C++ sequence points and changes to evaluation order in C++17 [duplicate]

Question

I am a bit confused about the way the changes to C++17 affect the rule that you should not modify one variable 2 times between sequence points.

For example is this specified, unspecified or undefined in C++17?

void func(int x, int y);
int x=47;
func(x++,x++);

My guess would be UB.

edit: extra datapoint: clang HEAD in c++2a mode gives

prog.cc:8:11: warning: multiple unsequenced modifications to 'x' [-Wunsequenced]

edit2: same with gcc HEAD in C++2a mode

prog.cc:8:15: warning: operation on 'x' may be undefined [-Wsequence-point]

edit3: with preincrement gcc and clang give different results

Answer 1

The example in your question

int x = 0;
f(x++, x++);

Is now unspecified behavior, rather than undefined behavior. That means that it’s valid code, but what actually happens may differ between compilers.