Brandt found this out while testing his implementation of the `match` statement. When a list or tuple are followed by a colon without an annotation, the old parser used to say "invalid syntax", while the new parser considers this an annotation and outputs something along the lines of "only single target (not tuple) can be annotated". For example:

➜  cpython git:(master) ./python.exe
Python 3.10.0a0 (heads/master:06a40d7359, Jun 26 2020, 01:33:34)
[Clang 11.0.3 (clang-1103.0.32.62)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> (a, b):
  File "<stdin>", line 1
    (a, b):
    ^
SyntaxError: only single target (not tuple) can be annotated
>>> [a, b]:
  File "<stdin>", line 1
    [a, b]:
    ^
SyntaxError: only single target (not list) can be annotated
>>> a,:
  File "<stdin>", line 1
    a,:
    ^
SyntaxError: only single target (not tuple) can be annotated

The behavior of the old parser seems more logical.

New changeset 4b85e60601489f9ee9dd2909e28d89a31566887c by Lysandros Nikolaou in branch 'master':
bpo-41119: Output correct error message for list/tuple followed by colon (GH-21160)
https://github.com/python/cpython/commit/4b85e60601489f9ee9dd2909e28d89a31566887c

New changeset d01a3e76ee0519cba3ccdb5de47b25ec2ed7bee4 by Lysandros Nikolaou in branch '3.9':
[3.9] bpo-41119: Output correct error message for list/tuple followed by colon (GH-21160) (GH-21172)
https://github.com/python/cpython/commit/d01a3e76ee0519cba3ccdb5de47b25ec2ed7bee4