Use case:

// test.proto syntax = "proto2"; package google.proto.test;

message TopMessage { message MidMessage { message BottomMessage { extensions 4 to max; }

optional group BottomGroup = 1 {
  extensions 4 to max;
}

}

optional group MidGroup = 2 { message BottomMessage { extensions 4 to max; }

optional group BottomGroup = 3 {
  extensions 4 to max;
}

} }

// test2.proto

syntax = "proto2";

import "test.proto";

extend .google.proto.test.TopMessage.MidMessage.BottomMessage { optional int32 v2_message_message_ext = 1; }

extend .google.proto.test.TopMessage.MidMessage.BottomGroup { optional int32 v2_message_group_ext = 2; }

extend .google.proto.test.TopMessage.MidGroup.BottomMessage { optional int32 v2_group_message_ext = 3; }

extend .google.proto.test.TopMessage.MidGroup.BottomGroup { optional int32 v2_group_group_ext = 4; }

r52a7fe6b2999