[quietbritishjim]

Ok, fair enough. I must admit I was looking at C99 as I thought that was most generally relevant, I don't follow recent C standards (as much as I do those for C++) and C23 hasn't been ratified yet. I've found your new snippet:

> In particular, all observable behavior (5.1.2.4) appears as specified in this document when it happens before an operation with undefined behavior in the execution of the program.

I consider that a change in the standard but, of course, that's allowed, especially as it's backwards compatible for well defined programs.

The wording is a little odd: it makes it sound a like you need some undefined behaviour in order to make the operations beforehand work, and, taken very literally, that operations between two undefined behaviours will work (because they're still "before an operation with undefined behavior"). But I suppose the intention is clear.

[uecker]

The definition of UB (which hasn't changed) is: "behavior, upon use of a nonportable or erroneous program construct or of erroneous data, for which this document imposes no requirement."

Note that the "for which" IMHO already makes this clear that this can not travel in time. When everything could be affected these words ("for which") would be meaningless.