Update class_ir.py

Author: BobTheBuidler

mypyc/ir/class_ir.py

@@ -341,7 +341,7 @@ def subclasses(self) -> set[ClassIR] | None:
         Return None if it is impossible to identify all subclasses, for example
         because we are performing separate compilation.
         """
-        if self.children is None or self.allow_interpreted_subclasses:
+        if not self.is_ext_class or self.allow_interpreted_subclasses or self.children is None:
             return None
         result = set(self.children)
         for child in self.children: