Proposed resolution:

This wording is relative to n4928.

1. Modify [range.chunk.fwd.iter] p13 as indicated:

   constexpr iterator& operator+=(difference_type x)
     requires random_access_range<Base>;
   

   -12- Preconditions: If x is positive, ranges::distance(current_, end_) > n_ * (x - 1) is true.

   [Note 1: If x is negative, the Effects paragraph implies a precondition. — end note]

   -13- Effects: Equivalent to:

   if (x > 0) {
     ranges::advance(current_, n_ * (x - 1));
     missing_ = ranges::advance(current_, n_ * x, end_);
   } else if (x < 0) {
     ranges::advance(current_, n_ * x + missing_);
     missing_ = 0;
   }
   return *this;
   

2. Modify [range.stride.iterator] p14 as indicated:

   constexpr iterator& operator+=(difference_type n) requires random_access_range<Base>;
   

   -13- Preconditions: If n is positive, ranges::distance(current_, end_) > stride_ * (n - 1) is true.

   [Note 1: If n is negative, the Effects paragraph implies a precondition. — end note]

   -14- Effects: Equivalent to:

   if (n > 0) {
     ranges::advance(current_, stride_ * (n - 1));
     missing_ = ranges::advance(current_, stride_ * n, end_);
   } else if (n < 0) {
     ranges::advance(current_, stride_ * n + missing_);
     missing_ = 0;
   }
   return *this;
   

[ 2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP. ]

[ Issaquah 2023-02-10; LWG issue processing ]

Move to Immediate for C++23

The intent was that the precondition allows the call to ranges::advance, which otherwise would have time linear in the argument of operator+=, to actually be implemented using operator+= or equivalent for all but the last step. This is at best very non-obvious and should be clarified.