From: Jonathan Wakely <jwakely@redhat.com>
Date: Wed, 22 Apr 2026 14:37:16 +0000 (+0100)
Subject: libstdc++: Constrain tuple(tuple&&) [PR78302]
X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=5154144864a669c722fbb2170e79416c3ae5ff50;p=thirdparty%2Fgcc.git

libstdc++: Constrain tuple(tuple&&) [PR78302]

Since C++20 the std::tuple move constructor should be constrained (as
modified by LWG 2899).

We already define the move constructor as defaulted, but it's not
implicitly defined as deleted for non-move-constructible element types
because the _Tuple_impl(_Tuple_impl&&) constructor is user-provided and
unconstrained. For C++20 and later we use a requires-clause to constrain
the defaulted tuple(tuple&&) constructor.

Ideally we'd make this change pre-C++20 as well, but that's harder to do
without using a requires-clause, so this change is only for C++20 and
later. I think that's OK, but if we need to change it for pre-C++20
later we can consider inheriting from _Enable_copy_move<..., tuple> to
make the defaulted move constructor defined as deleted.

libstdc++-v3/ChangeLog:

	PR libstdc++/78302
	PR libstdc++/71301
	* include/std/tuple [C++20] (tuple(tuple&&)): Add
	requires-clause.
	* testsuite/20_util/tuple/cons/78302.cc: New test.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
---

diff --git a/libstdc++-v3/include/std/tuple b/libstdc++-v3/include/std/tuple
index cbacd5a3c97..64b96fe4f59 100644
--- a/libstdc++-v3/include/std/tuple
+++ b/libstdc++-v3/include/std/tuple
@@ -954,7 +954,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
       constexpr tuple(const tuple&) = default;
 
-      constexpr tuple(tuple&&) = default;
+      constexpr
+      tuple(tuple&&) requires (is_move_constructible_v<_Elements> && ...)
+	= default;
 
       template<typename... _UTypes>
 	requires (__constructible<const _UTypes&...>())
diff --git a/libstdc++-v3/testsuite/20_util/tuple/cons/78302.cc b/libstdc++-v3/testsuite/20_util/tuple/cons/78302.cc
new file mode 100644
index 00000000000..6998a9e418a
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/tuple/cons/78302.cc
@@ -0,0 +1,17 @@
+// { dg-do compile { target c++20 } }
+
+// Bug 78302 is_move_constructible_v<tuple<nonmovable>> should be false
+// LWG 2899. is_(nothrow_)move_constructible and tuple, optional and unique_ptr
+
+#include <tuple>
+#include <type_traits>
+
+struct NotMovable { NotMovable(NotMovable&&) = delete; };
+static_assert(!std::is_move_constructible_v<std::tuple<NotMovable>>);
+static_assert(!std::is_move_constructible_v<std::tuple<int, NotMovable>>);
+static_assert(!std::is_move_constructible_v<std::tuple<int&, NotMovable>>);
+static_assert(!std::is_move_constructible_v<std::tuple<int&&, NotMovable>>);
+static_assert(std::is_nothrow_move_constructible_v<std::tuple<int>>);
+static_assert(std::is_nothrow_move_constructible_v<std::tuple<int&>>);
+static_assert(std::is_nothrow_move_constructible_v<std::tuple<int&&>>);
+static_assert(std::is_nothrow_move_constructible_v<std::tuple<int&&, int&>>);