issue a warning when Items and Collector form a diamond

addresses pytest-dev#8435

changelog/8447.deprecation.rst

@@ -0,0 +1,4 @@
+Warn when inheritance is used to bring Collectors into Items,
+it was never sanely supported and triggers hard to debug errors.
+
+Instead of inheritance composition should be used.

src/_pytest/nodes.py

@@ -34,6 +34,7 @@
 from _pytest.pathlib import absolutepath
 from _pytest.pathlib import commonpath
 from _pytest.store import Store
+from _pytest.warning_types import PytestWarning
 
 if TYPE_CHECKING:
     # Imported here due to circular import.
@@ -610,6 +611,19 @@ class Item(Node):
 
     nextitem = None
 
+    def __init_subclass__(cls):
+        problems = ", ".join(
+            base.__name__ for base in cls.__bases__ if issubclass(base, Collector)
+        )
+        if problems:
+            warnings.warn(
+                f"{cls.__name__} is a Item subclass and should not be a collector.\n"
+                f"however its bases {problems} are collectors\n"
+                "please split the collection and the items into 2 node types\n"
+                "TODO: doc link",
+                PytestWarning,
+            )
+
     def __init__(
         self,
         name,

testing/test_nodes.py

@@ -39,6 +39,16 @@ def test_node_from_parent_disallowed_arguments() -> None:
         nodes.Node.from_parent(None, config=None)  # type: ignore[arg-type]
 
 
+def test_subclassing_node_with_item_warns() -> None:
+
+    with pytest.warns(
+        PytestWarning, match="SoWrong is a Item subclass and should not be a collector"
+    ):
+
+        class SoWrong(nodes.Item, nodes.File):
+            pass
+
+
 @pytest.mark.parametrize(
     "warn_type, msg", [(DeprecationWarning, "deprecated"), (PytestWarning, "pytest")]
 )