How best to address dynamically selected grammar versions for a parse?

[kaby76]

It would be nice if we can dynamically select the version of the parser we want to use on some input. The GNU gcc compiler has an option to do that with -std=....

PR #4698 tries to do this, but breaks the target agnostic property required for .g4's.

For some languages, dynamically selecting a version to parse is a requirement. For the protobuf3 grammar, we're going to have to have this option because protobuf3 files can import protobuf2 files, and because protobuf has to have a symbol table for parsing. Unfortunately, it's one of those languages that makes the parse tree dependent on constructing a symbol table of the input.

There is also the issue on how to "merge" grammars between different versions.