Reuben Thomas <rrt@sc3d.org> writes:

> javascript-mode is an alias for js-mode, and some third-party modes (e.g.
> json-mode) derive from it.
>
> (provided-mode-derived-p 'js-mode 'prog-mode) == t
>
> but
>
> (provided-mode-derived-p 'javascript-mode 'prog-mode) == nil
>
> Hence in json-mode, (derived-mode-p 'prog-mode) evaluates to nil, which seems
> unfortunate.

Yup.  I've now changed the function to also work on MODE parameters that
are function aliases.