inherit maxchar of field value where needed (closes #14648)

Lib/test/test_unicode.py

@@ -1056,6 +1056,8 @@ class UnicodeTest(string_tests.CommonTest,
         self.assertEqual('%f' % INF, 'inf')
         self.assertEqual('%F' % INF, 'INF')
 
+        self.assertEqual(format("\u0410\u0411\u0412", "s"), "АБВ")
+
     def test_startswith_endswith_errors(self):
         for meth in ('foo'.startswith, 'foo'.endswith):
             with self.assertRaises(TypeError) as cm:

Python/formatter_unicode.c

@@ -747,6 +747,9 @@ format_string_internal(PyObject *value, const InternalFormatSpec *format)
         len = format->precision;
     }
 
+    if (len)
+        maxchar = PyUnicode_MAX_CHAR_VALUE(value);
+
     calc_padding(len, format->width, format->align, &lpad, &rpad, &total);
 
     if (lpad != 0 || rpad != 0)