1 files changed, 19 insertions, 26 deletions
diff --git a/src/google/protobuf/field_mask.proto b/src/google/protobuf/field_mask.proto
index 76e09f39..4015b1a3 100644
--- a/src/google/protobuf/field_mask.proto
+++ b/src/google/protobuf/field_mask.proto
@@ -38,6 +38,7 @@ option java_outer_classname = "FieldMaskProto";
 option java_multiple_files = true;
 option objc_class_prefix = "GPB";
 option go_package = "google.golang.org/genproto/protobuf/field_mask;field_mask";
+option cc_enable_arenas = true;
 
 // `FieldMask` represents a set of symbolic field paths, for example:
 //
@@ -107,57 +108,49 @@ option go_package = "google.golang.org/genproto/protobuf/field_mask;field_mask";
 // 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, the existing
-// repeated values in the target resource will be overwritten by the new values.
-// Note that a repeated field is only allowed in the last position of a `paths`
-// string.
+// 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 the existing sub-message in the target resource is
-// overwritten. Given the target message:
+// 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
+//         d: 1
+//         x: 2
 //       }
-//       c : 1
+//       c: [1]
 //     }
 //
 // And an update message:
 //
 //     f {
 //       b {
-//         d : 10
+//         d: 10
 //       }
+//       c: [2]
 //     }
 //
 // then if the field mask is:
 //
-//  paths: "f.b"
+//  paths: ["f.b", "f.c"]
 //
 // then the result will be:
 //
 //     f {
 //       b {
-//         d : 10
+//         d: 10
+//         x: 2
 //       }
-//       c : 1
+//       c: [1, 2]
 //     }
 //
-// However, if the update mask was:
-//
-//  paths: "f.b.d"
-//
-// then the result would be:
-//
-//     f {
-//       b {
-//         d : 10
-//         x : 2
-//       }
-//       c : 1
-//     }
+// An implementation may provide options to override this default behavior for
+// repeated and message fields.
 //
 // In order to reset a field's value to the default, the field must
 // be in the mask and set to the default value in the provided resource.