[ranges] Apply resolution of LWG-3502 to elements_view (#1878)

Co-authored-by: Casey Carter <[email protected]>
microsoft · May 1, 2021 · 4946c74 · 4946c74
1 parent 02ae70a
commit 4946c74
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Showing 2 changed files with 32 additions and 2 deletions.
diff --git a/stl/inc/ranges b/stl/inc/ranges
@@ -3701,10 +3701,14 @@ namespace ranges {
         // clang-format on
     };
 
+    template <class _Tuple, size_t _Index>
+    concept _Returnable_element = is_reference_v<_Tuple> || move_constructible<tuple_element_t<_Index, _Tuple>>;
+
     // clang-format off
     template <input_range _Vw, size_t _Index>
         requires view<_Vw> && _Has_tuple_element<range_value_t<_Vw>, _Index>
             && _Has_tuple_element<remove_reference_t<range_reference_t<_Vw>>, _Index>
+            && _Returnable_element<range_reference_t<_Vw>, _Index>
     class elements_view : public view_interface<elements_view<_Vw, _Index>> {
         // clang-format on
     private:
@@ -3767,10 +3771,18 @@ namespace ranges {
             }
 
             _NODISCARD constexpr decltype(auto) operator*() const
-                noexcept(noexcept(_STD get<_Index>(*_Current))) /* strengthened */ {
+                noexcept(noexcept(_STD get<_Index>(*_Current))) /* strengthened */
+                requires is_reference_v<range_reference_t<_Base>> {
                 return _STD get<_Index>(*_Current);
             }
 
+            _NODISCARD constexpr decltype(auto) operator*() const
+                noexcept(is_nothrow_move_constructible_v<tuple_element_t<_Index, range_reference_t<_Base>>> //
+                        && noexcept(_STD get<_Index>(*_Current))) /* strengthened */ {
+                using _ElemTy = remove_cv_t<tuple_element_t<_Index, range_reference_t<_Base>>>;
+                return static_cast<_ElemTy>(_STD get<_Index>(*_Current));
+            }
+
             constexpr _Iterator& operator++() noexcept(noexcept(++_Current)) /* strengthened */ {
                 ++_Current;
                 return *this;
@@ -3831,13 +3843,24 @@ namespace ranges {
 
             _NODISCARD constexpr decltype(auto) operator[](const difference_type _Idx) const
                 noexcept(noexcept(_STD get<_Index>(*(_Current + _Idx)))) /* strengthened */
-                requires random_access_range<_Base> {
+                requires random_access_range<_Base>&& is_reference_v<range_reference_t<_Base>> {
 #if _ITERATOR_DEBUG_LEVEL != 0
                 _Verify_offset(_Idx);
 #endif // _ITERATOR_DEBUG_LEVEL != 0
                 return _STD get<_Index>(*(_Current + _Idx));
             }
 
+            _NODISCARD constexpr decltype(auto) operator[](const difference_type _Idx) const
+                noexcept(is_nothrow_move_constructible_v<tuple_element_t<_Index, range_reference_t<_Base>>> //
+                        && noexcept(_STD get<_Index>(*(_Current + _Idx)))) /* strengthened */
+                requires random_access_range<_Base> {
+#if _ITERATOR_DEBUG_LEVEL != 0
+                _Verify_offset(_Idx);
+#endif // _ITERATOR_DEBUG_LEVEL != 0
+                using _ElemTy = remove_cv_t<tuple_element_t<_Index, range_reference_t<_Base>>>;
+                return static_cast<_ElemTy>(_STD get<_Index>(*(_Current + _Idx)));
+            }
+
             _NODISCARD friend constexpr bool operator==(const _Iterator& _Left, const _Iterator& _Right) noexcept(
                 noexcept(_Left._Current == _Right._Current)) /* strengthened */
                 requires equality_comparable<iterator_t<_Base>> {

diff --git a/tests/std/tests/P0896R4_views_elements/test.cpp b/tests/std/tests/P0896R4_views_elements/test.cpp
@@ -414,4 +414,11 @@ int main() {
         STATIC_ASSERT(test_one(s));
         test_one(s);
     }
+
+    { // Validate a view borrowed range
+        constexpr auto v = views::iota(0ull, ranges::size(expected_keys))
+                         | views::transform([](auto i) { return make_pair(expected_keys[i], expected_values[i]); });
+        STATIC_ASSERT(test_one(v));
+        test_one(v);
+    }
 }