what's this syntax?

["Michael Gong" <mwgong@xxxxxxxxxxxxxx>]

Hi, gcc develpers,

The following could be compiled by gcc. Can anyone tell me what's the 
feature of gcc to support the syntax ?

void foo() {
 if(
     ( __extension__
         (
             (
                 (
                     union {
                         int a;
                         int b;
                    }
                 ) { .a =7 }
             ) .b
          )
       ) == 7 ) {
             printf("abc\n");
   }else {
             printf("def\n");
  }
}

int main() {
foo();
}


Thanks.

Mike