I believe that's why Hoare said that the null reference, not null, was his billion dollar mistake.
It was a simple one-liner in his type checker "if getting a type error in a cast, if the source type is nulltype and the target type is any kind of reference, always let the cast happen". His (biased by hindsight) recollection is that he even felt it was a bit dirty at the time, but made a lot of code shorter.
It was a simple one-liner in his type checker "if getting a type error in a cast, if the source type is nulltype and the target type is any kind of reference, always let the cast happen". His (biased by hindsight) recollection is that he even felt it was a bit dirty at the time, but made a lot of code shorter.