encoding/json: Unmarshal inserts map keys when the value is not appropriate for the map type

[leventov]

package main

import (
	"fmt"
	"encoding/json"
)

func main() {
	var x map[string]int
	err := json.Unmarshal([]byte(`{"a":1,"b":"c"}`), &x)
	fmt.Printf("x = %#v, err = %s", x, err)
}

Output:

x = map[string]int{"a":1, "b":0}, err = json: cannot unmarshal string into Go value of type int

I think this violates the principle of least astonishment, "b" key shouldn't appear in the output x map.

The relevant part of the documentation for json.Unmarshal() is:

To unmarshal a JSON object into a map, Unmarshal first establishes a map to use. If the map is nil, Unmarshal allocates a new map. Otherwise Unmarshal reuses the existing map, keeping existing entries. Unmarshal then stores key-value pairs from the JSON object into the map. The map's key type must either be any string type, an integer, implement json.Unmarshaler, or implement encoding.TextUnmarshaler.

If a JSON value is not appropriate for a given target type, or if a JSON number overflows the target type, Unmarshal skips that field and completes the unmarshaling as best it can. If no more serious errors are encountered, Unmarshal returns an UnmarshalTypeError describing the earliest such error. In any case, it's not guaranteed that all the remaining fields following the problematic one will be unmarshaled into the target object.

It doesn't seem to me that it describes the above behaviour.

[mvdan]

I think this is a valid bug report, and the behavior also surprises me a little. I think it might be simply because the "init" operations (creating a zero value, inserting the key to the map) are out of order.

[AlexanderYastrebov]

I think it might be simply because the "init" operations (creating a zero value, inserting the key to the map) are out of order.

The fix does not seem to be trivial as the value is always initialized to the zero value:

go/src/encoding/json/decode.go

Lines 692 to 699 in 1b072b3

	if v.Kind() == reflect.Map {
	elemType := t.Elem()
	if !mapElem.IsValid() {
	mapElem = reflect.New(elemType).Elem()
	} else {
	mapElem.Set(reflect.Zero(elemType))
	}
	subv = mapElem

so at the assignment point

go/src/encoding/json/decode.go

Lines 815 to 817 in 1b072b3

	if kv.IsValid() {
	v.SetMapIndex(kv, subv)
	}

it is not possible to tell whether the value was changed (it could also be set to zero) by

go/src/encoding/json/decode.go

Lines 774 to 776 in 1b072b3

	if err := d.value(subv); err != nil {
	return err
	}

#39149 could be related