ENH: Automatic copy on overlap

Tracking issue as a continuation to gh-6166

• add copy-on-overlap to nditer, as optional flag gh-8043
• use it for ufuncs gh-8043
• check whether copy-on-overlap is respected in indexed assignment
• respect it for dot() gh-8539
• check other places in numpy that may need checking

Some initial code, https://github.com/numpy/numpy/compare/master...pv:copy-overlap

[ahaldane]

These seem OK:

• copyto is OK since it goes through PyArray_AssignArray, which has an overlap check.
• _ureduce code (eg, median/percentile) looks OK since it is pure python and delegates to ufuncs
• around is ok because it delegates to ufuncs

Problematic:

• np.take There is no overlap detection. Demonstration: np.take(x, [1,2,3], out=x[2:5], mode='wrap') -> array([1., 1., 1.]).
• np.choose no overlap detection. Demonstration: np.choose([0,0,0], [x[:3], x[:3], x[:3]], out=x[1:4], mode='wrap') -> array([1, 1, 1])
• I didn’t carefully check, but I expect put and others in that file are the same.

Note that for both of these, with mode='raise', which is the default, a copy of out is always made to avoid clobbering out if an error occurs during processing, so there will be no risk of overlap. (incidentally, this seems unexpectedly inefficient for most people!).

So if we think those last ones should be fixed, we should keep this open.

[ahaldane]

Reviewing this as part of my 10 random issues, as requested in a 3/20/19 request.

I am 95% sure this is done and just forgotten to be closed, because in 2019 I have seen these overlap checks in the assignment code, including in code involving assignment to slices of an array.

I’m going to go ahead and close this, please reopen if that’s wrong.