"alias that this" or "alias this : that"

[Tofu Ninja]

On Friday, 7 February 2014 at 07:52:22 UTC, Meta wrote:
> On Friday, 7 February 2014 at 07:36:57 UTC, Mike wrote:
>> Can someone please clarify for me which of these is the 
>> "preferred" syntax?
>>
>> alias that this;
>> alias this : that;
>>
>> The documentation here (http://dlang.org/class.html#AliasThis) 
>> doesn't mention the latter.  I'll do the unrewarding work of 
>> updating the docs if someone could just please tell me which 
>> syntax the core language folks prefer.
>>
>> Thanks.
>> Mike
>>
>> For context:
>> https://github.com/D-Programming-Language/dmd/pull/1413
>> https://github.com/D-Programming-Language/dmd/pull/1685
>
> `alias this : that` was discussed but has never been 
> implemented, so currently the only way to do it is `alias that 
> this`.

Is alias this = that not a thing? If not, why?