I thought this was neat and quirky. The implications of it are important, too. When you assign a macro variable in SAS, it doesn't resolve immediately. It resolves It will also warn you if the variable doesn't exist yet, too.
%let x = &y; %let y = z; %put &x;
Even though, when x was defined, y did not exist. We'll get the following warning, which can be ignored.
WARNING: Apparent symbolic reference Y not resolved.
As far as I can tell (I could be wrong!), I don't think there's a way of turning off that warning.