On 05/10/20 22:35 +0300, Ville Voutilainen via Libstdc++ wrote:
On Mon, 5 Oct 2020 at 01:15, Ville Voutilainen
<ville.voutilai...@gmail.com> wrote:
The patch is borked, doesn't pass tests, fixing...
Unborked, ok for trunk if full testsuite passes?
2020-10-05 Ville Voutilainen <ville.voutilai...@gmail.com>
PR libstdc++/95904
* include/std/variant (__deduce_visit_result): Add a nested ::type.
(__gen_vtable_impl</*base case*/>::_S_apply):
Check the visitor return type.
(__same_types): New.
(__check_visitor_result): Likewise.
(__check_visitor_results): Likewise.
(visit(_Visitor&&, _Variants&&...)): Use __check_visitor_results
in case we're visiting just one variant.
* testsuite/20_util/variant/visit_neg.cc: Adjust.
diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
index dd8847cf829..b32e564fd41 100644
--- a/libstdc++-v3/include/std/variant
+++ b/libstdc++-v3/include/std/variant
@@ -182,7 +182,7 @@ namespace __variant
// used for raw visitation with indices passed in
struct __variant_idx_cookie { using type = __variant_idx_cookie; };
// Used to enable deduction (and same-type checking) for std::visit:
- template<typename> struct __deduce_visit_result { };
+ template<typename _Tp> struct __deduce_visit_result { using type = _Tp; };
// Visit variants that might be valueless.
template<typename _Visitor, typename... _Variants>
@@ -1017,7 +1017,26 @@ namespace __variant
static constexpr auto
_S_apply()
- { return _Array_type{&__visit_invoke}; }
+ {
+ if constexpr (_Array_type::__result_is_deduced::value)
+ {
+ constexpr bool __visit_ret_type_mismatch =
+ !is_same_v<typename _Result_type::type,
+ decltype(__visit_invoke(std::declval<_Visitor>(),
+ std::declval<_Variants>()...))>;
+ if constexpr (__visit_ret_type_mismatch)
+ {
+ static_assert(!__visit_ret_type_mismatch,
+ "std::visit requires the visitor to have the same "
+ "return type for all alternatives of a variant");
+ return __nonesuch{};
+ }
+ else
+ return _Array_type{&__visit_invoke};
+ }
+ else
+ return _Array_type{&__visit_invoke};
+ }
};
template<typename _Result_type, typename _Visitor, typename... _Variants>
@@ -1692,6 +1711,26 @@ namespace __variant
std::forward<_Variants>(__variants)...);
}
+ template<typename _Tp, typename... _Types>
+ constexpr inline bool __same_types = (is_same_v<_Tp, _Types> && ...);
+
+ template <unsigned long int _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, unsigned long int... _Idxs>
index_sequence uses size_t not unsigned long. This parameter pack
needs to be size_t... _Idxs, and the NTTP for __check_visitor_result
should be size_t _Idx.
