[diath]

It has been available since GCC 12.1 (May 2022), Clang 19.1 (Sep 2024), and Visual Studio 17.13 (2022~): https://godbolt.org/z/on1v6qdf3

These days compiler developers implement accepted standard features pretty fast.

[noitpmeder]

And tl::expected (a largely identical impl) has been available similarly as long!