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

Using result of comma operator as l-value is not valid in C. But in C++, result of 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 C program, then it gives warning/error in compilation (Warning in Dev C++ and error in Code Blocks).

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#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);
  getchar();
  return 0;
}

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C++ Output:
b = 30

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