Result of comma operator as l-value in C and C++

Using the result of the comma operator as l-value is not valid in C. But in C++, the result of the comma operator can be used as l-value if the right operand of the comma operator is l-value. For example, if we compile the following program as a C++ program, then it works and prints b = 30. And if we compile the same program as a C program, then it gives a warning/error in the compilation (Warning in Dev C++ and error in Code Blocks). 

C

#include<stdio.h>

int main()
{
  int a = 10, b = 20;
  (a, b) = 30; // Since b is l-value, this statement is valid in C++, but not in C.
  printf("b = %d", b);
  return 0;
}

OUTPUT:
error: lvalue required as left operand of assignment

C++

#include <iostream>
using namespace std;

int main()
{

    int a = 10, b = 20;
    (a, b) = 30;
    cout << "a = " << a << endl;
    cout << "b = " << b << endl;
    return 0;
}

Output

a = 10
b = 30

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