[PATCH v6 2/2] libstdc++: Implement const copy-assignment for tuple<> [PR119721]

[Tomasz Kamiński]

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 constructor 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
        assigment.
---
v6:
 - add const assigment tuple test to this patch
 - test return type in const-assigment tests

 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 28a99ead499..c701164c84a 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