pythongh-122888: Fix crash on certain calls to str()

Fixes python#122888

Lib/test/test_str.py

@@ -1747,6 +1747,16 @@ def test_constructor_keyword_args(self):
         self.assertEqual(str(b'foo', errors='strict'), 'foo')  # not "b'foo'"
         self.assertEqual(str(object=b'foo', errors='strict'), 'foo')
 
+    def test_constructor_errors(self):
+        with self.assertRaises(TypeError):
+            str(b"x", b"ascii")
+        with self.assertRaises(TypeError):
+            str(b"x", encoding=b"ascii")
+        with self.assertRaises(TypeError):
+            str(b"x", "ascii", b"strict")
+        with self.assertRaises(TypeError):
+            str(b"x", "ascii", errors=b"strict")
+
     def test_constructor_defaults(self):
         """Check the constructor argument defaults."""
         # The object argument defaults to '' or b''.

Misc/NEWS.d/next/Core_and_Builtins/2024-08-10-12-44-03.gh-issue-122888.TUyu9r.rst

@@ -0,0 +1,2 @@
+Fix crash on certain calls to ``str()`` with positional arguments of the
+wrong type. Patch by Jelle Zijlstra.

Objects/unicodeobject.c

@@ -15121,7 +15121,16 @@ unicode_vectorcall(PyObject *type, PyObject *const *args,
         return PyObject_Str(object);
     }
     const char *encoding = arg_as_utf8(args[1], "encoding");
-    const char *errors = (nargs == 3) ? arg_as_utf8(args[2], "errors") : NULL;
+    if (encoding == NULL) {
+        return NULL;
+    }
+    const char *errors = NULL;
+    if (nargs == 3) {
+        errors = arg_as_utf8(args[2], "errors");
+        if (errors == NULL) {
+            return NULL;
+        }
+    }
     return PyUnicode_FromEncodedObject(object, encoding, errors);
 }