[pro] Sensible interpretation of declarations in CLOS

Dear all,

I am retaking an old question, which is the utility of type declarations in a language that has the flexibility of the MOP.

In an extremely open-minded setup, where any class is prone to change, not only their structure, but also their metaclass, it would seem that a local declaration of the kind (DECLARE (MY-CLASS FOO)) would be totally useless.

But... do you actually face that flexibility in real life? I mean, declarations such as (DECLARE (TYPE MY-INTEGER-TYPE FOO)) are used in actual code, where there is an implicit contract that the DEFTYPE type will not be changed in unsafe code. Can't we assume something similar when facing CLOS? What would be a reasonable optimization/safety/debug level to start considering that?

Best,

Juanjo