On Wed, May 6, 2026 at 10:10 PM Jonathan Wakely <[email protected]> wrote:
> On Wed, 6 May 2026 at 15:49, Tomasz Kaminski <[email protected]> wrote: > > > > > > > > On Wed, May 6, 2026 at 4:16 PM Jonathan Wakely <[email protected]> > wrote: > >> > >> 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. > >> --- > >> > >> Tested x86_64-linux. > >> > >> libstdc++-v3/include/std/tuple | 4 +++- > >> libstdc++-v3/testsuite/20_util/tuple/cons/78302.cc | 11 +++++++++++ > >> 2 files changed, 14 insertions(+), 1 deletion(-) > >> create mode 100644 libstdc++-v3/testsuite/20_util/tuple/cons/78302.cc > >> > >> diff --git a/libstdc++-v3/include/std/tuple > b/libstdc++-v3/include/std/tuple > >> index cbacd5a3c977..64b96fe4f599 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> && > ...) > > > > Could you add test for tuple of references (lvalue and rvalue)? I think > the traits > > gives correct result, but I am not sure. > >> > >> + = 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 000000000000..b3c6bd67fd27 > >> --- /dev/null > >> +++ b/libstdc++-v3/testsuite/20_util/tuple/cons/78302.cc > >> @@ -0,0 +1,11 @@ > >> +// { 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_nothrow_move_constructible_v<std::tuple<int>>); > > Is this good enough? > > > #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&>>); > Add them to test file, so I do not need to check is_move_constructible_v on reference, when looking at it. LGTM with that. > > (It passes) > >
