/cc @mvdan @dsnet

This is presumably a bug in json.indirect function (at encoding/json/decode.go). If a struct has an Unmarshal function—whether explicit or inherited—it returns that function which is further used to unmarshal the entire struct.

The obvious workaround is to redefine Unmarshal function for Q struct, however if the above behavior is intended, the compiler must warn about it.

I don't think this is a bug.

• You've implemented json.Marshaler on type Q
• Embedding Q to req also attaches that method to req.
• The req type of *struct{...} does not implement json.Marshaler

To fix, write:

	var req struct {
		Criteria string `json:"criteria"`
		Q        Q
	}

Note: I currently rely on the current behavior of Q hijacking the marshaling process for some functions.

You've implemented json.Marshaler on type Q

You'd mention json.Unmarshaler, I think. BTW, the godoc specifies:

To unmarshal JSON into a value implementing the Unmarshaler interface, Unmarshal calls that value's UnmarshalJSON method, including when the input is a JSON null.

So, yes! According to the document, since req is implementing json.Unmarshaler (borrowing it from Q), the behavior is expected. However, I'd recommend to raise a warning or be more specific in the document, since this behavior is prone to errors.

I've just looked at an example with encoding/xml for a similar scenario. The xml.Unmarshal would complain with an error:

xml: (*struct {...}).UnmarshalXML did not consume entire <X> element

This is much better, and the same can happen with encoding/json, e.g.:

json: (*struct {...}).UnmarshalJSON did not consume entire JSON object

And, if someone knows exactly what he/she is doing, can simply ignore it, using:

err := json.Unmarshal(data, &obj)
if err != nil && !json.IsNotEntireJSON(err) {
    panic(err)
}

CC @rsc @dsnet @bradfitz @mvdan for encoding/json.

As @as notes, this is working as documented.

On top of that, the encoding/json package has no way to know, in general, whether your UnmarshalJSON method consumed the entire JSON object or not. (For all we know, you're parsing some of the fields via a recursive call to json.Unmarshal and others by pulling out the fields directly.)

I don't think there is anything more to be done here. (You're welcome to propose a change to the behavior of the package, but that should be filed as a separate proposal.)