if(int a = 0) lowered to "if (int a = (int a = 1;) , a)" ?

[aliak]

Found this out while just looking at lowerings. Wondering if 
anyone knows the semantics involved here and where they are 
documented?

And, is there anyway to do it with multiple variables?

I.e.

if (int a = 0, int b = 0) {
  // ...
}


Cheers,
- Ali