[libcxx-commits] [PATCH] D128864: [libc++] Fix algorihtms which use reverse_iterator

[Hui via Phabricator via libcxx-commits]

huixie90 added inline comments.


================
Comment at: libcxx/include/__iterator/reverse_iterator.h:365-371
+  _LIBCPP_HIDE_FROM_ABI constexpr pointer operator->() const {
+    if constexpr (is_pointer_v<_Iter>) {
+      return std::prev(__iter_);
+    } else {
+      return std::prev(__iter_).operator->();
+    }
+  }
----------------
philnik wrote:
> huixie90 wrote:
> > philnik wrote:
> > > huixie90 wrote:
> > > > could you please clarify why SFIANE is removed and how do you prevent error on instantiating the class template with an `_Iter` that does not support `->`? (assuming all non-template member functions are generated when class template is instantiated)
> > > The new SFINAE in C++20 is the whole reason this PR exists at all. That's the part that breaks the classic algorithms. Regarding `operator->`: that has to exist for the classic algorithms. See https://en.cppreference.com/w/cpp/named_req/InputIterator.
> > Then this class is incorrectly constrained (the static assert bit) the input `_Iter` has to model the c++17 bidi iterator, because c++20 bidi iterator wouldn’t work. It may not have `->` , and it may have prvalue `reference`, which is not allowed in c++ 17 bidi
> > 
> > So the input `Iter` has to be c++17 and the result iterator is also c++17. Why not just call it c++17 reverse iterator. 
> > 
> > Regarding random access operations not sfinae , when you pass in c++17 bidi only , wouldn’t it error on instantiating those member functions?
> Thanks, I made a note for myself at the correct place.
> 
> I think `_Cpp17RevIter` sounds too much like it should be use pre-C++20 if you want a reverse iterator, not that this is added for compatibility and should only be used in specific circumstances (specifically, that you get a user-defined iterator which may be C++20-hostile).
> 
> SFINAE only applies when you try to check if a function exists. If you never call the function the compiler doesn't instantiate it and you don't get an error.
I totally agree that cpp17_xxx is really confusing. The standard uses these names 
https://eel.is/c++draft/iterator.traits#2
When the standard says cpp17-iterator, it really means the classic iterator that model the classic Iterator named requirement, which has nothing to do with c++ 17.  I don’t usually debate about names and I am ok with names that make sense for most people. for me I found the name cpp17, probably just because I used to how the spec calls those things

Regarding SFINAE, IIUC, for non template member function of a class template, the function body is instantiated as soon as the class template is instantiated, and this could trigger hard error in your case even you never call those functions 


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  https://reviews.llvm.org/D128864/new/

https://reviews.llvm.org/D128864