Why is f(i = -1, i = -1) undefined behavior?
Since the operations are unsequenced, there is nothing to say that the instructions performing the assignment cannot be interleaved. It might be optimal to do so, depending on CPU architecture. The referenced page states this:
If A is not sequenced before B and B is not sequenced before A, then two possibilities exist:
evaluations of A and B are unsequenced: they may be performed in any order and may overlap (within a single thread of execution, the compiler may interleave the CPU instructions that comprise A and B)
evaluations of A and B are indeterminately-sequenced: they may be performed in any order but may not overlap: either A will be complete before B, or B will be complete before A. The order may be the opposite the next time the same expression is evaluated.
That by itself doesn't seem like it would cause a problem - assuming that the operation being performed is storing the value -1 into a memory location. But there is also nothing to say that the compiler cannot optimize that into a separate set of instructions that has the same effect, but which could fail if the operation was interleaved with another operation on the same memory location.
For example, imagine that it was more efficient to zero the memory, then decrement it, compared with loading the value -1 in. Then this:
f(i=-1, i=-1)
might become:
clear i
clear i
decr i
decr i
Now i is -2.
It is probably a bogus example, but it is possible.
First, "scalar object" means a type like a int, float, or a pointer (see What is a scalar Object in C++?).
Second, it may seem more obvious that
f(++i, ++i);
would have undefined behavior. But
f(i = -1, i = -1);
is less obvious.
A slightly different example:
int i;
f(i = 1, i = -1);
std::cout << i << "\n";
What assignment happened "last", i = 1, or i = -1? It's not defined in the standard. Really, that means i could be 5 (see harmic's answer for a completely plausible explanation for how this chould be the case). Or you program could segfault. Or reformat your hard drive.
But now you ask: "What about my example? I used the same value (-1) for both assignments. What could possibly be unclear about that?"
You are correct...except in the way the C++ standards committee described this.
If a side effect on a scalar object is unsequenced relative to another side effect on the same scalar object, the behavior is undefined.
They could have made a special exception for your special case, but they didn't. (And why should they? What use would that ever possibly have?) So, i could still be 5. Or your hard drive could be empty. Thus the answer to your question is:
It is undefined behavior because it is not defined what the behavior is.
(This deserves emphasis because many programmers think "undefined" means "random", or "unpredictable". It doesn't; it means not defined by the standard. The behavior could be 100% consistent, and still be undefined.)
Could it have been defined behavior? Yes. Was it defined? No. Hence, it is "undefined".
That said, "undefined" doesn't mean that a compiler will format your hard drive...it means that it could and it would still be a standards-compliant compiler. Realistically, I'm sure g++, Clang, and MSVC will all do what you expected. They just wouldn't "have to".
A different question might be Why did the C++ standards committee choose to make this side-effect unsequenced?. That answer will involve history and opinions of the committee. Or What is good about having this side-effect unsequenced in C++?, which permits any justification, whether or not it was the actual reasoning of the standards committee. You could ask those questions here, or at programmers.stackexchange.com.
A practical reason to not make an exception from the rules just because the two values are the same:
// config.h
#define VALUEA 1
// defaults.h
#define VALUEB 1
// prog.cpp
f(i = VALUEA, i = VALUEB);
Consider the case this was allowed.
Now, some months later, the need arises to change
#define VALUEB 2
Seemingly harmless, isn't it? And yet suddenly prog.cpp wouldn't compile anymore. Yet, we feel that compilation should not depend on the value of a literal.
Bottom line: there is no exception to the rule because it would make successful compilation depend on the value (rather the type) of a constant.
EDIT
@HeartWare pointed out that constant expressions of the form A DIV B are not allowed in some languages, when B is 0, and cause compilation to fail. Hence changing of a constant could cause compilation errors in some other place. Which is, IMHO, unfortunate. But it is certainly good to restrict such things to the unavoidable.