Add support.built_with_c_assertions()

Fix also a compiler warning on Windows:

    Objects\object.c(3629,39): warning C4244: 'function': conversion
    from 'long' to 'un signed char', possible loss of data

Author: vstinner

Lib/test/pythoninfo.py

@@ -611,14 +611,6 @@ def collect_sysconfig(info_add):
         value = normalize_text(value)
         info_add('sysconfig[%s]' % name, value)
 
-    PY_CFLAGS = sysconfig.get_config_var('PY_CFLAGS')
-    NDEBUG = (PY_CFLAGS and '-DNDEBUG' in PY_CFLAGS)
-    if NDEBUG:
-        text = 'ignore assertions (macro defined)'
-    else:
-        text= 'build assertions (macro not defined)'
-    info_add('build.NDEBUG',text)
-
     for name in (
         'WITH_DOC_STRINGS',
         'WITH_DTRACE',
@@ -844,6 +836,8 @@ def collect_support(info_add):
              support.check_sanitizer(memory=True))
     info_add('support.check_sanitizer(ub=True)',
              support.check_sanitizer(ub=True))
+    info_add('support.built_with_c_assertions',
+             support.built_with_c_assertions())
 
 
 def collect_support_os_helper(info_add):

Lib/test/support/__init__.py

@@ -3492,3 +3492,14 @@ def check_immutable_type(testcase, type):
     else:
         flags = type_getflags(type)
         testcase.assertTrue(flags & Py_TPFLAGS_IMMUTABLETYPE)
+
+
+def built_with_c_assertions():
+    # Check if Python was built in debug mode or using --with-assertions
+    if MS_WINDOWS:
+        Py_DEBUG = hasattr(sys, 'gettotalrefcount')
+        return Py_DEBUG
+
+    PY_CFLAGS = sysconfig.get_config_var('PY_CFLAGS')
+    NDEBUG = (PY_CFLAGS and '-DNDEBUG' in PY_CFLAGS)
+    return (not NDEBUG)

Lib/test/test_capi/test_misc.py

@@ -3094,6 +3094,9 @@ def test_ceval_decref(self):
         self.assertEqual(lines.count("DESTROY list"), 2)
 
 
+# _Py_CheckSingletons() is only built if the NDEBUG macro is not defined
+@unittest.skipUnless(support.built_with_c_assertions(),
+                     'Python built without C assertions')
 class TestCheckSingleton(unittest.TestCase):
     # Test _Py_CheckSingletons() which is called by gc.collect()
     #

Objects/object.c

@@ -3626,7 +3626,7 @@ _Py_CheckSingletons(void)
 
     for (ival=0; ival <= 255; ival++) {
         obj = (PyObject*)&_Py_SINGLETON(bytes_characters)[ival];
-        check_singleton_bytes(obj, 1, ival);
+        check_singleton_bytes(obj, 1, (unsigned char)ival);
     }
 
     // Empty Unicode string ('')