keymap inheritance for non-sparse keymaps

[Miles Bader]

If `button-buffer-map' defines TAB to be `forward-button', and I do:

   (progn
     (setq map (make-sparse-keymap))
     (set-keymap-parent map button-buffer-map)
     (use-local-map map))

Then the TAB key in the current buffer invokes `forward-button', as I'd
expect.  However, if I do:

   (progn
     (setq map (make-keymap))
     (set-keymap-parent map button-buffer-map)
     (use-local-map map))

[the same, but with `make-keymap' instead of `make-sparse-keymap']

Is it a known problem that keymap inheritance doesn't work properly for
non-sparse keymaps?  I'd think it was a bug, but I vaguely remember
this being a misfeature kept around for historical reasons or something.

I don't see any mention of this in `(elisp)Inheritance and Keymaps'.

-Miles
-- 
I'm beginning to think that life is just one long Yoko Ono album; no rhyme
or reason, just a lot of incoherent shrieks and then it's over.  --Ian Wolff