get this error context.

[bradfitz]

Can you try Go 1.8beta1? We're no longer making changes to Go 1.7.

[LeGEC]

In Go 1.8beta1 :

• json.Unmarshal() will not raise any error when trying to fit a null value into a non pointer time.Time field (question: is this behaviour as expected ?)
• json.Unmarshal() will still return a raw &time.ParseError{...} when trying to unmarshal a value with an incorrect time format

[bradfitz]

Yes, the Unmarshal of null behavior was changed in 1.8. See https://beta.golang.org/doc/go1.8#encoding_json

[bradfitz]

I'm going to close this bug, but if you have a concrete JSON change proposal, please file a proposal per https://github.com/golang/proposal#readme