Unexpectedly nice case of auto return type

[Basile B.]

On Tuesday, 3 December 2019 at 09:58:36 UTC, Jonathan M Davis 
wrote:
> On Tuesday, December 3, 2019 12:12:18 AM MST Basile B. via 
> Digitalmars-d- learn wrote:
>> I wish something like this was possible, until I change the 
>> return type of `alwaysReturnNull` from `void*` to `auto`.
>>
>>
>> ---
>> class A {}
>> class B {}
>>
>> auto alwaysReturnNull() // void*, don't compile
>> {
>>      writeln();
>>      return null;
>> }
>>
>> A testA()
>> {
>>      return alwaysReturnNull();
>> }
>>
>> B testB()
>> {
>>      return alwaysReturnNull();
>> }
>>
>> void main()
>> {
>>      assert( testA() is null );
>>      assert( testB() is null );
>> }
>> ---
>>
>> OMG, isn't it nice that this works ?
>>
>> I think that this illustrates an non intuitive behavior of auto
>> return types.
>> One would rather expect auto to work depending on the inner
>> return type.
>
> The void* version doesn't work, because void* doesn't 
> implicitly convert to a class type. It has nothing to do with 
> null. auto works thanks to the fact that typeof(null) was added 
> to the language a while back, and since class references can be 
> null, typeof(null) implicitly converts to the class type. 
> Before typeof(null) was added to the language, null by itself 
> had no type, since it's just a literal representing the null 
> value for any pointer or class reference. The result was that 
> using null in generic code or with auto could run into issues. 
> typeof(null) was added to solve those problems.
>
> - Jonathan M Davis

That's interesting details of D developement. Since you reply to 
the first message I think you have not followed but in the last 
reply I told that maybe we should be able to name the type of 
null. I think this relates to TBottom too a bit.