libstdc++: Implement P1994R1 changes to ranges::elements_view

The example from the paper doesn't compile without the proposed
resolution for LWG 3406, so we'll add a testcase for this once the
proposed resolution is implemented.

libstdc++-v3/ChangeLog:

	* include/std/ranges (elements_view::end): Replace these two
	overloads with four new overloads.
	(elements_view::_Iterator::operator==): Remove.
	(elements_view::_Iterator::operator-): Likewise.
	(elements_view::_Sentinel): Define.

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index 9d22b138082939bc164db5fd878f3dfef067f4ff..efa8d2cf9f4a7930241c2897a22ddc62c3d925cc 100644 (file)
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -3366,12 +3366,20 @@ namespace views
       { return _Iterator<true>(ranges::begin(_M_base)); }
 
       constexpr auto
-      end() requires (!__detail::__simple_view<_Vp>)
-      { return ranges::end(_M_base); }
+      end()
+      { return _Sentinel<false>{ranges::end(_M_base)}; }
 
       constexpr auto
-      end() const requires __detail::__simple_view<_Vp>
-      { return ranges::end(_M_base); }
+      end() requires common_range<_Vp>
+      { return _Iterator<false>{ranges::end(_M_base)}; }
+
+      constexpr auto
+      end() const requires range<const _Vp>
+      { return _Sentinel<true>{ranges::end(_M_base)}; }
+
+      constexpr auto
+      end() const requires common_range<const _Vp>
+      { return _Iterator<true>{ranges::end(_M_base)}; }
 
       constexpr auto
       size() requires sized_range<_Vp>
@@ -3382,6 +3390,9 @@ namespace views
       { return ranges::size(_M_base); }
 
     private:
+      template<bool _Const>
+       struct _Sentinel;
+
       template<bool _Const>
        struct _Iterator
        {
@@ -3484,10 +3495,6 @@ namespace views
            requires equality_comparable<iterator_t<_Base>>
          { return __x._M_current == __y._M_current; }
 
-         friend constexpr bool
-         operator==(const _Iterator& __x, const sentinel_t<_Base>& __y)
-         { return __x._M_current == __y; }
-
          friend constexpr bool
          operator<(const _Iterator& __x, const _Iterator& __y)
            requires random_access_range<_Base>
@@ -3536,15 +3543,54 @@ namespace views
            requires random_access_range<_Base>
          { return __x._M_current - __y._M_current; }
 
-         friend constexpr difference_type
-         operator-(const _Iterator<_Const>& __x, const sentinel_t<_Base>& __y)
+         friend _Sentinel<_Const>;
+       };
+
+      template<bool _Const>
+       struct _Sentinel
+       {
+       private:
+         constexpr bool
+         _M_equal(const _Iterator<_Const>& __x) const
+         { return __x._M_current == _M_end; }
+
+         using _Base = __detail::__maybe_const_t<_Const, _Vp>;
+         sentinel_t<_Base> _M_end = sentinel_t<_Base>();
+
+       public:
+         _Sentinel() = default;
+
+         constexpr explicit
+         _Sentinel(sentinel_t<_Base> __end)
+           : _M_end(std::move(__end))
+         { }
+
+         constexpr
+         _Sentinel(_Sentinel<!_Const> __other)
+           requires _Const
+             && convertible_to<sentinel_t<_Vp>, sentinel_t<_Base>>
+           : _M_end(std::move(__other._M_end))
+         { }
+
+         constexpr sentinel_t<_Base>
+         base() const
+         { return _M_end; }
+
+         friend constexpr bool
+         operator==(const _Iterator<_Const>& __x, const _Sentinel& __y)
+         { return __y._M_equal(__x); }
+
+         friend constexpr range_difference_t<_Base>
+         operator-(const _Iterator<_Const>& __x, const _Sentinel& __y)
            requires sized_sentinel_for<sentinel_t<_Base>, iterator_t<_Base>>
-         { return __x._M_current - __y; }
+         { return __x._M_current - __y._M_end; }
 
-         friend constexpr difference_type
-         operator-(const sentinel_t<_Base>& __x, const _Iterator<_Const>& __y)
+         friend constexpr range_difference_t<_Base>
+         operator-(const _Sentinel& __x, const _Iterator<_Const>& __y)
            requires sized_sentinel_for<sentinel_t<_Base>, iterator_t<_Base>>
-         { return -(__y - __x); }
+         { return __x._M_end - __y._M_current; }
+
+         friend _Sentinel<!_Const>;
        };
 
       _Vp _M_base = _Vp();