This patch adds support for constructing and assigning tuple<> from
other empty tuple-like types (e.g., array<T, 0>), completing the C++23
tuple-like interface for the zero-element tuple specialization.
The implementation includes:
- Constructor from forwarding reference to tuple-like types
- Allocator-aware constructor from tuple-like types
- Assignment operator from tuple-like types
- Const assignment operator from tuple-like types
- Const copy assignment operator for consistency with swap
- Defaulted copy assignment to maintain trivial copyability
The defaulted copy assignment operator is crucial to:
1. Maintain trivial copyability of tuple<> (all 5 trivial traits)
2. Support non-const assignment expressions: tuple<>& r = (t = u)
3. Avoid API/ABI breaks with existing code
The const copy assignment operator is placed near swap() as it serves
a similar purpose for proxy reference semantics in C++23.
PR libstdc++/119721
libstdc++-v3/ChangeLog:
* include/std/tuple (tuple<>): Add constructor and allocator-aware
constructor from tuple-like types. Add assignment operators from
tuple-like types. Add const copy assignment operator for proxy
reference semantics. Add defaulted copy assignment to maintain
trivial copyability.
* testsuite/23_containers/tuple/cons/119721.cc: New test for
constructors and assignments with empty tuple-like types.
* testsuite/20_util/tuple/element_access/trivial_empty_tuple.cc:
New test verifying tuple<> remains trivially copyable.
Signed-off-by: Osama Abdelkader <[email protected]>
---
v2:
- Fix trivial copyability of tuple<>.
- Add test for trivial copyability of tuple<>.
---
libstdc++-v3/include/std/tuple | 40 ++++++-
.../element_access/trivial_empty_tuple.cc | 19 +++
.../23_containers/tuple/cons/119721.cc | 113 ++++++++++++++++++
3 files changed, 171 insertions(+), 1 deletion(-)
create mode 100644
libstdc++-v3/testsuite/20_util/tuple/element_access/trivial_empty_tuple.cc
create mode 100644 libstdc++-v3/testsuite/23_containers/tuple/cons/119721.cc
diff --git a/libstdc++-v3/include/std/tuple b/libstdc++-v3/include/std/tuple
index c064a92df..af7ffdf40 100644
--- a/libstdc++-v3/include/std/tuple
+++ b/libstdc++-v3/include/std/tuple
@@ -1989,9 +1989,21 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
#if __cpp_lib_ranges_zip // >= C++23
constexpr void swap(const tuple&) const noexcept { /* no-op */ }
#endif
+
+#if __cpp_lib_tuple_like // >= C++23
+ // Const copy assignment operator for consistency with swap and other
+ // const member functions in C++23. This supports proxy reference
semantics.
+ constexpr const tuple&
+ operator=(const tuple&) const noexcept
+ { return *this; }
+#endif
+
// We need the default since we're going to define no-op
// allocator constructors.
tuple() = default;
+ // Defaulted copy assignment operator to maintain trivial copyability
+ // and support non-const assignment expressions.
+ tuple& operator=(const tuple&) = default;
// No-op allocator constructors.
template<typename _Alloc>
_GLIBCXX20_CONSTEXPR
@@ -2001,7 +2013,33 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
tuple(allocator_arg_t, const _Alloc&, const tuple&) noexcept { }
#if __cpp_lib_tuple_like // >= C++23
- // Comparison operators for tuple<> with other empty tuple-like types
+ template<__tuple_like _UTuple>
+ requires (!is_same_v<remove_cvref_t<_UTuple>, tuple>
+ && !is_same_v<remove_cvref_t<_UTuple>, allocator_arg_t>
+ && tuple_size_v<remove_cvref_t<_UTuple>> == 0)
+ constexpr
+ tuple(_UTuple&&) noexcept { }
+
+ template<typename _Alloc, __tuple_like _UTuple>
+ requires (!is_same_v<remove_cvref_t<_UTuple>, tuple>
+ && tuple_size_v<remove_cvref_t<_UTuple>> == 0)
+ constexpr
+ tuple(allocator_arg_t, const _Alloc&, _UTuple&&) noexcept { }
+
+ template<__tuple_like _UTuple>
+ requires (!is_same_v<remove_cvref_t<_UTuple>, tuple>
+ && tuple_size_v<remove_cvref_t<_UTuple>> == 0)
+ constexpr tuple&
+ operator=(_UTuple&&) noexcept
+ { return *this; }
+
+ template<__tuple_like _UTuple>
+ requires (!is_same_v<remove_cvref_t<_UTuple>, tuple>
+ && tuple_size_v<remove_cvref_t<_UTuple>> == 0)
+ constexpr const tuple&
+ operator=(_UTuple&&) const noexcept
+ { return *this; }
+
template<__tuple_like _UTuple>
requires (!__is_tuple_v<_UTuple> && tuple_size_v<_UTuple> == 0)
[[nodiscard]]
diff --git
a/libstdc++-v3/testsuite/20_util/tuple/element_access/trivial_empty_tuple.cc
b/libstdc++-v3/testsuite/20_util/tuple/element_access/trivial_empty_tuple.cc
new file mode 100644
index 000000000..7ec74740b
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/tuple/element_access/trivial_empty_tuple.cc
@@ -0,0 +1,19 @@
+// { dg-do compile { target c++11 } }
+
+// Test that tuple<> remains trivially copyable with defaulted assignment
+
+#include <tuple>
+#include <type_traits>
+
+// Check that tuple<> has the expected trivial properties
+static_assert(std::is_trivially_copyable<std::tuple<>>::value,
+ "tuple<> should be trivially copyable");
+static_assert(std::is_trivially_copy_constructible<std::tuple<>>::value,
+ "tuple<> should be trivially copy constructible");
+static_assert(std::is_trivially_move_constructible<std::tuple<>>::value,
+ "tuple<> should be trivially move constructible");
+static_assert(std::is_trivially_copy_assignable<std::tuple<>>::value,
+ "tuple<> should be trivially copy assignable");
+static_assert(std::is_trivially_move_assignable<std::tuple<>>::value,
+ "tuple<> should be trivially move assignable");
+
diff --git a/libstdc++-v3/testsuite/23_containers/tuple/cons/119721.cc
b/libstdc++-v3/testsuite/23_containers/tuple/cons/119721.cc
new file mode 100644
index 000000000..e87845c6b
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/tuple/cons/119721.cc
@@ -0,0 +1,113 @@
+// { dg-do compile { target c++23 } }
+// { dg-options "-std=c++23" }
+
+// Test for PR libstdc++/119721: tuple<> construction/assignment with array<T,
0>
+
+#include <tuple>
+#include <array>
+#include <memory>
+#include <testsuite_hooks.h>
+
+constexpr void
+test01()
+{
+ std::array<int, 0> a{};
+
+ // Constructor from array<int, 0>
+ std::tuple<> t1(a);
+ std::tuple<> t2(std::move(a));
+
+ // Assignment from array<int, 0>
+ std::tuple<> t3;
+ t3 = a;
+ t3 = std::move(a);
+
+ VERIFY( t1 == a );
+ VERIFY( t2 == a );
+ VERIFY( t3 == a );
+}
+
+constexpr void
+test02()
+{
+ // Test with non-comparable element type
+ struct NonComparable
+ {
+ void operator==(const NonComparable&) const = delete;
+ void operator<=>(const NonComparable&) const = delete;
+ };
+
+ std::array<NonComparable, 0> a{};
+
+ std::tuple<> t1(a);
+ std::tuple<> t2(std::move(a));
+
+ std::tuple<> t3;
+ t3 = a;
+ t3 = std::move(a);
+
+ VERIFY( t1 == a );
+}
+
+constexpr void
+test03()
+{
+ // Test assignment return type (non-const assignment)
+ std::tuple<> t, u;
+ std::tuple<>& r = (t = u);
+ VERIFY( &r == &t );
+
+ std::array<int, 0> a{};
+ std::tuple<>& r2 = (t = a);
+ VERIFY( &r2 == &t );
+}
+
+constexpr void
+test04()
+{
+ std::array<int, 0> a{};
+ const std::tuple<> t1;
+
+ // Const copy assignment from tuple
+ std::tuple<> t2;
+ t1 = t2;
+
+ // Const assignment from array
+ t1 = a;
+ t1 = std::move(a);
+
+ VERIFY( t1 == t2 );
+ VERIFY( t1 == a );
+}
+
+void
+test05()
+{
+ std::array<int, 0> a{};
+ std::allocator<int> alloc;
+
+ // Allocator constructor from array
+ std::tuple<> t1(std::allocator_arg, alloc, a);
+ std::tuple<> t2(std::allocator_arg, alloc, std::move(a));
+
+ VERIFY( t1 == a );
+ VERIFY( t2 == a );
+}
+
+int main()
+{
+ auto test_all = [] {
+ test01();
+ test02();
+ test03();
+ test04();
+ };
+
+ test_all();
+ static_VERIFY( test_all() );
+
+ // allocator test is not constexpr
+ test05();
+ return 0;
+}
+
--
2.43.0
