Question about google.protobuf.FieldMask in update operations

[Shouichi Kamiya]

The following block is the excerpt from google.protobuf.FieldMask.

// # Field Masks in Update Operations

//

// A field mask in update operations specifies which fields of the

// targeted resource are going to be updated. The API is required

// to only change the values of the fields as specified in the mask

// and leave the others untouched. If a resource is passed in to

// describe the updated values, the API ignores the values of all

// fields not covered by the mask.

//

// If a repeated field is specified for an update operation, new values will

// be appended to the existing repeated field in the target resource. Note that

// a repeated field is only allowed in the last position of a `paths` string.

//

// If a sub-message is specified in the last position of the field mask for an

// update operation, then new value will be merged into the existing sub-message

// in the target resource.

//

// For example, given the target message:

//

//     f {

//       b {

//         d: 1

//         x: 2

//       }

//       c: [1]

//     }

//

// And an update message:

//

//     f {

//       b {

//         d: 10

//       }

//       c: [2]

//     }

//

// then if the field mask is:

//

//  paths: ["f.b", "f.c"]

//

// then the result will be:

//

//     f {

//       b {

//         d: 10

//         x: 2

//       }

//       c: [1, 2]

//     }

In the example, the field `f.b.x` is unchanged after merge. Is this behavior

possible in proto3? Because `f.b.x` is an integer and whether `f.b.x` is absent

or is the zero value cannot be distinguished.

Thank you