Summary

Given a custom type whose underlying type is a map and an attached UnmarshalBSON method with a pointer receiver:

type MyMap map[string]interface{}

func (m *MyMap) UnmarshalBSON(byts []byte) error {
	//Implementaion not relevant
	return nil
}

Then creating a struct with a field the type of which is the custom type (importantly not a pointer to the type)

type MyStruct struct {
	MyMap MyMap
}

Any attempt to unmarshal a BSON document into an instance of *MyStruct where the MyMap field in the BSON document is null will cause a panic in bson.DefaultValueDecoders#ValueUnmarshalerDecodeValue

panic: reflect: reflect.Value.Set using unaddressable value

Prior to v1.9.0, this code would run through bson.Unmarshal without error.

Please provide the version of the driver. If applicable, please provide the MongoDB server version and topology (standalone, replica set, or sharded cluster).

Driver Version: 1.9.0+

MongoDB version: N/A

How to Reproduce

package mainimport (	"fmt"	"go.mongodb.org/mongo-driver/bson")type MyMap map[string]interface{}func (m *MyMap) UnmarshalBSON(byts []byte) error {	// Implementation not relevant	return nil}type MyStruct struct {	MyMap MyMap}func main() {	// Making some BSON so I don't have to craft it by hand	x := &MyStruct{}	byts, err := bson.Marshal(x)	if err != nil {		panic(err)	}	// Immediately unmarshal to show the panic	y := &MyStruct{}	err = bson.Unmarshal(byts, y) // Panic will occur in here	if err != nil {		panic(err)	}}

Additional Background

N/A