I can imagine that is "confusing" to you but "what programmers expect" is not what the people who talk about "spills" (like you did) expect. I understand where you come from when you mention them. But spills etc are all irrelevant to how we started this discussion:

As cpreciva stated that Kahan said that the behavior of the given example (the code where two sums of two some numbers are summed to doubles and compared to equality) should never give "false" you wrote "it seems to contradict his attitude ..." and your "proof" was Kahan's Java paper.

I showed that there's nowhere anything in Kahan's Java paper or K&R that would support such behavior. I quoted explicit lines form Kahan's paper which explicitly claim that the explicit evaluation to doubles should properly round away the bits that don't fit. And you continued to claim "but but FLT_EVAL_METHOD" whatever.

> We're talking past each other at this point.

I agree about that. You also gave false example 5/2 and you now explain it was your "intention" even if it's neither K&R nor what Kahan argued. You also use the name of the "optimization" for the basic design property of IEEE 754 operations, probably the best feature of IEEE 754 compared to all "common practices" of the times it was designed. I'd be glad to explain you what it is, you'd actually have to make the effort to understand the design principles of IEEE 754. If you're really interested, please post some place where we could start a new thread for that topic, I won't post in this thread more. There we can also discuss, if you are really interested, what Kahan actually says in the paper you posted.

> the point of what is being argued via a vis refactoring of expression trees changing calculation results

That was not a topic why the discussion started and I never discussed that here. Check for yourself. It started with pcwalton's (I simplify) "the bug is because of Kahan, see his Java paper" and I showed the opposite: whoever reads Kahan's paper can see that following his suggestions in the paper the bug can only be a bug. I also cited the exact pages.

I know there are a lot of people who would like to handle FP arithmetic identically to the integer one, but that can't work by the definition of FP. And please see my other post here -- it's surely time to continue the discussion somewhere else.