NonNull template

[kdevel]

On Sunday, 20 April 2025 at 22:19:39 UTC, Jonathan M Davis wrote:
>> I consider nonconforming generally inacceptable.
>
> Writing a program which doesn't behave properly is always a 
> problem and should be consider unacceptable.

The problematic word is "behave". Only recently there was a
thread on reddit where the user Zde-G pinpointed the
problem while discussing a "new name" for undefined
behavior (UB) [5]:

     '90% of confusion about UB comes from the simple fact
     that something is called behavior. Defined, undefined,
     it doesn't matter: layman observes world behavior,
     layman starts thinking about what kind of behavior can
     there be.

     The mental model every programmer which observes that
     term for the first time is “some secret behavior which
     is too complex to write in the description of the
     language… but surely I can glean it from the compiler
     with some experiments”.

     This is entirely wrong mental model even for C and doubly
     so for Rust or Zig. And it takes insane amount of effort
     to teach **every single newcomer** that it's wrong model.
     I have seen **zero** exceptions.

     New name should talk about code, not about behavior.
     “Invalid code” or “forbidden code” or maybe “erroneous
     construct”, but something, anything which is not related
     to what happens in runtime.

     There are no runtime after UB, it's as simple as that.
     The only option if your code have UB is to go and fix
     the code… and yet the name doesn't include anything
     related to code at all and concentrates on entirely wrong
     thing.'

> [...]
>
> If you create a reference from a null pointer, you have a bug 
> whether the program is written in C++ or D.

That is not true. A D program like this:

     void main ()
     {
         int *p = null;
         ref int i = *p; // DMD v2.111.0
     }

is a valid program and there is no UB, no crash and no bug.
I already pointed this out earlier with reference to the D spec.
[3].

[3] https://dlang.org/spec/type.html#pointers
     "When a pointer to T is dereferenced, it must either contain 
a null
     value, or point to a valid object of type T."

[5] Zde-G kommentiert Blog Post: UB Might Be a Wrong Term for 
Newer Languages:
     
https://old.reddit.com/r/rust/comments/129mz8z/blog_post_ub_might_be_a_wrong_term_for_newer/jep231f/