Feature request: kwarg defaults in terms of destructured argument names

[pablosanjose]

We cannot currently combine positional-argument-destructuring and kwarg-with-defaults-in-terms-of-arguments. Namely, this works

f(x; y = x) = (x, y)

but this doesn't

julia> g((x, x´); y = x) = (x,y)
g (generic function with 1 method)

julia> g((2, 2))
ERROR: UndefVarError: x not defined

I have needed this functionality today, and I think it makes sense to provide it. The use case was something like this

function process_filtered_pairs((xs, ys); xmin = minimum(xs), xmax = maximum(xs))
    pairs = [(x, y) for (x, y) in zip(xs, ys) if xmin <= x <= xmax]
    # process pairs
end

[JeffBezanson]

Yes I'm pretty sure this should work. Seems like a lowering pass ordering kind of issue.