Not completely tested yet. This does fix the problem of converting
incompatible pointer-to-function types, and thus gets rid of the suggestion
that compiling the code with -fpermissive is a possibility. There
is a special-casing for visit() for visitation of a single variant, and there
we don't even instantiate the whole table mechanism. We should really
entertain the possibility of flattening the whole visitation table; then
this check could (at least in theory) be uniformly written as just
an iteration of all table elements, which is not so convenient to do
with the nested multitable. This seems like a worthy incremental
improvement to me.
2020-09-29 Ville Voutilainen
PR libstdc++/95904
* include/std/variant (__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.
(__gen_vtable_impl::_S_apply):
Check the visitor return type.
diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
index dd8847cf829..56de78407c4 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 struct __deduce_visit_result { };
+ template struct __deduce_visit_result { using type = _Tp; };
// Visit variants that might be valueless.
template
@@ -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(),
+ 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
@@ -1692,6 +1707,27 @@ namespace __variant
std::forward<_Variants>(__variants)...);
}
+ template
+struct __same_types : public std::bool_constant<
+std::__and_...>::value> {};
+
+ template
+decltype(auto) __check_visitor_result(_Visitor&& __vis,
+ _Variant&& __variant)
+{
+ return std::forward<_Visitor>(__vis)(
+std::get<_Idx>(std::forward<_Variant>(__variant)));
+}
+
+ template
+constexpr bool __check_visitor_results(std::index_sequence<_Idxs...>)
+{
+ return __same_types(
+ std::declval<_Visitor>(),
+ std::declval<_Variant>()))...>::value;
+}
+
+
template
constexpr decltype(auto)
visit(_Visitor&& __visitor, _Variants&&... __variants)
@@ -1704,8 +1740,28 @@ 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...>::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
