} // namespace __variant
} // namespace __detail
-#define __cpp_lib_variant 201606L
+#define __cpp_lib_variant 202102L
template<typename... _Types> class tuple;
template<typename... _Types> class variant;
std::forward<_Variants>(__variants)...);
}
+ // The __as function templates implement the exposition-only "as-variant"
+
+ template<typename... _Types>
+ constexpr std::variant<_Types...>&
+ __as(std::variant<_Types...>& __v)
+ { return __v; }
+
+ template<typename... _Types>
+ constexpr const std::variant<_Types...>&
+ __as(const std::variant<_Types...>& __v) noexcept
+ { return __v; }
+
+ template<typename... _Types>
+ constexpr std::variant<_Types...>&&
+ __as(std::variant<_Types...>&& __v) noexcept
+ { return std::move(__v); }
+
+ template<typename... _Types>
+ constexpr const std::variant<_Types...>&&
+ __as(const std::variant<_Types...>&& __v) noexcept
+ { return std::move(__v); }
+
// _Uninitialized<T> is guaranteed to be a trivially destructible type,
// even if T is not.
template<typename _Type, bool = std::is_trivially_destructible_v<_Type>>
std::index_sequence<__indices...>>
: _Base_dedup<__indices, __poison_hash<remove_const_t<_Types>>>... { };
- template<size_t _Np, typename _Variant>
- using __get_t = decltype(std::get<_Np>(std::declval<_Variant>()));
+ // Equivalent to decltype(get<_Np>(as-variant(declval<_Variant>())))
+ template<size_t _Np, typename _Variant,
+ typename _AsV = decltype(__variant::__as(std::declval<_Variant>())),
+ typename _Tp = variant_alternative_t<_Np, remove_reference_t<_AsV>>>
+ using __get_t
+ = conditional_t<is_lvalue_reference_v<_Variant>, _Tp&, _Tp&&>;
// Return type of std::visit.
template<typename _Visitor, typename... _Variants>
constexpr __detail::__variant::__visit_result_t<_Visitor, _Variants...>
visit(_Visitor&& __visitor, _Variants&&... __variants)
{
- if ((__variants.valueless_by_exception() || ...))
+ namespace __variant = std::__detail::__variant;
+
+ if ((__variant::__as(__variants).valueless_by_exception() || ...))
__throw_bad_variant_access("std::visit: variant is valueless");
using _Result_type
if constexpr (sizeof...(_Variants) == 1)
{
+ using _Vp = decltype(__variant::__as(std::declval<_Variants>()...));
+
constexpr bool __visit_rettypes_match = __detail::__variant::
- __check_visitor_results<_Visitor, _Variants...>(
- std::make_index_sequence<
- std::variant_size<remove_reference_t<_Variants>...>::value>());
+ __check_visitor_results<_Visitor, _Vp>(
+ make_index_sequence<variant_size_v<remove_reference_t<_Vp>>>());
if constexpr (!__visit_rettypes_match)
{
static_assert(__visit_rettypes_match,
else
return std::__do_visit<_Tag>(
std::forward<_Visitor>(__visitor),
- std::forward<_Variants>(__variants)...);
+ static_cast<_Vp>(__variants)...);
}
else
return std::__do_visit<_Tag>(
std::forward<_Visitor>(__visitor),
- std::forward<_Variants>(__variants)...);
+ __variant::__as(std::forward<_Variants>(__variants))...);
}
#if __cplusplus > 201703L
constexpr _Res
visit(_Visitor&& __visitor, _Variants&&... __variants)
{
- if ((__variants.valueless_by_exception() || ...))
+ namespace __variant = std::__detail::__variant;
+
+ if ((__variant::__as(__variants).valueless_by_exception() || ...))
__throw_bad_variant_access("std::visit<R>: variant is valueless");
return std::__do_visit<_Res>(std::forward<_Visitor>(__visitor),
- std::forward<_Variants>(__variants)...);
+ __variant::__as(std::forward<_Variants>(__variants))...);
}
#endif
--- /dev/null
+// { dg-do compile { target c++17 } }
+
+#include <variant>
+
+// P2062R2 Inheriting from std::variant (resolving LWG 3052)
+
+#if __cpp_lib_variant < 202102L
+#error __cpp_lib_variant has the wrong value in <variant>
+#endif
+
+struct V : std::variant<int> {
+ using std::variant<int>::variant;
+};
+
+constexpr int
+test01()
+{
+ V v = 42;
+ return std::visit([](int&){ return 17; }, v);
+}
+static_assert( test01() == 17 );
+
+constexpr int
+test02()
+{
+ const V c = 77;
+ std::variant<char*, long> x = 88L;
+ return std::visit([](auto&& a, auto&& b) {
+ if constexpr (std::is_same_v<decltype(a), const int&&>)
+ if constexpr (std::is_same_v<decltype(b), long&&>)
+ return 99;
+ return 0;
+ },
+ std::move(c), std::move(x));
+}
+static_assert( test02() == 99 );