[libc++]Implement LWG-3646 std::ranges::view_interface::size returns a signed type

[Mordante]

Thanks for the patch, LGTM! I like to have somebody more familiar with ranges to have a second look, especially whether we're happy with the test coverage. @philnik can you have a look?

[philnik]

LGTM too. I don't remember if you have commit access. If not: Name and E-Mail, same as last time.

[fsb4000]

@philnik, I have the commit access.
Thank you both for the review.

libcxx/docs/Status/Cxx2bIssues.csv

 "`3711 <https://wg21.link/LWG3711>`__","Missing preconditions for slide_view constructor","July 2022","","","|ranges|"
 "`3712 <https://wg21.link/LWG3712>`__","``chunk_view`` and ``slide_view`` should not be ``default_initializable``","July 2022","","","|ranges|"
 "`3713 <https://wg21.link/LWG3713>`__","Sorted with respect to comparator (only)","July 2022","",""
 "`3715 <https://wg21.link/LWG3715>`__","``view_interface::empty`` is overconstrained","July 2022","","","|ranges|"
 "`3719 <https://wg21.link/LWG3719>`__","Directory iterators should be usable with default sentinel","July 2022","","","|ranges|"
 "`3721 <https://wg21.link/LWG3721>`__","Allow an ``arg-id`` with a value of zero for ``width`` in ``std-format-spec``","July 2022","|Complete|","16.0","|format|"
 "`3724 <https://wg21.link/LWG3724>`__","``decay-copy`` should be constrained","July 2022","|Complete|","14.0"
 "","","","",""
+"`3646 <https://wg21.link/LWG3646>`__","``std::ranges::view_interface::size`` returns a signed type","November 2022","|Complete|","16.0"
+"","","","",""
 "`3629 <https://wg21.link/LWG3629>`__","``make_error_code`` and ``make_error_condition`` are customization points","Not voted in","|Complete|","16.0",""
 "`3631 <https://wg21.link/LWG3631>`__","``basic_format_arg(T&&)`` should use ``remove_cvref_t<T>`` throughout","Not voted in","|Complete|","15.0",""
 "`3645 <https://wg21.link/LWG3645>`__","``resize_and_overwrite`` is overspecified to call its callback with lvalues","Not voted in","|Complete|","14.0",""
 "`3636 <https://wg21.link/LWG3636>`__","``formatter<T>::format`` should be ``const``-qualified","Not voted in","|Complete|","16.0",""
 "","","","",""

libcxx/include/__ranges/view_interface.h

constexpr auto data() const
     return std::to_address(ranges::begin(__derived()));
   }
 
   template<class _D2 = _Derived>
   _LIBCPP_HIDE_FROM_ABI
   constexpr auto size()
     requires forward_range<_D2> && sized_sentinel_for<sentinel_t<_D2>, iterator_t<_D2>>
   {
-    return ranges::end(__derived()) - ranges::begin(__derived());
+    return std::__to_unsigned_like(ranges::end(__derived()) - ranges::begin(__derived()));
   }
 
   template<class _D2 = _Derived>
   _LIBCPP_HIDE_FROM_ABI
   constexpr auto size() const
     requires forward_range<const _D2> && sized_sentinel_for<sentinel_t<const _D2>, iterator_t<const _D2>>
   {
-    return ranges::end(__derived()) - ranges::begin(__derived());
+    return std::__to_unsigned_like(ranges::end(__derived()) - ranges::begin(__derived()));
   }
 
   template<class _D2 = _Derived>
   _LIBCPP_HIDE_FROM_ABI
   constexpr decltype(auto) front()
     requires forward_range<_D2>
   {
     _LIBCPP_ASSERT(!empty(),

libcxx/test/std/ranges/range.utility/view.interface/view.interface.pass.cpp

 template<class T>
 concept SizeInvocable = requires (T const& obj) { obj.size(); };
 
 constexpr bool testSize() {
   static_assert(!SizeInvocable<InputRange>);
   static_assert(!SizeInvocable<NotSizedSentinel>);
   static_assert( SizeInvocable<ForwardRange>);
 
+  using SignedSize = std::common_type_t<std::ptrdiff_t, std::make_signed_t<std::size_t>>;
   ForwardRange forwardRange;
   assert(forwardRange.size() == 8);
   assert(static_cast<ForwardRange const&>(forwardRange).size() == 8);
 
   assert(std::ranges::size(forwardRange) == 8);
+  static_assert(std::same_as<decltype(std::ranges::size(std::declval<ForwardRange>())), std::size_t>);
+  static_assert(std::same_as<decltype(std::ranges::ssize(std::declval<ForwardRange>())), SignedSize>);
+
   assert(std::ranges::size(static_cast<ForwardRange const&>(forwardRange)) == 8);
+  static_assert(std::same_as<decltype(std::ranges::size(std::declval<ForwardRange const>())), std::size_t>);
+  static_assert(std::same_as<decltype(std::ranges::ssize(std::declval<ForwardRange const>())), SignedSize>);
 
   SizeIsTen sizeTen;
   assert(sizeTen.size() == 10);
   assert(static_cast<SizeIsTen const&>(sizeTen).size() == 10);
   assert(sizeTen.std::ranges::view_interface<SizeIsTen>::size() == 8);
 
   assert(std::ranges::size(sizeTen) == 10);
   assert(std::ranges::size(static_cast<SizeIsTen const&>(sizeTen)) == 10);