[krull10]

I've nearly exclusively used Julia since 2017. I don't think this is a perverse use of such functions -- long ago I naturally guessed I could use `cumsum!` on the same input and output and it would correctly overwrite the values (which now gives a similar warning in the documentation). However, when I first used it that way I tested if it did what I expected to verify my assumption.

It is good the documentation is now explicit that the behavior is not guaranteed in this case, but even better would be if aliasing were detected and handled (at least for base Julia arrays, so that the warning would only be needed for non-base types).

Still, the lesson is that when using generic functions one should look at what they expect of their input, and if this isn't documented one should at least test what they are giving thoroughly and not assume it just works. I've always worked this way, and never run into surprises like the types of issues reported in the blog post.

Currently there is no documentation on what properties an input to `sum!` must support in the doc string, so one needs to test its correctness when using it outside of base Julia data types (I haven't checked the broader docs for an interface specification, but if there is one it really should be linked in the docstring).

[leephillips]

But your use of cumsum!() seems natural; I can see using it that way, and might have done so myself. The use of sum!() under discussion seems weird, though.