Why does this syntax compile on gcc?

Hi all you C gurus!

I've recently found a strange piece of C syntax in our codebase which I can't quite explain... (simplified version below)


#define ARR(a, i) (*({assert(i>0); &arr;}))

Note the semicolons and {} in the definition!
Is this code standard C ? What is this behaviour called?
It definitely compiles and runs as expected with gcc.

I'd only have known hot to write this using the comma-operator:


#define ARR(a, i) (*(assert(i>0), &arr))

Just wondering, how much of the C standard I don't know...

Phobon