[ranges] Apply resolution of LWG3502 to elements_view

fixes #1685
microsoft · Apr 23, 2021 · a7965e5 · a7965e5
1 parent f675d68
commit a7965e5
Show file tree

Hide file tree

Showing 2 changed files with 24 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:
@@ -3768,7 +3772,12 @@ namespace ranges {
 
             _NODISCARD constexpr decltype(auto) operator*() const
                 noexcept(noexcept(_STD get<_Index>(*_Current))) /* strengthened */ {
-                return _STD get<_Index>(*_Current);
+                if constexpr (is_reference_v<range_reference_t<_Base>>) {
+                    return _STD get<_Index>(*_Current);
+                } else {
+                    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 */ {
@@ -3835,7 +3844,12 @@ namespace ranges {
 #if _ITERATOR_DEBUG_LEVEL != 0
                 _Verify_offset(_Idx);
 #endif // _ITERATOR_DEBUG_LEVEL != 0
-                return _STD get<_Index>(*(_Current + _Idx));
+                if constexpr (is_reference_v<range_reference_t<_Base>>) {
+                    return _STD get<_Index>(*(_Current + _Idx));
+                } else {
+                    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(

diff --git a/tests/std/tests/P0896R4_views_elements/test.cpp b/tests/std/tests/P0896R4_views_elements/test.cpp
@@ -385,6 +385,14 @@ constexpr void instantiation_test() {
 #endif // TEST_EVERYTHING
 }
 
+constexpr bool test_dangling() {
+    constexpr int input[] = {1, 5, 42, 1307};
+    const auto r = input | std::views::transform([](auto c) { return std::make_tuple(c, c); }) | std::views::keys;
+
+    assert(ranges::equal(input, r));
+    return true;
+}
+
 int main() {
     { // Validate copyable views
         constexpr span<const P> s{some_pairs};