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libstdc++: Do not use deduced return type for std::visit [PR 100384]

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This avoids errors outside the immediate context when std::visit is an
overload candidate because of ADL, but not actually viable.

The solution is to give std::visit a non-deduced return type. New
helpers are introduced for that, and existing ones refactored slightly.

libstdc++-v3/ChangeLog:

	PR libstdc++/100384
	* include/std/variant (__get_t): New alias template yielding the
	return type of std::get<N> on a variant.
	(__visit_result_t): New alias template yielding the result of
	std::visit.
	(__same_types): Move into namespace __detail::__variant.
	(__check_visitor_results): Likewise. Use __invoke_result_t and
	__get_t.
	(__check_visitor_result): Remove.
	(visit): Use __visit_result_t for return type.
	* testsuite/20_util/variant/100384.cc: New test.
This commit is contained in:
Jonathan Wakely 2021-05-04 12:16:46 +01:00
parent ad0a3be4df
commit af5b2b911d
2 changed files with 34 additions and 25 deletions
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50
libstdc++-v3/include/std/variant
View File

@ -1063,6 +1063,25 @@ namespace __variant
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>()));
// Return type of std::visit.
template<typename _Visitor, typename... _Variants>
using __visit_result_t
= invoke_result_t<_Visitor, __get_t<0, _Variants>...>;
template<typename _Tp, typename... _Types>
constexpr inline bool __same_types = (is_same_v<_Tp, _Types> && ...);
template <typename _Visitor, typename _Variant, size_t... _Idxs>
constexpr bool __check_visitor_results(std::index_sequence<_Idxs...>)
{
return __same_types<
invoke_result_t<_Visitor, __get_t<_Idxs, _Variant>>...
>;
}
} // namespace __variant
} // namespace __detail
@ -1248,7 +1267,8 @@ namespace __variant
#endif
template<typename _Visitor, typename... _Variants>
constexpr decltype(auto) visit(_Visitor&&, _Variants&&...);
constexpr __detail::__variant::__visit_result_t<_Visitor, _Variants...>
visit(_Visitor&&, _Variants&&...);
template<typename... _Types>
inline enable_if_t<(is_move_constructible_v<_Types> && ...)
@ -1715,41 +1735,21 @@ namespace __variant
std::forward<_Variants>(__variants)...);
}
template<typename _Tp, typename... _Types>
constexpr inline bool __same_types = (is_same_v<_Tp, _Types> && ...);
template <size_t _Idx, typename _Visitor, typename _Variant>
decltype(auto)
__check_visitor_result(_Visitor&& __vis, _Variant&& __variant)
{
return std::__invoke(std::forward<_Visitor>(__vis),
std::get<_Idx>(std::forward<_Variant>(__variant)));
}
template <typename _Visitor, typename _Variant, size_t... _Idxs>
constexpr bool __check_visitor_results(std::index_sequence<_Idxs...>)
{
return __same_types<decltype(__check_visitor_result<_Idxs>(
std::declval<_Visitor>(),
std::declval<_Variant>()))...>;
}
template<typename _Visitor, typename... _Variants>
constexpr decltype(auto)
constexpr __detail::__variant::__visit_result_t<_Visitor, _Variants...>
visit(_Visitor&& __visitor, _Variants&&... __variants)
{
if ((__variants.valueless_by_exception() || ...))
__throw_bad_variant_access("std::visit: variant is valueless");
using _Result_type = std::invoke_result_t<_Visitor,
decltype(std::get<0>(std::declval<_Variants>()))...>;
using _Result_type
= __detail::__variant::__visit_result_t<_Visitor, _Variants...>;
using _Tag = __detail::__variant::__deduce_visit_result<_Result_type>;
if constexpr (sizeof...(_Variants) == 1)
{
constexpr bool __visit_rettypes_match =
constexpr bool __visit_rettypes_match = __detail::__variant::
__check_visitor_results<_Visitor, _Variants...>(
std::make_index_sequence<
std::variant_size<remove_reference_t<_Variants>...>::value>());

9
libstdc++-v3/testsuite/20_util/variant/100384.cc Normal file
View File

@ -0,0 +1,9 @@
// { dg-do compile { target c++17 } }
#include <variant>
int visit(int*, std::true_type) { return 0; }
const std::true_type dat;
int i = visit(nullptr, dat);