On Tue, 29 Sep 2020 at 14:20, Jonathan Wakely <jwak...@redhat.com> wrote:
> I think this is what we want:
>
> template<typename _Tp, typename... _Types>
> constexpr inline __same_types = (is_same_v<_Tp, _Types> && ...);
>
> is_same_v is very cheap, it uses the built-in directly, so you don't
> need to instantiate any class templates at all.
>
> >+
> >+ template <unsigned long int _Idx, class _Visitor, class _Variant>
>
> typename not class please.
>
> >+ decltype(auto) __check_visitor_result(_Visitor&& __vis,
>
> New line after the decltype(auto) please, not in the middle of the
> parameter list.
Aye.
diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
index dd8847cf829..6f647d622c4 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 +1017,22 @@ namespace __variant
static constexpr auto
_S_apply()
- { return _Array_type{&__visit_invoke}; }
+ {
+ constexpr bool __visit_ret_type_mismatch =
+ _Array_type::__result_is_deduced::value
+ && !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};
+ }
};
template<typename _Result_type, typename _Visitor, typename... _Variants>
@@ -1692,6 +1707,26 @@ 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::forward<_Visitor>(__vis)(
+ std::get<_Idx>(std::forward<_Variant>(__variant)));
+ }
+
+ template <typename _Visitor, typename _Variant, unsigned long int... _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)
visit(_Visitor&& __visitor, _Variants&&... __variants)
@@ -1704,8 +1739,28 @@ namespace __variant
using _Tag = __detail::__variant::__deduce_visit_result<_Result_type>;
- return std::__do_visit<_Tag>(std::forward<_Visitor>(__visitor),
- std::forward<_Variants>(__variants)...);
+ if constexpr (sizeof...(_Variants) == 1)
+ {
+ constexpr bool __visit_rettypes_match =
+ __check_visitor_results<_Visitor, _Variants...>(
+ std::make_index_sequence<
+ std::variant_size<remove_reference_t<_Variants>...>::value>());
+ if constexpr (!__visit_rettypes_match)
+ {
+ static_assert(__visit_rettypes_match,
+ "std::visit requires the visitor to have the same "
+ "return type for all alternatives of a variant");
+ return;
+ }
+ else
+ return std::__do_visit<_Tag>(
+ std::forward<_Visitor>(__visitor),
+ std::forward<_Variants>(__variants)...);
+ }
+ else
+ return std::__do_visit<_Tag>(
+ std::forward<_Visitor>(__visitor),
+ std::forward<_Variants>(__variants)...);
}
#if __cplusplus > 201703L
