how does isInputRange(T) actually work?

[kevin via Digitalmars-d-learn]

On Tuesday, 21 April 2015 at 19:13:34 UTC, Meta wrote:
> 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.

Thanks for your responses. Don't lambdas need a => token? Also, 
what is the purpose of typeof? I would have expected a simple 
is() to work just fine.