[jiayq84]

So what we do is to keep syntax=proto2, but allow users to compile with both protobuf 2.x and protobuf 3.x libraries. Minumum need is 2.6.1. We kind of feel that this gives maximum flexibility for people who have already chosen a protobuf library version.

[haberman]

Makes sense! All I'm saying is that there is no need to avoid using extensions if that is your strategy. Extensions will work in all versions of the library you wish to support.