>
>
> >           (generate-new-buffer-name "foo")
> >                => "foo<5>"
> >           (generate-new-buffer-name "foo" "foo<3>")
> >                => "foo<3>"
> >           (generate-new-buffer-name "foo" "foo<6>")
> >                => "foo<5>"
> >
> > However, for me all these forms evaluate to "foo"
>
> This might be obvious, but: Do you actually have existing buffers named
> "foo", "foo<2>", "foo<3>", and "foo<4>" as described in the paragraph before
> that example code?
>

I did not create the buffers with the given names.

alin