stubtest: understand override (#18815)

Fixes #18814

Author: hauntsaninja

mypy/stubtest.py

@@ -858,7 +858,7 @@ def from_overloadedfuncdef(stub: nodes.OverloadedFuncDef) -> Signature[nodes.Arg
 
         all_args: dict[str, list[tuple[nodes.Argument, int]]] = {}
         for func in map(_resolve_funcitem_from_decorator, stub.items):
-            assert func is not None
+            assert func is not None, "Failed to resolve decorated overload"
             args = maybe_strip_cls(stub.name, func.arguments)
             for index, arg in enumerate(args):
                 # For positional-only args, we allow overloads to have different names for the same
@@ -1330,6 +1330,7 @@ def apply_decorator_to_funcitem(
         if (
             decorator.fullname in ("builtins.staticmethod", "abc.abstractmethod")
             or decorator.fullname in mypy.types.OVERLOAD_NAMES
+            or decorator.fullname in mypy.types.OVERRIDE_DECORATOR_NAMES
             or decorator.fullname in mypy.types.FINAL_DECORATOR_NAMES
         ):
             return func