Re: Source file '.../killer-source.el' newer than byte-compiled file

[Eli Zaretskii <eliz@gnu.org>]

> From: Emanuel Berg <moasenwood@zoho.eu>
> Date: Tue, 23 Apr 2019 19:39:34 +0200
> 
> Often when I do some changes to Elisp files and
> then run the byte-compiler with the following
> Makefile [1] I get this message for several
> files, but not all of them, and not always:
> 
>     Source file '~/path/to/killer-source.el' newer
>     than byte-compiled file
> 
> But what is the purpose of this message and why
> is it so capricious in terms of when
> it appears?

Its purpose is to tell you that Emacs loads a .elc file that is older
than the corresponding .el file.  See load-prefer-newer.

> Why am I told this specifically, and why for
> just certain files, not every one I edit?

It depends on the order of byte-compilation.

> Has it to do with files being `require'd before
> compilation has gotten that far? So if a.el is
> compiled into a.elc, but a requires b, only at
> that time b.elc hasn't been compiled *but* b.el
> has been edited?

Yes.