[mypyc] fix: isinstance_native fast path conflict w/ interp. subclasses [1/1]

mypyc/ir/class_ir.py

@@ -345,6 +345,10 @@ def subclasses(self) -> set[ClassIR] | None:
             return None
         result = set(self.children)
         for child in self.children:
+            if child.allow_interpreted_subclasses:
+                return None
+            if not (child.is_ext_class or child.is_final_class):
+                return None
             if child.children:
                 child_subs = child.subclasses()
                 if child_subs is None:

mypyc/irbuild/ll_builder.py

@@ -695,8 +695,8 @@ def isinstance_native(self, obj: Value, class_ir: ClassIR, line: int) -> Value:
         """Fast isinstance() check for a native class.
 
         If there are three or fewer concrete (non-trait) classes among the class
-        and all its children, use even faster type comparison checks `type(obj)
-        is typ`.
+        and all its children, and none of them allow interpreted subclasses, use
+        even faster type comparison checks `type(obj) is typ`.
         """
         concrete = all_concrete_classes(class_ir)
         if concrete is None or len(concrete) > FAST_ISINSTANCE_MAX_SUBCLASSES + 1: