That's because it's in gcc 13,

On Mon, Dec 12, 2022, 11:43 Pavel Vazharov via Std-Proposals <> 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
However, I don't see the above implemented in the libstdc++ for GCC 12.1.
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