Fix @inject on classmethod returning wrong cls in subclass hierarchy

docs/wiring.rst

@@ -55,6 +55,11 @@ the wiring works appropriately. This will also contribute to the performance of
 Specifying the ``@inject`` as a first decorator is also crucial for FastAPI, other frameworks
 using decorators similarly, for closures, and for any types of custom decorators with the injections.
 
+.. note:: Note on complex class hierarchies
+
+    If you have complex class hierarchies with ``@inject``, when wiring modules, make sure to include all
+    modules with decorator. Otherwise partially wired classes might violate Liskov Substitution Principle.
+
 FastAPI example:
 
 .. code-block:: python

src/dependency_injector/wiring.py

@@ -625,6 +625,13 @@ def _patch_method(
         method = cls.__dict__[name]
         fn = method.__func__
     else:
+        # For inherited methods, check if the underlying function is already
+        # patched on a parent class. If so, skip to preserve the classmethod
+        # descriptor protocol (cls binding) for subclasses.
+        # See: https://github.com/ets-labs/python-dependency-injector/issues/947
+        underlying = getattr(method, "__func__", None)
+        if underlying is not None and _is_patched(underlying):
+            return
         fn = method
 
     if not _is_patched(fn):

tests/unit/wiring/test_classmethod_inject_inheritance_py36.py

@@ -0,0 +1,48 @@
+"""Test that @inject on classmethods preserves correct cls in subclasses.
+
+See issue for details: https://github.com/ets-labs/python-dependency-injector/issues/947
+"""
+
+import sys
+
+from pytest import fixture
+from typing_extensions import Annotated
+
+from dependency_injector import providers
+from dependency_injector.containers import DeclarativeContainer
+from dependency_injector.wiring import Provide, inject
+
+
+class Container(DeclarativeContainer):
+    singleton = providers.Singleton(lambda: object())
+
+
+class Base:
+    @classmethod
+    @inject
+    def injected_factory(cls, singleton: Annotated[object, Provide["singleton"]]):
+        return cls, singleton
+
+
+class Sub1(Base):
+    pass
+
+
+class Sub2(Sub1):
+    pass
+
+
+@fixture
+def container():
+    container = Container()
+    container.wire(modules=[sys.modules[__name__]])
+    yield container
+    container.unwire()
+
+
+def test_base_injected_classmethod(container):
+    sentinel = container.singleton()
+
+    for cls in [Sub2, Sub1, Base]:
+        result = cls.injected_factory()
+        assert result == (cls, sentinel)