c++ - So why is i = ++i + 1 well-defined in C++11?

Question

I've seen the other similar questions and read the defect about it. But I still don't get it. Why is i = ++i + 1 well-defined in C++11 when i = i++ + 1 is not? How does the standard make this well defined?

By my working out, I have the following sequenced before graph (where an arrow represents the sequenced before relationship and everything is a value computation unless otherwise specified):

i = ++i + 1
     ^
     |
assignment (side effect on i)
 ^      ^
 |      |
☆i   ++i + 1
     ||    ^
    i+=1   |
     ^     1
     |
★assignment (side effect on i)
  ^      ^
  |      |
  i      1

I've marked a side effect on i with a black star and value computation of i with a white star. These appear to be unsequenced with respect to each other (according to my logic). And the standard says:

If a side effect on a scalar object is unsequenced relative to either another side effect on the same scalar object or a value computation using the value of the same scalar object, the behavior is undefined.

The explanation in the defect report didn't help me understand. What does the lvalue-to-rvalue conversion have to do with anything? What have I gotten wrong?

Answer 1

... or a value computation using the value of the same scalar object ...

The important part is bolded here. The left hand side does not use the value of i for the value computation. What is being computed is a glvalue. Only afterwards (sequenced after), the value of the object is touched and replaced.

Unfortunately this is a very subtle point :)