gh-112345: Let failed protocol subclasscheck show non-method members

Lib/test/test_typing.py

@@ -4091,6 +4091,25 @@ def method(self) -> None: ...
         self.assertIsInstance(Foo(), ProtocolWithMixedMembers)
         self.assertNotIsInstance(42, ProtocolWithMixedMembers)
 
+    def test_protocol_issubclass_error_message(self):
+        class Vec2D(Protocol):
+            x: float
+            y: float
+
+            def square_norm(self) -> float:
+                return self.x ** 2 + self.y ** 2
+
+        assert Vec2D.__protocol_attrs__ == {'x', 'y', 'square_norm'}
+        expected_error_message = (
+            "Protocols with non-method members don't support issubclass()."
+            " Non-method members: {'x', 'y'}."
+        )
+
+        try:
+            issubclass(int, Vec2D)
+        except TypeError as exc:
+            self.assertEqual(str(exc), expected_error_message)
+
 
 class GenericTests(BaseTestCase):