bug#42147: 28.0.50; pure vs side-effect-free, missing optimizations?

[Philipp Stephani]

Am Mi., 29. Juli 2020 um 22:39 Uhr schrieb Stefan Monnier
<monnier@iro.umontreal.ca>:
>
> > Regarding package-get-version: perhaps we should drop the 'pure' property
> > and just let callers wrap it in eval-when-compile?
>
> I'd rather not: the benefit is too subtle, I'd expect most users won't
> know/bother to use `eval-when-compile` around it even though I'd expect
> a vast majority of the uses can benefit from compile-time evaluation.
> In contrast the cases where the impurity will get in the way
> should be rare.

Arguments relating to probabilities ("most users", "vast majority",
"rare") don't really apply here, as the discussion is about
correctness. A pure function must be pure 100% of the time, no
exceptions allowed. `package-get-version' can't possibly be pure: it
doesn't take any argument, so if it were pure, it would have to return
a constant in the mathematical sense.