Re: loading and evaluating the buffer are not the same thing.

[Richard Stallman]

    (eval-when-compile (require 'cl))

    If I do load-library <RET> gud <RET>

    then gud is loaded and cl is not, as you would expect.

That is because you're loading the compiled file.

    However, if I put gud.el in a buffer and do eval-buffer, cl *is* loaded.
    Even if I just evaluate the above expression cl is loaded.

That is because you are loading the source.
It would do the same if you load gud.el.

Does the manual's explanation eval-when-compile not make this clear?