[PATCH] libstdc++: Restrict engaged -> disengaged std::optional fix [PR124910]

[Patrick Palka]

On Tue, 21 Apr 2026, Jonathan Wakely wrote:

> 
> 
> On Mon, 20 Apr 2026, 22:25 Patrick Palka, <ppalka@redhat.com> wrote:
>       Tested on x86_64-pc-linux-gnu, does this look OK for trunk?
> 
> 
> OK
> 
> (We could use addressof instead of __addressof as they're equally efficient nowadays, and I think we could just use & here? But it's fine as is.)

True, I went with __addressof to be locally consisten, and to minimize
cognitive overhead (using & would at first glance seem potentially buggy
whereas addressof is definitely safe).

> 
> 
>       -- >8 --
> 
>       It turns out a union without an active member does not violate C++20 core
>       constant expression rules and r16-8748 was really just a workaround for
>       a front end bug.  The actual underlying problem -- that the constexpr
>       evaluator treated an explicitly destroyed union member as still active
>       -- has been fixed by r16-8767 which makes this workaround unnecessary
>       for GCC.  Rather than remove the workaround, restrict it to Clang which
>       seems to have a similar bug.
> 
>               PR c++/124910
> 
>       libstdc++-v3/ChangeLog:
> 
>               * include/std/optional (_Optional_payload_base::_M_destroy):
>               Restrict r16-8748 workaround to Clang, and adjust comment.
>       ---
>        libstdc++-v3/include/std/optional | 5 ++---
>        1 file changed, 2 insertions(+), 3 deletions(-)
> 
>       diff --git a/libstdc++-v3/include/std/optional b/libstdc++-v3/include/std/optional
>       index 49ba7b6b45dd..b9cc019feff7 100644
>       --- a/libstdc++-v3/include/std/optional
>       +++ b/libstdc++-v3/include/std/optional
>       @@ -321,10 +321,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>              {
>               _M_engaged = false;
>               _M_payload._M_value.~_Stored_type();
>       -#if __cpp_lib_optional >= 202106L // full constexpr support
>       +#if defined(__clang__) && __cpp_lib_optional >= 202106L // full constexpr support
>               if (std::is_constant_evaluated())
>       -         // Ensure union _M_payload always has an active member, for sake
>       -         // of the core constant expression requirements.
>       +         // Work around PR124910 for Clang.
>                 std::construct_at(std::__addressof(_M_payload._M_empty));
>        #endif
>              }
>       --
>       2.54.0.rc1.54.g60f07c4f5c
> 
> 
>