From 4ef8c1a4ccf41cfa0c9303c3d248cee08ab735af Mon Sep 17 00:00:00 2001
From: Ronny Pfannschmidt <opensource@ronnypfannschmidt.de>
Date: Sun, 14 Mar 2021 23:01:58 +0100
Subject: [PATCH] issue a warning when Items and Collector form a diamond

addresses #8435
---
 changelog/8447.deprecation.rst |  4 ++++
 src/_pytest/nodes.py           | 14 ++++++++++++++
 testing/test_nodes.py          | 10 ++++++++++
 3 files changed, 28 insertions(+)
 create mode 100644 changelog/8447.deprecation.rst

diff --git a/changelog/8447.deprecation.rst b/changelog/8447.deprecation.rst
new file mode 100644
index 00000000000..098ca89be75
--- /dev/null
+++ b/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.
diff --git a/src/_pytest/nodes.py b/src/_pytest/nodes.py
index 99b7eb1a61c..f5774a728e2 100644
--- a/src/_pytest/nodes.py
+++ b/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.
@@ -600,6 +601,19 @@ 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,
diff --git a/testing/test_nodes.py b/testing/test_nodes.py
index fbdbce3950d..de6c4595c96 100644
--- a/testing/test_nodes.py
+++ b/testing/test_nodes.py
@@ -37,6 +37,16 @@ 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")]
 )