[libcxx-commits] [libcxx] [libc++] optimization on ranges::drop_view::begin (#72883) (PR #72929)

[via libcxx-commits]

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@@ -104,7 +108,8 @@ namespace ranges {
     constexpr auto begin() const
       requires random_access_range<const _View> && sized_range<const _View>
     {
-      return ranges::next(ranges::begin(__base_), __count_, ranges::end(__base_));
+      range_difference_t<_View> __dist = ranges::distance(__base_);
+      return ranges::begin(__base_) + std::min(__count_, __dist);
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huixie90 wrote:

> For my own edification, I attempted to answer this question ... it looks like the type is stripped of `const`/`&`?
> 
> ```c++
> template <range _Rp>
> using range_difference_t = iter_difference_t<iterator_t<_Rp>>;
> 
> // ...
> 
> template <class _Ip>
> using iter_difference_t = typename conditional_t<__is_primary_template<iterator_traits<remove_cvref_t<_Ip> > >::value,
>                                                  incrementable_traits<remove_cvref_t<_Ip> >,
>                                                  iterator_traits<remove_cvref_t<_Ip> > >::difference_type;
> ```

I think this is unrelated. off course these traits needs to strip off `cvref`, but `iterator_t<_Rp>` and `iterator_t<const _Rp>` can well be two different types.  (although one would argue that it is evil to let them have two different difference_type)

https://github.com/llvm/llvm-project/pull/72929