Why are compile-time constraints checked with an (inout int = 0) lambda in Phobos?

[Dicebot]

On Friday, 28 March 2014 at 11:17:10 UTC, Atila Neves wrote:
> Seriously, nobody? I'd've thought this was quite important.
>
> On Thursday, 27 March 2014 at 05:27:50 UTC, Atila Neves wrote:
>> Why the (inout int = 0) instead of an empty parameter list? I
>> checkout how isInputRange was implemented and I copied the 
>> idiom,
>> but I'd like to know why it's like that instead of cargo 
>> culting.
>> Thanks,
>>
>> Atila

Friend of mine (learning D) has recently asked same question and 
I failed to answer :( I never use it in constraint checks (empty 
parameter list instead) and have not encountered any issue.