Template constraints: opCmp and opUnary!"++"

[Timon Gehr]

On 12/20/2013 09:42 PM, monarch_dodra wrote:
> On Friday, 20 December 2013 at 17:48:03 UTC, Philippe Sigaud wrote:
>> On Fri, Dec 20, 2013 at 6:33 PM, Francesco Cattoglio
>> <francesco.cattoglio at gmail.com> wrote:
>>
>>> Is there any difference between "is(typeof(<somecode>))" and
>>> __traits(compiles, <somecode>)?
>>
>> I find the latter cleaner: its intent is more apparent. I use
>> is(typeof()) only for really testing for type existence.
>
> AFAIK, there is no real difference,  but "is(typeof())" is more
> "idiomatic" in phobos.

I used is(typeof(...)) as that was used in the original post, but I 
think it is actually better to use __traits(compiles,...). The 
difference is that typeof will not check if all referenced symbols are 
indeed accessible in the given scope. (Currently __traits(compiles, ...) 
won't either, but I assume this is a bug.)

Most non-trivial templates that use is(typeof(...)) in the constraint 
can be broken. (In the sense that it is possible to instantiate them 
even though their body does not compile.)