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

[John Colvin]

On Friday, 30 August 2013 at 14:51:40 UTC, Namespace wrote:
> 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. :/

I believe typedef was deprecated due to no-one being able to 
agree on the semantics (although it's a bit before my time). I 
think it needs to come back as a very simple concept:

A type duplication. e.g. typedef int myInt; (or typedef myInt = 
int;) creates a new type that is exactly identical to int, but is 
a distinct type in the type system.

It should provide *exactly* identical behaviour to e.g. having 
two structs with identical contents but different names. E.g.

struct A{}

typedef A tA;
alias A aA;
alias tA atA;
typedef aA taA;

assert(!is(tA == A));
assert(is(aA == A));
assert(is(atA == tA));
assert(!is(taA == atA));
etc....

It's a really basic feature that D ought to have.