gh-118404: Fix inspect.signature() for non-comparable callables (GH-118405)

Lib/inspect.py

@@ -2179,8 +2179,10 @@ def _signature_is_builtin(obj):
             ismethoddescriptor(obj) or
             isinstance(obj, _NonUserDefinedCallables) or
             # Can't test 'isinstance(type)' here, as it would
-            # also be True for regular python classes
-            obj in (type, object))
+            # also be True for regular python classes.
+            # Can't use the `in` operator here, as it would
+            # invoke the custom __eq__ method.
+            obj is type or obj is object)
 
 
 def _signature_is_functionlike(obj):

Lib/test/test_inspect/test_inspect.py

@@ -4690,6 +4690,16 @@ class TestSignatureObject(unittest.TestCase):
 
         self.assertEqual(inspect.signature(D2), inspect.signature(D1))
 
+    def test_signature_on_non_comparable(self):
+        class NoncomparableCallable:
+            def __call__(self, a):
+                pass
+            def __eq__(self, other):
+                1/0
+        self.assertEqual(self.signature(NoncomparableCallable()),
+                         ((('a', ..., ..., 'positional_or_keyword'),),
+                          ...))
+
 
 class TestParameterObject(unittest.TestCase):
     def test_signature_parameter_kinds(self):

Misc/NEWS.d/next/Library/2024-04-29-22-11-54.gh-issue-118404.GYfMaD.rst

@@ -0,0 +1 @@
+Fix :func:`inspect.signature` for non-comparable callables.