Many attributes (including the UseDefs attribute itself) are unaffected by the setting of the UseDefs attribute, and default values will always be used without error for such attributes. The "Applicability:" section below lists the attributes which are affected by the setting of UseDefs.
Note, UseDefs only affects access to attributes internally within AST. The public accessor functions such as astGetC is unaffected by the UseDefs attribute - default values will always be returned if no value has been set. Application code should use the astTestastTest function if required to determine if a value has been set for an attribute.