Hello Erik, all,
I think the option should instead be to disable the old parser. The new parser already helped us find bugs in ccl files, and the new parser is the future. There is no known instance where the new parser silently have wrong results. Disabling it is being too cautious.
Right now it already is using the new parser.
I believe the intention during the call was rather that the option and help text would be there so that users who update Cactus, want to compile there (existing) code and get errors due to the parser, can at least revert to the old parser and get what they got before.
This may be either b/c we forgot to support a corner case or b/c their code relied on a specific behaviour of the old parser that no longer exists (or was simply buggy). This option would only exist for one release (think of it as the deprecation warning for parameters that will go away).
Yours, Roland