Compiled file behaves differently than the uncompiled file (Was Re: bug#19582)

[Jay Belanger <jay.p.belanger@gmail.com>]

Wolfgang Jenkner <wjenkner@inode.at> writes:
...
> In emacs -Q, do `M-x calc' then convert, say 1m, to cm, i.e.,
>
> ' 1 m <return> u c c m <return>
>
> This gives `100 cm' on top of the stack (a plain integer scalar).
>
> Next, start another emacs -Q instance, do `M-x calc' and then, before
> anything else, `M-: (load-library "calc-units")'.
>
> Now the same conversion exercise as before gives `100. cm' on the stack
> instead (a calc float).

The float appears because Calc does units simplifications differently if the
calc-units.elc file is loaded too soon.  I'm not sure why.  I could
probably narrow the problem down a little more, but not too far since
the problem is in the elc file, not the el file.

If the calc-units.elc file is loaded when it is supposed to be, the float doesn't
appear.
If the calc-units.elc file is loaded right away, then the float appears.
If the calc-units.el file is loaded right away, then the float doesn't appear.

What could cause the elc file to behave differently than the el file?