bug#23926: defcustom with STANDARD=<non-pure-expression> gives confusing results

[Drew Adams]

Calling such an expression "non-pure" can be misleading.

The concept you are looking for is apparently just non-constant,
IOW, an expression whose value is not always the same.

IOW the expression is not referentially transparent.

But in Lisp non-pure often refers to or suggests side effects,
esp. mutation of structure.  That's not required for this bug
to manifest.  All that's needed is for the expression to not
necessarily return the same value each time it is evaluated. 

Anyway, yes, I agree that filing another bug for this
(separate from #4755) is good.