If you allow shadowing then surely the local definition of the name will take precedence over the new name introduced in the stdlib (or wherever), and thus the

If you allow shadowing then surely the local definition of the name will take precedence over the new name introduced in the stdlib (or wherever), and thus the program will keep behaving how it did before the new builtin was introduced. I think I must be misreading you, because it sounds like you're arguing for the idea that adding new names to an API should be considered a breaking change? I'll accept that maybe reflection will catch those changes, but with that exception why would any code even notice?