[libcxx][ranges][nfc] Factor out common logic in the copy algorithms.

[philnik]

Summary

Factors out common logic from std::copy, std::copy_backward, std::copy_if, and std::copy_n into __copy_*_impl. Also adds __in_out_result which is the return type for the impl function. This will make adding support for the ranges overloads super easy.

This is an example of how this will be done for all algorithms. Once this lands, I'll update all the algorithms, so let's get this review right.

[cjdb]

We're going to need to hold off on this patch until we determine what's breaking Fuchsia.

[cjdb]

This is blocked by https://bugs.llvm.org/show_bug.cgi?id=50592.

[• Quuxplusone]

libcxx/include/__algorithm/copy.h

83

_VSTD:: — but also I worry a little bit about the codegen here. As you've written it so far, ranges::copy could just be

return __in_out_result{__last, _VSTD::copy(__first, __last, __result)};

and save a lot of modifications. Which is obviously not C++20-compliant. So for C++20, we're going to have to write a new ranges::copy algorithm that returns the computed iterator, generated by incrementing __first, which ended up comparing equal to _Sentinel __last. That's a much different-looking algorithm from what we've got today for std::copy, and might not even be able to share any code with it.

I strongly strongly strongly ask that you continue working on this PR until it succeeds at implementing the C++20 ranges::copy semantics, and then see if you still think this is a good direction. I strongly strongly strongly oppose merging this PR in its current state.

libcxx/test/std/algorithms/robust_against_adl.pass.cpp

15

This is testing that your library code is robust against ADL.
I assume it's failing at the moment because you didn't ADL-proof your call to __copy above.
Remember, every time you call a function in namespace _VSTD that would be subject to ADL, you need to qualify it. (The one common exception we make is for declval, because it takes no arguments.)

[philnik]

The ranges algorithms have been implemented and the code for copy etc. have been massively changed, so I'll abandon this patch.