There may be differences between C and C++ here (and I worked with C++ more recently than C), but: if that enum doesn't specify an underlying type it would be UB to have a value that isn't in the "member list" of the enum. In that case it should be possible to determine if all cases are covered.

If an underlying type is specified, then it should error since it is legal to have those values (unless the whole range of the underlying type is covered by the cases.

Again, that is what would be sensible from a C++ perspective, I don't know if C differs here.

EDIT: Also, and now I'm talking with my Rust user hat on: it is better to not have pointless UB. Yes some is needed to practically allow for optimisation. But C and C++ had a lot of UB that doesn't really help with making your code faster, such as this.