[libcxx-commits] [PATCH] D103335: [libcxx][ranges] Adds `common_iterator`.

[Zoe Carver via Phabricator via libcxx-commits]

zoecarver added inline comments.


================
Comment at: libcxx/include/__iterator/common_iterator.h:169
+
+    if (__x_index == __y_index)
+      return true;
----------------
tcanens wrote:
> zoecarver wrote:
> > ldionne wrote:
> > > How is this behavior really what we want? IIUC, that means that if you compare two `common_iterators` that both contain different iterators (say `It1` and `It2`) that are not comparable to each other, the `common_iterator`s will compare equal to each other even if `It1` and `It2` are "pointing" to entirely different elements. Am I misunderstanding something here, or that's an incredibly subtle (and broken) behavior to have at runtime? @tcanens Can you shed some light on this?
> > > 
> > > In all cases, we need a test where we exercise that.
> > We should assert these aren't both zero, even though it's non-conforming, people will thank us.
> This is meant for C++20 input iterators that aren't comparable with each other. Because incrementing an input iterator invalidates all copies, you can't have two valid iterators pointing to different elements in the same range. 
> 
> > We should assert these aren't both zero, even though it's non-conforming, people will thank us.
> 
> Please don't. `i == i` had better work.
> This is meant for C++20 input iterators that aren't comparable with each other. Because incrementing an input iterator invalidates all copies, you can't have two valid iterators pointing to different elements in the same range.

Shouldn't this assert (or require) that these are input iterators then?

> Please don't. i == i had better work.

But this is not i == i, it's x == y. What you're saying makes sense for input iterators, but what about forward iterators, or even contiguous iterators? 

(Thanks for all the help with interpreting the standard's wording here, by the way.)


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  https://reviews.llvm.org/D103335/new/

https://reviews.llvm.org/D103335