Your code can't work. You are implementing the unmarshaling of Test by unmarshaling into Test. That will simply lead to an infinite recursion. The package is saving you from that by returning an error.

Do something like this instead: https://play.golang.org/p/KvZogax4MRQ

Can you elaborate why encoding/json looks at the UnmarshalText implementation when unmarshaling an object?

It's possible that I've misdescribed what the encoding/json package is doing, but as far as I can see it's still the case that your original program can't work.

I see no reason why encoding/json would need to look at its UnmarshalText function when mapping a json object to a struct. Nothing in the documentation indicates this, and I honestly expected it to perform the json parsing as usual, without looking at UnmarshalText. If I was implementing json.Unmarshaler, I'd be on your side with the infinite recursion, but not like this.

OK, maybe I see what you mean. Given a type that implements the UnmarshalText method and an input that is not a JSON quoted string, we can either ignore the UnmarshalText or we can return an error. Since UnmarshalText was implemented, we've returned an error (https://golang.org/cl/12703043). I suppose that perhaps the question is which operation is less confusing: honoring UnmarshalJSON but ignoring UnmarshalText, or honoring UnmarshalJSON and giving an error for UnmarshalText.

IMHO, it the latter makes more sense - when unmarshaling a JSON value, I'd assume that the decoder would look at the function that was specifically designed to handle JSON representations (json.Unmarshaler), and not the generic function unmarshaling it from an arbitrary text format.

Suppose I need to provide a type implementing encoding.TextMarshaler and encoding.TextUnmarshaler to a library's function, but don't want to serialize/deserialize the struct's fields myself, I expected to be able to simply call the json.Marshal/json.Unmarshal functions when implementing these interfaces, but due to this unmarshaling behaviour, that's not possible.