gh-106078: Convert _decimal types to heap types

Lib/test/test_decimal.py

@@ -34,7 +34,8 @@
 import locale
 from test.support import (run_unittest, run_doctest, is_resource_enabled,
                           requires_IEEE_754, requires_docstrings,
-                          requires_legacy_unicode_capi, check_sanitizer)
+                          requires_legacy_unicode_capi, check_sanitizer,
+                          check_disallow_instantiation)
 from test.support import (TestFailed,
                           run_with_locale, cpython_only,
                           darwin_malloc_err_warning, is_emscripten)
@@ -5661,6 +5662,24 @@ def test_maxcontext_exact_arith(self):
             self.assertEqual(Decimal(4) / 2, 2)
             self.assertEqual(Decimal(400) ** -1, Decimal('0.0025'))
 
+    def test_immutable_types(self):
+        decimal = C
+        siganldict_type = type(decimal.Context().flags)
+        self.dataset = (
+            siganldict_type.__bases__[0],  # SignalDictMixin type
+            type(decimal.localcontext()),  # ContextManager type
+            decimal.Decimal,
+            decimal.Context,
+        )
+        for tp in self.dataset:
+            with self.subTest(tp=tp):
+                with self.assertRaisesRegex(TypeError, "immutable"):
+                    tp.foo = 1
+
+    def test_disallow_instantiation(self):
+        decimal = C
+        ctxmanager_type = type(decimal.localcontext())
+        check_disallow_instantiation(self, ctxmanager_type)
 
 @requires_docstrings
 @unittest.skipUnless(C, "test requires C version")