Tested x86_64-linux. Pushed to trunk.
Thanks to Nina Ranns for noticing this bug.
Removing the base class isn't an ABI change. It's an empty base class
with a type that's unique to the std::variant that derives from it, so
doesn't change whether two std::variant subobjects can be overlapping.
-- >8 --
The standard says the default ctor should be constrained, not deleted.
Our use of a defaulted default ctor and _Enable_default_constructor base
class results in it being deleted.
libstdc++-v3/ChangeLog:
* include/std/variant (variant): Remove derivation from
_Enable_default_constructor base class.
(variant::variant()): Constrain.
* testsuite/20_util/variant/default_ctor.cc: New test.
---
libstdc++-v3/include/std/variant | 24 +++++++++----------
.../testsuite/20_util/variant/default_ctor.cc | 11 +++++++++
2 files changed, 23 insertions(+), 12 deletions(-)
create mode 100644 libstdc++-v3/testsuite/20_util/variant/default_ctor.cc
diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
index 36eec8d829b..c0e41740dcf 100644
--- a/libstdc++-v3/include/std/variant
+++ b/libstdc++-v3/include/std/variant
@@ -1332,9 +1332,6 @@ namespace __variant
template<typename... _Types>
class variant
: private __detail::__variant::_Variant_base<_Types...>,
- private _Enable_default_constructor<
- __detail::__variant::_Traits<_Types...>::_S_default_ctor,
- variant<_Types...>>,
private _Enable_copy_move<
__detail::__variant::_Traits<_Types...>::_S_copy_ctor,
__detail::__variant::_Traits<_Types...>::_S_copy_assign,
@@ -1355,10 +1352,6 @@ namespace __variant
"variant must have no void alternative");
using _Base = __detail::__variant::_Variant_base<_Types...>;
- using _Default_ctor_enabler =
- _Enable_default_constructor<
- __detail::__variant::_Traits<_Types...>::_S_default_ctor,
- variant<_Types...>>;
template<typename _Tp>
static constexpr bool __not_self
@@ -1396,7 +1389,16 @@ namespace __variant
= !__is_in_place_tag<__remove_cvref_t<_Tp>>::value;
public:
- variant() = default;
+#if __cpp_concepts
+ variant() requires is_default_constructible_v<__to_type<0>> = default;
+#else
+ template<typename _Tp0 = __to_type<0>,
+ typename = enable_if_t<is_default_constructible_v<_Tp0>>>
+ constexpr
+ variant() noexcept(is_nothrow_default_constructible_v<__to_type<0>>)
+ { }
+#endif
+
variant(const variant& __rhs) = default;
variant(variant&&) = default;
variant& operator=(const variant&) = default;
@@ -1441,8 +1443,7 @@ namespace __variant
typename = enable_if_t<is_constructible_v<_Tp, _Args...>>>
constexpr explicit
variant(in_place_index_t<_Np>, _Args&&... __args)
- : _Base(in_place_index<_Np>, std::forward<_Args>(__args)...),
- _Default_ctor_enabler(_Enable_default_constructor_tag{})
+ : _Base(in_place_index<_Np>, std::forward<_Args>(__args)...)
{ }
template<size_t _Np, typename _Up, typename... _Args,
@@ -1453,8 +1454,7 @@ namespace __variant
constexpr explicit
variant(in_place_index_t<_Np>, initializer_list<_Up> __il,
_Args&&... __args)
- : _Base(in_place_index<_Np>, __il, std::forward<_Args>(__args)...),
- _Default_ctor_enabler(_Enable_default_constructor_tag{})
+ : _Base(in_place_index<_Np>, __il, std::forward<_Args>(__args)...)
{ }
template<typename _Tp>
diff --git a/libstdc++-v3/testsuite/20_util/variant/default_ctor.cc
b/libstdc++-v3/testsuite/20_util/variant/default_ctor.cc
new file mode 100644
index 00000000000..d0d9235a59c
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/variant/default_ctor.cc
@@ -0,0 +1,11 @@
+// { dg-do compile { target c++17 } }
+
+#include <variant>
+
+struct A{ A(int); };
+struct B{ B(){}; };
+
+void f(std::variant<A>);
+int f(B);
+
+int unambiguous = f({}); // { dg-bogus "ambiguous" }
--
2.41.0
