It seems like you're approaching this from the wrong end. I think C23
got that one right.

First, you should ask yourself
> What optimizations is [[pure]] supposed to enable?

Then, you can come up with a set of conditions that the attribute is
supposed to have. C23 does this right by formulating the attributes in
terms of optimizations they enable:
- [[reproducible]] functions can be called multiple times or once; the
same result would have been reproduced anyway. (Enable coalescing
calls)
- [[unsequenced]] functions can be treated as if they were
unsequenced. (Enable reordering and a whole lot more).

You cannot design [[pure]] in a vacuum.