Dependency on copy isn't detected

[frycast]

As expected, the below assigns 5 to v[1].

# ╔═╡ cc0c1db0-f4dc-11ea-2f85-dd812a3de9b1
v = [1 2 3]

# ╔═╡ d2905c00-f4dc-11ea-367a-a5073a34c7db
begin
w = v
w[1] = 5
v
end

But if now we change the second cell above, to make a copy of v:

# ╔═╡ d2905c00-f4dc-11ea-367a-a5073a34c7db
begin
w = copy(v)
w[2] = 5
v
end

Then my expectation is that when I run the updated second cell, the first cell should automatically be rerun so that the output of the second cell reflects what it would be if the whole notebook was run from scratch. Instead, after updating and running the second cell, I get

1×3 Array{Int64,2}:
 5  5  3

[fonsp]

This is very very difficult. Even the simple task of knowing which functions mutate which arguments is hard:
#316 (comment)
@lungben