On Fri, 10 Jul 2026 at 12:25, Tomasz Kamiński <[email protected]> wrote:
>
> This implements remaining part of P2679R2, "Fixing std::start_lifetime_as
> and std::start_lifetime_as_array".
>
> For incomplete type _Tp, the is_implicit_lifetime_v<_Tp> check inside
> start_lifetime_as already leads to following "invalid use of incomplete
> type 'struct Incomplete'" error.
>
> libstdc++-v3/ChangeLog:
>
> * include/bits/stl_construct.h (std::start_lifetime_as_array):
> static_assert that _Tp is complete type.
> * testsuite/std/memory/start_lifetime_as/neg.cc: New test.
> ---
> Testing on x86_64-linux. *start_lifetime* test already passed.
> OK for trunk when all tests passes?
OK
>
> libstdc++-v3/include/bits/stl_construct.h | 8 ++++
> .../std/memory/start_lifetime_as/neg.cc | 45 +++++++++++++++++++
> 2 files changed, 53 insertions(+)
> create mode 100644 libstdc++-v3/testsuite/std/memory/start_lifetime_as/neg.cc
>
> diff --git a/libstdc++-v3/include/bits/stl_construct.h
> b/libstdc++-v3/include/bits/stl_construct.h
> index 4a0650f3cb8..9608d7fd8ca 100644
> --- a/libstdc++-v3/include/bits/stl_construct.h
> +++ b/libstdc++-v3/include/bits/stl_construct.h
> @@ -346,6 +346,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> inline _Tp*
> start_lifetime_as_array(void* __p, size_t __n) noexcept
> {
> + static_assert(sizeof(_Tp), "template argument must be a complete
> type");
> +
> auto __q = reinterpret_cast<_Tp*>(__p);
> if (!__n)
> return __q;
> @@ -360,6 +362,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> inline const _Tp*
> start_lifetime_as_array(const void* __p, size_t __n) noexcept
> {
> + static_assert(sizeof(_Tp), "template argument must be a complete
> type");
> +
> auto __q = reinterpret_cast<const _Tp*>(__p);
> if (!__n)
> return __q;
> @@ -376,6 +380,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> inline volatile _Tp*
> start_lifetime_as_array(volatile void* __p, size_t __n) noexcept
> {
> + static_assert(sizeof(_Tp), "template argument must be a complete
> type");
> +
> auto __q = reinterpret_cast<volatile _Tp*>(__p);
> if (!__n)
> return __q;
> @@ -392,6 +398,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> inline const volatile _Tp*
> start_lifetime_as_array(const volatile void* __p, size_t __n) noexcept
> {
> + static_assert(sizeof(_Tp), "template argument must be a complete
> type");
> +
> auto __q = reinterpret_cast<const volatile _Tp*>(__p);
> if (!__n)
> return __q;
> diff --git a/libstdc++-v3/testsuite/std/memory/start_lifetime_as/neg.cc
> b/libstdc++-v3/testsuite/std/memory/start_lifetime_as/neg.cc
> new file mode 100644
> index 00000000000..1adbd2f266d
> --- /dev/null
> +++ b/libstdc++-v3/testsuite/std/memory/start_lifetime_as/neg.cc
> @@ -0,0 +1,45 @@
> +// { dg-do compile { target c++23 } }
> +
> +#include <bit>
> +#include <memory>
> +
> +#include <testsuite_hooks.h>
> +#include <testsuite_allocator.h>
> +
> +struct Incomplete;
> +struct NonTrivial
> +{
> + NonTrivial() {}
> + ~NonTrivial() {}
> +};
> +
> +void test(void* p)
> +{
> + (void)std::start_lifetime_as<Incomplete>(p); // { dg-error
> "here" }
> + (void)std::start_lifetime_as_array<Incomplete>(p, 2); // { dg-error
> "here" }
> + (void)std::start_lifetime_as<NonTrivial>(p); // { dg-error
> "here" }
> + // Array are implicit-lifetime regardless of they element type
> + (void)std::start_lifetime_as_array<NonTrivial>(p, 2);
> +
> + const void* cp = p;
> + (void)std::start_lifetime_as<Incomplete>(cp); // { dg-error
> "here" }
> + (void)std::start_lifetime_as_array<Incomplete>(cp, 2); // { dg-error
> "here" }
> + (void)std::start_lifetime_as<NonTrivial>(cp); // { dg-error
> "here" }
> + (void)std::start_lifetime_as_array<NonTrivial>(cp, 2);
> +
> + volatile void* vp = p;
> + (void)std::start_lifetime_as<Incomplete>(vp); // { dg-error
> "here" }
> + (void)std::start_lifetime_as_array<Incomplete>(vp, 2); // { dg-error
> "here" }
> + (void)std::start_lifetime_as<NonTrivial>(vp); // { dg-error
> "here" }
> + (void)std::start_lifetime_as_array<NonTrivial>(vp, 2);
> +
> + const volatile void* cvp = vp;
> + (void)std::start_lifetime_as<Incomplete>(cvp); // { dg-error
> "here" }
> + (void)std::start_lifetime_as_array<Incomplete>(cvp, 2); // { dg-error
> "here" }
> + (void)std::start_lifetime_as<NonTrivial>(cvp); // { dg-error
> "here" }
> + (void)std::start_lifetime_as_array<NonTrivial>(cvp, 2);
> +}
> +
> +// { dg-prune-output "invalid use of incomplete type" }
> +// { dg-prune-output "template argument must be a complete type" }
> +// { dg-prune-output "static assertion" }
> --
> 2.54.0
>
