how does isInputRange(T) actually work?

[Meta via Digitalmars-d-learn]

On Tuesday, 21 April 2015 at 19:11:43 UTC, John Colvin wrote:
> On Tuesday, 21 April 2015 at 19:06:39 UTC, kevin wrote:
>> enum bool isInputRange = is(typeof(
>>    (inout int = 0)
>>    {
>>        R r = R.init;     // can define a range object
>>        if (r.empty) {}   // can test for empty
>>        r.popFront();     // can invoke popFront()
>>        auto h = r.front; // can get the front of the range
>>    }));
>>
>>
>> ... is the current implementation in Phobos. But I can't seem 
>> to understand this syntax. What is (inout int = 0)? Why can a 
>> block follow it?
>>
>> My guess is that this is declaring some sort of function and 
>> testing if it is syntactically valid, but this is still 
>> strange to me.
>
> It's defining a lambda function and checking that it is 
> *semantically* valid.
>
> No idea what the `(inout int = 0)` is there for, I would have 
> thought it would be fine without it.

`inout int = 0` is just `inout int n = 0` without the variable 
name, which is just `inout int n` with a default argument of 0.