Simen Kjaeraas:
I believe at least part of the explanation is that Walter wants NotNull to implemented in a library. That's part of the reason for introducing @disable this().
Yes, I remember part of the discussions. And I agree that generally it's better to put meta-features in a language that allow library code to implement the desired features.
That's why recently in the main D newsgroup I have said that built-in vector ops may be better replaced by library code (what's missing is some built-in trick to avoid the creation of intermediate arrays in complex expression).
But implementing good non-null types in library code is hard (rather harder than implementing vector ops in library code on library defined vectors). I think @disable isn't enough to cover what Spec# shows good non-null types are meant to be.
Bye, bearophile
