Emit warning for broken object

changelog/5404.bugfix.rst

@@ -0,0 +1,2 @@
+Emit a warning when attempting to unwrap a broken object raises an exception,
+for easier debugging (`#5080 <https://github.com/pytest-dev/pytest/issues/5080>`__).

src/_pytest/doctest.py

@@ -3,6 +3,7 @@
 import platform
 import sys
 import traceback
+import warnings
 from contextlib import contextmanager
 
 import pytest
@@ -12,6 +13,7 @@
 from _pytest.compat import safe_getattr
 from _pytest.fixtures import FixtureRequest
 from _pytest.outcomes import Skipped
+from _pytest.warning_types import PytestWarning
 
 DOCTEST_REPORT_CHOICE_NONE = "none"
 DOCTEST_REPORT_CHOICE_CDIFF = "cdiff"
@@ -368,10 +370,21 @@ def _patch_unwrap_mock_aware():
     else:
 
         def _mock_aware_unwrap(obj, stop=None):
-            if stop is None:
-                return real_unwrap(obj, stop=_is_mocked)
-            else:
-                return real_unwrap(obj, stop=lambda obj: _is_mocked(obj) or stop(obj))
+            try:
+                if stop is None or stop is _is_mocked:
+                    return real_unwrap(obj, stop=_is_mocked)
+                else:
+                    return real_unwrap(
+                        obj, stop=lambda obj: _is_mocked(obj) or stop(obj)
+                    )
+            except Exception as e:
+                warnings.warn(
+                    "Got %r when unwrapping %r.  This is usually caused "
+                    "by a violation of Python's object protocol; see e.g. "
+                    "https://github.com/pytest-dev/pytest/issues/5080",
+                    PytestWarning,
+                )
+                raise
 
         inspect.unwrap = _mock_aware_unwrap
         try: