[stubtest] Respect @final runtime decorator, enforce it in stubs

mypy/stubtest.py

@@ -419,6 +419,16 @@ class SubClass(runtime):  # type: ignore[misc]
         # Examples: ctypes.Array, ctypes._SimpleCData
         pass
 
+    # Runtime class might be annotated with `@final`:
+    if getattr(runtime, "__final__", False) and not stub.is_final:
+        yield Error(
+            object_path,
+            "has `__final__` attribute, but isn't marked with @final in the stub",
+            stub,
+            runtime,
+            stub_desc=repr(stub),
+        )
+
 
 def _verify_metaclass(
     stub: nodes.TypeInfo, runtime: type[Any], object_path: list[str]

mypy/test/teststubtest.py

@@ -1139,6 +1139,45 @@ def test_not_subclassable(self) -> Iterator[Case]:
             error="CannotBeSubclassed",
         )
 
+    @collect_cases
+    def test_has_runtime_final_decorator(self) -> Iterator[Case]:
+        yield Case(
+            stub="from typing_extensions import final",
+            runtime="from typing_extensions import final",
+            error=None,
+        )
+        yield Case(
+            stub="""
+            @final
+            class A: ...
+            """,
+            runtime="""
+            @final
+            class A: ...
+            """,
+            error=None,
+        )
+        yield Case(  # Runtime can miss `@final` decorator
+            stub="""
+            @final
+            class B: ...
+            """,
+            runtime="""
+            class B: ...
+            """,
+            error=None,
+        )
+        yield Case(  # Stub cannot miss `@final` decorator
+            stub="""
+            class C: ...
+            """,
+            runtime="""
+            @final
+            class C: ...
+            """,
+            error="C",
+        )
+
     @collect_cases
     def test_name_mangling(self) -> Iterator[Case]:
         yield Case(