It's been implemented in GCC(in review)[1], onramp[2] and my own slimcc[3]!

[1] https://patchwork.ozlabs.org/project/gcc/list/?series=470822

[2] https://github.com/ludocode/onramp

[3] https://github.com/fuhsnn/slimcc

Jen's macro that this was based on was an implementation of his own proposal (N3434) for `defer`, which was one of a few preceding what finally became TS25755! So, yes, C2y is lined up to have "defer: the feature", but until then, we can explore "defer: the macro" (at least on GCC builds, as formulated).