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

[Mike]

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