Generic objects, when realized to strings, raise TypeError on concatenation.

[pschanely]

CrossHair's counterexample does not reproduce in this example:

def arithmetic_op(a, b):
    """
    post: True
    """
    result = a + b
    return result

# actual: /tmp/main.py:5: error: TypeError:  when calling arithmetic_op('', '')
# expect: counterexamples like arithmetic_op('', 0)

See also the original report in #234