[3.11] gh-106368: Increase Argument Clinic test coverage (#106369) (#106374)

Add tests for 'self' and 'defining_class' converter requirements.
(cherry picked from commit 7f4c812)

Author: erlend-aasland

Lib/test/test_clinic.py

@@ -812,6 +812,63 @@ def test_other_bizarre_things_in_annotations_fail(self):
         )
         self.assertEqual(s, expected_failure_message)
 
+    def test_self_param_placement(self):
+        expected_error_msg = (
+            "Error on line 0:\n"
+            "A 'self' parameter, if specified, must be the very first thing "
+            "in the parameter block.\n"
+        )
+        block = """
+            module foo
+            foo.func
+                a: int
+                self: self(type="PyObject *")
+        """
+        out = self.parse_function_should_fail(block)
+        self.assertEqual(out, expected_error_msg)
+
+    def test_self_param_cannot_be_optional(self):
+        expected_error_msg = (
+            "Error on line 0:\n"
+            "A 'self' parameter cannot be marked optional.\n"
+        )
+        block = """
+            module foo
+            foo.func
+                self: self(type="PyObject *") = None
+        """
+        out = self.parse_function_should_fail(block)
+        self.assertEqual(out, expected_error_msg)
+
+    def test_defining_class_param_placement(self):
+        expected_error_msg = (
+            "Error on line 0:\n"
+            "A 'defining_class' parameter, if specified, must either be the "
+            "first thing in the parameter block, or come just after 'self'.\n"
+        )
+        block = """
+            module foo
+            foo.func
+                self: self(type="PyObject *")
+                a: int
+                cls: defining_class
+        """
+        out = self.parse_function_should_fail(block)
+        self.assertEqual(out, expected_error_msg)
+
+    def test_defining_class_param_cannot_be_optional(self):
+        expected_error_msg = (
+            "Error on line 0:\n"
+            "A 'defining_class' parameter cannot be marked optional.\n"
+        )
+        block = """
+            module foo
+            foo.func
+                cls: defining_class(type="PyObject *") = None
+        """
+        out = self.parse_function_should_fail(block)
+        self.assertEqual(out, expected_error_msg)
+
     def parse(self, text):
         c = FakeClinic()
         parser = DSLParser(c)