This patch completes the implementation of P2321R2, giving tuple proper proxy
reference semantics.
The assignment operator is implemented as a template constrained to accept only
tuple<>. Consequently, the language does not consider it a copy assignment
operator, which prevents tuple<> from losing its trivially copyable status.
The _Tuple template parameter is defaulted, ensuring the operator remains
a viable candidate for assignment with an empty brace-init list.
PR libstdc++/119721
libstdc++-v3/ChangeLog:
* include/std/tuple (tuple<>::operator=(const _Tuple&) const)
[__cpp_lib_ranges_zip]: Define.
* testsuite/23_containers/tuple/cons/119721.cc: Test const
assignment.
Reviewed-by: Jonathan Wakely <[email protected]>
Signed-off-by: Tomasz Kamiński <[email protected]>
---
v7 applie corrections from review.
Pushed to trunk.
libstdc++-v3/include/std/tuple | 5 +++++
.../23_containers/tuple/cons/119721.cc | 20 ++++++++++++++-----
2 files changed, 20 insertions(+), 5 deletions(-)
diff --git a/libstdc++-v3/include/std/tuple b/libstdc++-v3/include/std/tuple
index 6edcf5e5553..d4db12557ab 100644
--- a/libstdc++-v3/include/std/tuple
+++ b/libstdc++-v3/include/std/tuple
@@ -1996,6 +1996,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
void swap(tuple&) noexcept { /* no-op */ }
#if __cpp_lib_ranges_zip // >= C++23
+ template<same_as<tuple> _Tuple = tuple>
+ constexpr const tuple&
+ operator=(const _Tuple&) const noexcept
+ { return *this; }
+
constexpr void swap(const tuple&) const noexcept
{ /* no-op */ }
#endif
diff --git a/libstdc++-v3/testsuite/23_containers/tuple/cons/119721.cc
b/libstdc++-v3/testsuite/23_containers/tuple/cons/119721.cc
index 240f1a5e8d7..1d152386fb7 100644
--- a/libstdc++-v3/testsuite/23_containers/tuple/cons/119721.cc
+++ b/libstdc++-v3/testsuite/23_containers/tuple/cons/119721.cc
@@ -69,12 +69,22 @@ test04()
{
std::array<int, 0> a{};
const std::tuple<> t1;
-
+
// Const assignment from array
- t1 = a;
- t1 = std::move(a);
-
- VERIFY( t1 == a );
+ std::tuple<> t2;
+ const std::tuple<>& r1 = (t1 = t2);
+ VERIFY( &r1 == &t1 );
+ const std::tuple<>& r2 = (t1 = std::move(t2));
+ VERIFY( &r2 == &t1 );
+
+ const std::tuple<>& r3 = (t1 = {});
+ VERIFY( &r3 == &t1 );
+
+ // Const assignment from array
+ const std::tuple<>& r4 = (t1 = a);
+ VERIFY( &r4 == &t1 );
+ const std::tuple<>& r5 = (t1 = std::move(a));
+ VERIFY( &r5 == &t1 );
}
void
--
2.51.0
