[pjmlp]

If I remember correctly, Timur Doumler does some remarks on that sense on his presentation:

"Standard Attributes in C and C++"

https://youtu.be/TDKqAWtvH9c?si=b5quQkMe7XUvBbG7

[fluoridation]

Could you least give some timestamps? It's nearly two hours.

Anyway, while it is possible that some attributes can cause UB if misused, I very much doubt that's possible with [[likely]] and [[unlikely]], as they are just hints for the optimizer, and the optimizer is supposed to preserve semantical guarantees.