DIP 1017--Add Bottom Type--Final Review

[Jonathan Marler]

On Thursday, 17 January 2019 at 20:15:20 UTC, H. S. Teoh wrote:
> On Thu, Jan 17, 2019 at 06:51:13PM +0000, Jonathan Marler via 
> Digitalmars-d wrote: [...]
>> To summarize, the bottom type allows you to declare a special 
>> pointer with no storage!
>> 
>>     TBottom*.sizeof == 0
>> 
>> This is also something that should be in the DIP and Walter 
>> should chime in with whether or not he thinks these semantics 
>> can actually be implemented.
>
> This would introduce an asymmetry with the rest of the pointer 
> types in the language which all have equal sizes, and this 
> asymmetry will percolate down to other language constructs, 
> causing special cases and inconsistencies everywhere.  I don't 
> recommend doing this.

Yes that's kind of the point :)  It's a new pointer type that has 
a unique size from all other pointer types.  This means you can 
no-longer assume all pointers types are the same size, you have a 
special case where it's size can be zero. This might warrant some 
code changes to handle this case, but does enable some new 
semantics which can be useful for library writers.  Case in point 
was you can now have a template parameter T* that the user can 
instantiate in such a way as to eliminate the pointer alltogether.

>
> A better approach might be to make TBottom* always equal to 
> null -- i.e., it's always illegal to dereference it because no 
> instances of TBottom can exist.
>

That's conceptually the same thing.  Saying that TBottom* is 
"always equal to null" is the same as saying it's a Unit Type, 
which is the same as saying that it contains no information so 
the storage needed is 0 (same as void).