These are two completely different things.

One is the provider of the API promising that the function supports compile-time evaluation.

The second is the consumer of the API ensuring that an expression has been compile-time evaluated. It's nice that the consumer can make sure, but that's not helpful if the provider of the API never specifically intended for a function to be evaluatable at compile-time.

> Whereas right now it's not allowed to.

Of course it's allowed, but you're at the mercy of the compiler's decisions.