Using the collecting variable of iterable unpacking causes "used before definition" error

[brettcannon]

microsoft/vscode-python#3543

a, *b, c = range(5)
print(b)

b ends up being reported as used before definition.

[jakebailey]

My guess (without looking too hard) is that we aren't handling this style of unpack properly (see #412, #403), and we're considering it to be a use like it's a mul or a args/kwargs unpack.

[md2perpe]

I got the same error in a loop construction like

for first, *rest in data:
    use(first, rest)

Here, rest gets reported as used before definition on the use line.