This occurred when attempting to decode invalid UTF-8 bytes using "errors='replace'", then attempting to lowercase the produced unicode string.

This was also tested in python 2.7, but it doesn't occur there.

Code to reproduce:

x = b'\xe2\xb3\x99\xb3\xd1\x9f\xe0vjGd|\x12\xf2\x84\xac\xae&$\xa4\xae+\xa4sbtf$&fG\xfb\xe6?.\xe2sbv\x14\xcb\x89\x98\xda\xd9\x99\xda\xb9d9\x1bY\x99\xb7\xb3\x1b9\xa2y*B\xa3\xba\xefj&g\xe2\x92Et\x85~\xbf\x8a\xe3\x919\x8bvc\xfb#$$.\xber6D&b.#4\xa4.\x13RtI\x10\xed\x9c\xd0\x98\xb8\x18\x91\x99\\\nC\x13\x8dV\xccL\xf4\x89\x9c\x90'

x = x.decode('utf-8', errors='replace')

x.lower()


Output:
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
SystemError: invalid maximum character passed to PyUnicode_New

Minimal example:

>>> '\U00010000\U00100000'.lower()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
SystemError: invalid maximum character passed to PyUnicode_New

It happens due to use fast MAX_MAXCHAR() which can produce maxchar out of range (0x10000 | 0x100000 > MAX_UNICODE).

>>> a = chr(0x84b2e)+chr(0x109710)
>>> a.lower()
SystemError: invalid maximum character passed to PyUnicode_New

The MAX_MAXCHAR() macro only works for 'maxchar' values, like 0xff, 0xffff...  in do_upper_or_lower() it's used with arbitrary UCS4 values.

New changeset 89b106d298a9 by Benjamin Peterson in branch '3.3':
remove MAX_MAXCHAR because it's unsafe for computing maximum codepoitn value (see #18183)
http://hg.python.org/cpython/rev/89b106d298a9

New changeset 668aba845fb2 by Benjamin Peterson in branch 'default':
merge 3.3 (#18183)
http://hg.python.org/cpython/rev/668aba845fb2

I simply removed the MAX_MAXCHAR micro-optimization, since it seems fairly unsafe. Interested parties can restore it safely if they wish.

Oops, my MAX_MAXCHAR macro was too optimized :-) (the result is incorrect)

It shows us that the test suite does not have enough test on non-BMP characters.

Here are additional tests for this issue.

+        '\U00010000\U00100000'.lower()

Why not checking the result of these calls?

The result is trivial. Is not checking the result distract an attention from the main issue?

New changeset b11507395ce4 by Serhiy Storchaka in branch '3.3':
Add tests for issue #18183.
http://hg.python.org/cpython/rev/b11507395ce4

New changeset 17c9f1627baf by Serhiy Storchaka in branch 'default':
Add tests for issue #18183.
http://hg.python.org/cpython/rev/17c9f1627baf