how does isInputRange(T) actually work?

[John Colvin via Digitalmars-d-learn]

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.