Suggestion: Ignore B006 if the variable in question is immediately copied

I would argue that these two functions:

def foo(x=[]):
    x = list(x)
    x.reverse()
    return x

def bar(x={}):
    x = dict(x)
    x.pop('k', None)
    return x

are far more readable than the way B006 forces me to write them

def foo_bad(x=None):
    if x is None:
        x = []
    x.reverse()
    return x

def bar_bad(x=None):
    if x is None:
        x = {}
    x.pop('k', None)
    return x

Furthermore, the rewrite encouraged by B006 now actually leads to a bug - x = [1]; foo_bad(x) leads to x being mutated.

Would it be possible/sensible to add an exemption to B006 along the lines of "if the only use of the mutable default is to pass into a non-mutating function call, then emit no warning"?

Review B006's current handling of mutable defaults and the examples in the issue. Determine whether an immediately copied default can be distinguished safely from later mutation, then define tests covering foo, bar, and the existing B006 behavior.