While the fix works as advertised, it breaks a similar existing test case:

======================================================================
ERROR: test_positional_args (ctypes.test.test_structures.StructureTestCase)
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Traceback (most recent call last):
  File "/Users/jeff/sandbox/src/python3.7/cpython/Lib/ctypes/test/test_structures.py", line 390, in test_positional_args
    z = Z(1, 2, 3, 4, 5, 6)
IndexError: list index out of range

----------------------------------------------------------------------

Below is the relevant test code:

        class W(Structure):
            _fields_ = [("a", c_int), ("b", c_int)]
        class X(W):
            _fields_ = [("c", c_int)]
        class Y(X):
            pass
        class Z(Y):
            _fields_ = [("d", c_int), ("e", c_int), ("f", c_int)]

        z = Z(1, 2, 3, 4, 5, 6)

It similar but slightly different from the provided test case:

        libm = CDLL(find_library('m'))

        class Base(Structure):
            _fields_ = [('y', c_double),
                        ('x', c_double)]

        class Mid(Base):
            pass

        Mid._fields_ = []

        class Vector(Mid): pass

        libm.atan2.argtypes = [Vector]
        libm.atan2.restype = c_double

        arg = Vector(y=0.0, x=-1.0)

I will do some more digging.