assert() vs. enforce(), invariant() vs. ... ?

[Namespace]

On Friday, 30 August 2013 at 12:18:21 UTC, John Colvin wrote:
> On Friday, 30 August 2013 at 11:50:20 UTC, Jacob Carlborg wrote:
>> On 2013-08-30 12:56, John Colvin wrote:
>>
>>> alias this, but without implicit conversion. It just 
>>> implements all the
>>> op**** functions (including opDispatch).
>>
>> So what's the difference to Typedef, declared just below?
>
> Typedef uses Proxy to do the work. Proxy is a mixing template 
> for adding in to a struct/class and Typedef is a very simple 
> struct making use of it to implement a library typedef.
>
> Unfortunately, Typedef is rather lacking as far as being a 
> typedef is concerned, but that's not due to problems with Proxy.

What was the initial reason, to move typedef from the language 
into the library? I assume, that there is something special about 
typedef that can not be done with alias. If so, why was it moved?
It seems to be a trend, to move incomplete features from the 
language into the library (See also scope -> scoped). I do not 
like that. :/