[Bug libstdc++/100824] New: ranges::size should treat the subexpression as an lvalue

[hewillk at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100824

            Bug ID: 100824
           Summary: ranges::size should treat the subexpression as an
                    lvalue
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hewillk at gmail dot com
  Target Milestone: ---

The CPO described in [range.access] treats the subexpression E as an lvalue,
but ranges::size, ranges::empty, and ranges::data forward E perfectly, which
makes the following case behavior wrong:

https://godbolt.org/z/7KoT3WE41

#include <ranges>
#include <array>

struct R : std::ranges::view_base {
  constexpr int* begin() & { return nullptr; }
  constexpr int* end() & { return nullptr; }
  constexpr int size() & { return 42; }
};
template <>
inline constexpr bool std::ranges::enable_borrowed_range<R> = true;

// call ranges::end(e) - ranges::begin(e) incorrectly
static_assert(std::ranges::size(R{}));

int main() {
  std::array r{42, 42, 42};
  // call e.data() correctly
  (r | std::views::all).data();

  // call std::to_address(ranges::begin(e)) incorrectly
  std::ranges::data(r | std::views::all);
}