gh-154936: Fix JSON control character error position (GH-154941)

Report the index of the invalid literal control character instead of
the following index in the pure Python decoder.
(cherry picked from commit adf836e)

Co-authored-by: yangbaechu <45089264+yangbaechu@users.noreply.github.com>

Author: yangbaechu

Lib/json/decoder.py

@@ -97,7 +97,7 @@ def py_scanstring(s, end, strict=True,
             if strict:
                 #msg = "Invalid control character %r at" % (terminator,)
                 msg = "Invalid control character {0!r} at".format(terminator)
-                raise JSONDecodeError(msg, s, end)
+                raise JSONDecodeError(msg, s, end - 1)
             else:
                 _append(terminator)
                 continue

Lib/test/test_json/test_unicode.py

@@ -44,7 +44,13 @@ def test_ascii_non_printable_decode(self):
                          '\b\t\n\f\r')
         s = ''.join(map(chr, range(32)))
         for c in s:
-            self.assertRaises(self.JSONDecodeError, self.loads, f'"{c}"')
+            with self.subTest(control_character=ord(c)):
+                with self.assertRaises(self.JSONDecodeError) as cm:
+                    self.loads(f'"a{c}b"')
+                error = cm.exception
+                self.assertEqual(error.pos, 2)
+                self.assertEqual(error.lineno, 1)
+                self.assertEqual(error.colno, 3)
         self.assertEqual(self.loads(f'"{s}"', strict=False), s)
         self.assertEqual(self.loads('"\x7f"'), '\x7f')
 

Misc/NEWS.d/next/Library/2026-07-30-23-29-56.gh-issue-154936.a90443.rst

@@ -0,0 +1,2 @@
+Fix the pure Python :mod:`json` decoder to report the correct position for
+invalid literal control characters in JSON strings.