If either of these lines are added to test_annotations() in Lib/test/test_future.py, a SystemError will be raised:

        eq("(x:=10)")
        eq("f'{(x:=10):=10}'")

Traceback (most recent call last):
  File "/Users/eric/local/python/cpython/Lib/test/test_future.py", line 258, in test_annotations
    eq("(x:=10)")
  File "/Users/eric/local/python/cpython/Lib/test/test_future.py", line 134, in assertAnnotationEqual
    actual = self.getActual(annotation)
  File "/Users/eric/local/python/cpython/Lib/test/test_future.py", line 121, in getActual
    exec(self.template.format(ann=annotation), {}, scope)
SystemError: unknown expression kind