Yes, but turning a single use of a lexical variable into a warning will raise warnings for code like: ... This could maybe circumvented by only raising the warning when my is missing

Or it isn't a blessed Guard :) Yes, but this new scope pragma could also notice its a Guard singleton and not issue a warning -- sure it won't work for all singletons, but the popular ones could be listed, and the new ones could register themselves like you register warnings or even autodie::hints