Proposed resolution:

This wording is relative to N4878.

1. Modify [range.split.inner] as indicated:

   constexpr decltype(auto) operator++(int) {
     if constexpr (forward_range<VBase>) {
       auto tmp = *this;
       ++*this;
       return tmp;
     } else
       ++*this;
   }
   

[ 2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP. ]

[ 2021-04-20; Reflector poll ]

Set status to Tentatively Ready after five votes in favour during reflector poll.

split_view<V, P>::inner-iterator<Const>::operator++(int) is specified directly in the synopsis in [range.split.inner] as:

constexpr decltype(auto) operator++(int) {
  if constexpr (forward_range<V>) {
    auto tmp = *this;
    ++*this;
    return tmp;
  } else
    ++*this;
}

The dependency on the properties of V here is odd given that we are wrapping an iterator obtained from a maybe-const<Const, V> (aka Base). It seems like this function should instead be concerned with forward_range<Base>.