how does isInputRange(T) actually work?

[kevin via Digitalmars-d-learn]

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.