Virtual functions
Lack of support for virtual functions makes this much less useful than if could be. Yeah, I know, I'm old fashioned and like my polymorphism.
I was (2 years retired) a very enthusiastic user of the assert macro with pretty much the same intent as this extension. Pre/post and inline predicates that should always hold. Absolutely invaluable in making your assumptions explicit and executable. "Don't call this function unless you can provide parameters in this shape". "After you call me I promise the following will hold".
Co-workers hated me for rubbing their noses in their laziness and stupidity. The number of times I've had a an email begging to be allowed to remove an assertion so that their code can "work". Some would just go ahead and remove an assertion as part of a PR without even bothering to ask. The entire concept of correctness seems to be lost on many programmers.
Problem of course is when the assertion turns out to be wrong or too strict and a case is discovered that brings everything to a screeching halt. The standard does seem to have considered that with ability to handle failures or ignore evaluation. Devil is in the details...