I knew I shouldn't even try them, especially taking into account that I dislike all these novelties. But then CPy stdlib was parsed by pure accident - they seem to start using it heavily.

Each of cases below is a failure:

def f1(a, *, b):
    print(a, b)

f1(1, b=2)

def f2(a, *, b=[]):
    print(a, b)

def f3(a, *, b=1):
    print(a, b)