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* [gcc r11-3760] libstdc++: Diagnose visitors with different return types [PR95904]
@ 2020-10-09 17:49 Ville Voutilainen
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From: Ville Voutilainen @ 2020-10-09 17:49 UTC (permalink / raw)
To: gcc-cvs, libstdc++-cvs
https://gcc.gnu.org/g:3427e31331677ca826c5588c87924214f7e5c54b
commit r11-3760-g3427e31331677ca826c5588c87924214f7e5c54b
Author: Ville Voutilainen <ville.voutilainen@gmail.com>
Date: Fri Oct 9 20:47:01 2020 +0300
libstdc++: Diagnose visitors with different return types [PR95904]
libstdc++-v3/ChangeLog:
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:
---
libstdc++-v3/include/std/variant | 67 ++++++++++++++++++++--
.../testsuite/20_util/variant/visit_neg.cc | 2 +-
2 files changed, 64 insertions(+), 5 deletions(-)
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>
+ 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 +1743,28 @@ 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
diff --git a/libstdc++-v3/testsuite/20_util/variant/visit_neg.cc b/libstdc++-v3/testsuite/20_util/variant/visit_neg.cc
index 6279dec5aa2..748eb21c1ad 100644
--- a/libstdc++-v3/testsuite/20_util/variant/visit_neg.cc
+++ b/libstdc++-v3/testsuite/20_util/variant/visit_neg.cc
@@ -21,7 +21,7 @@
#include <variant>
#include <testsuite_hooks.h>
-// { dg-error "invalid conversion" "" { target *-*-* } 0 }
+// { dg-error "same return type for all alternatives" "" { target *-*-* } 0 }
// { dg-prune-output "in 'constexpr' expansion" }
void
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