safety: null checks

[Ola Fosheim Grøstad]

On Monday, 23 November 2020 at 12:28:39 UTC, Patrick Schluter 
wrote:
> No. null is not a trap representation as a C standard would 
> call it. It is a valid value for a pointer. Dereferencing it is 
> an entirely other thing.

A trap is an interrupt at the hardware level. It has nothing to 
do with C.

>> Trivially invalid.
>
> Nope.

Yes. "nothing" in not a valid value for "int". That is trivially 
invalid.

Try to think of null as an empty set.

> Nope. In the case of D. The error generated by dereferencing a 
> null pointer is a defined behaviour. As defined as is calling 
> abort() in a C program.

You are speaking of DMD, not the spec?

> The issue with null pointer dereferencing has nothing to do 
> with its definition but with its implementation as the defined 
> behaviour of aborting the program is not guaranteed in all 
> circumstances (null pointer + offset big enough to hit a real 
> page).

My argument is based on the spec and what is required to get to 
something that is consistent/sound and portable.

Also, as it has been pointed out, it does not work this way in 
shipping compilers so the spec should provide implemtation notes 
on what actually happens in shipping compilers.