DEP: deprecate ndarray.conjugate's no-op fall through for non-numeric types

[longjon]

While np.conjugate errors on non-numeric types, ndarray.conjugate silently returns its argument, even though both are documented as being equivalent. This is due to a fall-through condition in ndarray.conjugate which is useful for real data (e.g., https://github.com/numpy/numpy/blob/master/numpy/lib/function_base.py#L3087) but not meaningful for non-numeric data.

This PR deprecates the fall-through for non-numeric dtypes, bringing ndarray.conjugate closer to np.conjugate by favoring its fail-fast behavior.

This was originally part of #9003.

[eric-wieser]

Excellent, this exhibits the same UI bug

[eric-wieser]

Nitpick: should have another blank line before

[longjon]

Fixed; was copying the style immediately above which is single-spaced!

[eric-wieser]

Mirroring my approval from the other PR. Would be good to have a second opinion on the deprecation before merging, but otherwise I'd hope this can make 1.13

[longjon]

Sounds good! The deprecation and bugfix commits are actually independent now; would it be helpful if I split them into separate PRs?

[eric-wieser]

Yeah, I guess if you did that I could at least pull the trigger on the bugfix without any further opinions

[homu]

☔ The latest upstream changes (presumably #8939) made this pull request unmergeable. Please resolve the merge conflicts.

[charris]

I copied over the description from #9003.

[charris]

Needs rebase on account of the release notes.

[charris]

Yikes,

In [1]: a = ones(3)

In [2]: b = a.conjugate()

In [3]: b[...] = 0

In [4]: b
Out[4]: array([ 0.,  0.,  0.])

In [5]: a
Out[5]: array([ 0.,  0.,  0.])

That definitely looks like an unexpected result that needs a future warning and fix.

Although I see that this does not affect that behavior.

[eric-wieser]

Although I see that this does not affect that behavior.

The original patch did, but this came with a hefty performance cost for np.cov.

It would be nice if np.conjugate(x, writable=False) were a thing, which allows short-circuiting of no-op operators, and could be used in np.cov. (#9069)

[longjon]

I've rebased and rewritten the description, since the old one didn't make a lot of sense here.

(Keeping in mind that this PR is about something else:)
@charris, right, this is the original behavior, it's currently relied upon as @eric-wieser notes, and I'm not sure that it's not the "right" behavior. E.g., transpose also returns a view, and one often wants to compute the Hermitian conjugate, .conj().T, which one would generally like to still be a view on real data. ndarray.real also returns a view on real data (though perhaps its property-ness is more suggestive of that).