Complex if statement C/C++

I stumbled upon the ability to do this.

#include <iostream>
using namespace std;

int main() {
    if ( ({int i = 1024; i == 10;}) ) {
        cout << "In" << endl;
    }
}

The important disassembly area seems to be:

->  0x10000118f <+15>: movl   $0x400, -0x18(%rbp)       ; imm = 0x400 
    0x100001196 <+22>: cmpl   $0xa, -0x18(%rbp)
    0x10000119a <+26>: sete   %al
    0x10000119d <+29>: andb   $0x1, %al
    0x10000119f <+31>: movb   %al, -0x19(%rbp)
    0x1000011a2 <+34>: testb  $0x1, -0x19(%rbp)
    0x1000011a6 <+38>: je     0x1000011d9               ; <+89> at main.cpp:37

From examining this, it does seem like it takes the last statement (the comparison i == 10) as the boolean for the if statement.

I understand that this case doesn't allow me to use variable i within the if statement because of the scope operator, but wanted to know why the if statement decides to use i == 10 as the boolean statement.

For alternatives to this, I understand that a function call might be cleaner which returns a boolean that I can use to set to a variable for the if statement. However, I see MACROs that expand to this very similar style within glibc source code.

Is it an old style of programming with MACROs?

Is there a benefit to this I am missing?