Fix cubic-time from_type() on abstract class hierarchies#4749
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Resolving an abstract type whose subclasses reference the base class in their own annotations re-resolved the entire hierarchy once per reference, taking time cubic in the number of subclasses. Track the abstract types currently being resolved in a guard, and defer any recursive reference back to them by returning the cached from_type() strategy (sharing one object lets the recursion in e.g. is_empty checks terminate), so each type is resolved only once. This guard is kept separate from _recurse_guard so that it catches references arriving by any route (e.g. as a union arg) and doesn't make from_type_guarded treat a subclass's required field of the abstract type as unresolvable. Also filter the registered-subtype lookup before sorting it, so the expensive repr-based sort only runs on the (usually empty) matching set rather than the whole global type lookup on every resolution. https://claude.ai/code/session_01MdLX8p4tdAUaDjSHBnveVy
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Resolving an abstract type whose subclasses reference the base class in their own annotations re-resolved the entire hierarchy once per reference, taking time cubic in the number of subclasses. Guard against re-entry so each type is resolved only once: if we're already resolving an abstract type higher up the stack, return the cached from_type() strategy (sharing one object lets the recursion in e.g. is_empty checks terminate) instead of re-resolving it. We reuse the existing _recurse_guard, which also catches references that reach us as a union arg rather than a field.
Also filter the registered-subtype lookup before sorting it, so the expensive repr-based sort only runs on the (usually empty) matching set rather than the whole global type lookup on every resolution.
Fixes #4729.