That's because it's in gcc 13,
https://gcc.gnu.org/gcc-13/changes.html


   - Support for the <experimental/scope> header from v3 of the Library
   Fundamentals Technical Specification.


On Mon, Dec 12, 2022, 11:43 Pavel Vazharov via Std-Proposals <
std-proposals_at_[hidden]> wrote:

>
> I propose that we should be able to achieve this in C++26 with some
>> simple syntax as follows:
>>
>
> There is similar functionality already proposed by wg21.link/P0052.
> https://en.cppreference.com/w/cpp/experimental/scope_exit
> However, I don't see the above implemented in the libstdc++ for GCC 12.1.
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