为什么C ++ 17 GCC编译器会发出关于undefined的警告？

Question

According to C++17, there is no guarantee for order of evaluation in following expression. It is called unspecified behaviour.

int i = 0;
std::cout<<i<<i++<<std::endl;

C++17 GCC compiler gives following warning: Live Demo

prog.cc: In function 'int main()':
prog.cc:6:20: warning: operation on 'i' may be undefined [-Wsequence-point]
     std::cout<<i<<i++<<std::endl;

I don't understand, in c++17 above express no longer undefined behaviour, then Why does compiler gives warning about undefined?

Answer 1

Seems like gcc gives a warning because this is a corner case, or at least very close to being one. Portability seems to be one concern.

From the page https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html

The C++17 standard will define the order of evaluation of operands in more cases: in particular it requires that the right-hand side of an assignment be evaluated before the left-hand side, so the above examples are no longer undefined. But this warning will still warn about them, to help people avoid writing code that is undefined in C and earlier revisions of C++.

The standard is worded confusingly, therefore there is some debate over the precise meaning of the sequence point rules in subtle cases. Links to discussions of the problem, including proposed formal definitions, may be found on the GCC readings page, at http://gcc.gnu.org/readings.html .