bug#34506: 27.0.50: push-button bug with basic text-property button

[Eli Zaretskii <eliz@gnu.org>]

> From: Robert Weiner <rsw@gnu.org>
> Date: Sun, 17 Feb 2019 18:46:09 -0500
> Cc: 34506@debbugs.gnu.org
> 
>  button-activate and push-button already include that additional code,
>  but you are trying to invoke them on a kind of "button" that they
>  don't know how to handle.  The problem is that "button" is overloaded
>  here: buttons created by Customize are not of the kind supported by
>  functions from button.el, you need to invoke functions described in
>  widget.info instead.  The "text-property buttons" mentioned in the
>  documentation of button-at etc. are those created by make-text-button
>  and insert-text-button, not those created by Customize.
> 
> I really don't fully understand what you are saying here.

I'm saying that a "button" created by Customize is radically different
from "buttons" that button.el functions can handle.  They are
different objects, and it's unfortunate that both kinds are named the
same.

> There is a section in widget.info which describes use of the (push-button)
> function which is defined in button.el

No, push-button described in widget.info is a type of widget, not a
function.  Again, the same name used for two very different things.

> Maybe the custom widgets use text-property buttons.

Custom widget button do indeed use text properties, but those
properties are entirely different from the properties used by
text-buttons created by button.el.  You can see that yourself by using
the describe-text-properties command with point on each type of
button.

> If you could, for each of the two types of buttons,
> just show code that finds the button at point and then
> activates it, so I can see the difference clearly.  That
> would be most helpful.

If you type "C-h c RET" on a button in a Customize buffer, you will
see that RET invokes Custom-newline there -- this is the way to "push"
the button widgets that Custom uses.  By contrast, if you do the same
on a button created by button.el, like a hyperlink in the *Help*
buffer, you will see that RET invokes the command push-button there, a
different command.  These are the two ways of pushing these two
different kinds of buttons.

> And what about (button-type (button-at (point))) returning
> nil when button-at returns non-nil.  Both of these functions
> operate on push-buttons as the button.el code reflects, right?
> If so, then that should be a bug.  If not, then it could use
> some explanation.

button-type requires a button as an argument, whereas button-at is
documented to return a marker for text-buttons.  So you cannot safely
invoke button-type if the button at point might be of the text-button
type.  IOW, the text-button is not an object, it is a place in the
buffer where the button starts.