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

[via libcxx-commits]

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@@ -90,6 +90,10 @@ namespace ranges {
       requires (!(__simple_view<_View> &&
                   random_access_range<const _View> && sized_range<const _View>))
     {
+      if constexpr (random_access_range<_View> && sized_range<_View>) {
+        range_difference_t<_View> __dist = ranges::distance(__base_);
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huixie90 wrote:

> > maybe
> > `range_difference_t<_View> __dist = ranges::size(__base_);`
> > is the actual intent of the code?
> 
> The standard does not guarantee that the above implicit conversion is well-formed. `ranges::distance` is always preferred over `ranges::size` when deal with iterators.

Only narrowing conversion can be explicit-only.  IIUC if  size_t -> difeerence_t is narrowing conversion, I think we have other problems.  The problem of ranges::distance is that it is not for the purpose of calculating the size in constant time. It is a utility tool to calculate the size in case you cannot caluculate it in constant time, which is the wrong tool here imo. And I strongly against using ‘auto’ when dealing with difference type for the exact reason we are discussing: narrowing conversion. 
If the spec does not say size -> difference_t cannot be narrowing, I’d prefer to use either parenthesis or static_cast to do the comversion

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