What's the result of += in C and C++?

I've got the following code:

#include <stdio.h>
int main(int argc, char **argv) {
    int i = 0;
    (i+=10)+=10;
    printf("i = %d\n", i);
    return 0;
}

If I try to compile it as a C source using gcc I get an error:

error: lvalue required as left operand of assignment

But if I compile it as a C++ source using g++ I get no error and when i run the executable:

i = 20

Why the different behavior?

Semantics of the compound assignment operators is different in C and C++:

C99 standard, 6.5.16, part 3:

> An assignment operator stores a value in the object designated by the left operand. An assignment expression has the value of the left operand after the assignment, but is not an lvalue.

In C++ 5.17.1:

> The assignment operator (=) and the compound assignment operators all group right-to-left. All require a modifiable lvalue as their left operand and return an lvalue with the type and value of the left operand after the assignment has taken place.

EDIT : The behavior of (i+=10)+=10 in C++ is undefined in C++98, but well defined in C++11. See this answer to the question by NPE for the relevant portions of the standards.

In addition to being invalid C code, the line

> (i+=10)+=10;

would result in undefined behaviour in both C and C++03 because it would modify i twice between sequence points.

As to why it's allowed to compile in C++:

> [C++N3242 5.17.1] The assignment operator (=) and the compound assignment operators all group right-to-left. All require a modifiable lvalue as their left operand and return an lvalue referring to the left operand.

The same paragraph goes on to say that

> In all cases, the assignment is sequenced after the value computation of the right and left operands, and before the value computation of the assignment expression.

This suggests that in C++11, the expression no longer has undefined behaviour.