Is `x^=y^=x^=y;` correct for swapping integers in C/C++?

Question

x^=y^=x^=y; is a tricky/amusing implementation of the XOR swap algorithm in C and C++. It parses as x^=(y^=(x^=y)); and uses the fact that assignment operators return the assigned value. But is it correct? The GCC 10.3.0 C compiler gives me the warning operation on ‘x’ may be undefined [-Wsequence-point] and clang 12.0.0 warning: unsequenced modification and access to 'x' [-Wunsequenced]. Compiling as C++, clang continues to warn the same way, and GCC stops. So is this code correct in either language? It looks rather sequenced to me, but maybe it's illegal to modify a variable two times in the same statement?

As pointed out in this answer, clang++ -std=c++17 does not give the warning. With -std=c++11 the situation is as described above. So maybe my question should be further broken down into C/C++11/C++17.

Answer 1

Add --std=c++17 to your compiler and you will not get warning anymore.

There is a part that is added to C++17 that prevents undefined behavior and you need that part for it:

In every simple assignment expression E1=E2 and every compound assignment expression E1@=E2, every value computation and side-effect of E2 is sequenced before every value computation and side effect of E1

Though, I suggest that you never use it in your code too.