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.