Is there any standard way to create a transient class in CLOS; that is, a class which can be GC'd after all its instances are dead?
In SBCL, I tried (setf test (defclass #:foo () ())), i.e. using an uninterned symbol under the assumption that it would normally only be the class' name which caused the class to be retained by the GC, but installing a SBCL finalization function on that class and then setting the symbol value of TEST to NIL shows that the class is never GC'd, no matter how many times I run (gc :full t). Which leaves me wondering if it's possible, and if so, how.
It does not matter that the class name, a symbol, is not interned in a package. FIND-CLASS will find the class by looking at some internal registry data structure. Clozure Common Lisp uses for example a normal hash table CCL::%FIND-CLASSES%.
There is no standard way. The Common Lisp standard provides no mechanism. Typically a CLOS implementation will want to provide a list of all subclasses of a certain class. For that it needs a reference from a class to its subclasses. It is not specified that this should be a weak reference. For example CLISP implements it as a weak references, other Common Lisp implementations may not.
Sketch of a solution:
So in a finalizer you would need calls to REMOVE-DIRECT-SUBCLASS (in what package that function may be, often in package CLOS) to remove the class from its superclasses.
You also need to call (setf (find-class 'my-class-to-be-removed) nil).
You also better look that the class does not have subclasses itself.
So you may be able to build something using the widely supported MOP and implementation specific finalizers.