Interface decoder is not compatible with standard library

[arekziobrowski]

When trying to unmarshal into the interface, the interface decoder is expecting the UnmarshalJSON method to be defined on the underlying struct [source]. Otherwise, it checks if the value is null and ultimately fails if neither condition applies with:

json: cannot unmarshal X into Go value of type X

The standard library behaviour though is handling such situations and does not fail.

I'm using Go version 1.19.

Code to reproduce this issue

package main

import (
	"github.com/goccy/go-json"
	"log"
	"time"
)

type ExampleInterface interface {
	Method(a string) error
}

type ExampleStruct struct {
	Value     string    `json:"value"`
	CreatedAt time.Time `json:"createdAt"`
}

func (e *ExampleStruct) Method(a string) error {
	return nil
}

var bb = []byte(`{"value":"abc","createdAt":"2023-03-20T08:15:07.526300877Z"}`)

func main() {
	unmarshalled, err := unmarshal(&ExampleStruct{})
	if err != nil {
		log.Fatal(err)
	}
	log.Println(unmarshalled)
}

// This fails.
func unmarshal(a ExampleInterface) (ExampleInterface, error) {
	data := bb
	if err := json.Unmarshal(data, &a); err != nil {
		return a, err
	}
	return a, nil
}

// This works.
func unmarshalStruct(a *ExampleStruct) (*ExampleStruct, error) {
	data := bb
	if err := json.Unmarshal(data, &a); err != nil {
		return a, err
	}
	return a, nil
}

Output of the main function run is:

json: cannot unmarshal main.ExampleInterface into Go value of type main.ExampleInterface

I can contribute some example test cases if this would be helpful!