bug#43593: 27.1; `customize-set-variable': wrong prompt for Boolean value

[Drew Adams <drew.adams@oracle.com>]

This bug exists since at least Emacs 22.

emacs -Q

(defcustom bar nil
  "@@@"
  :type 'boolean :group 'convenience)

M-x customize-set-variable bar
Set variable: bar

The prompt for the Boolean value is this:

[boolean] Set customized value for bar to: (y or n)

Set it to WHAT?

It turns out that `y' sets the value to `t' and `n' sets the value to
`nil'.  But there's no way of knowing that.

The fault is with function `custom-prompt-variable', or more precisely,
with `widget-prompt-value', and then with `widget-boolean-prompt-value'.
It's the latter that calls `y-or-n-p'.

The value returned by `y-or-n-p' is t or nil, which is OK.  But the
prompt and the expected inputs are wrong.

Even if the prompt said "[boolean] Set customized value for bar to `t':
(y or n)" it would be inadequate, because it suggests that if you reply
`n' then the variable value remains unchanged.  `y-or-n-p' is just wrong
here.

The comment for `widget-boolean-prompt-value' says that it toggles.  But
it does NOT toggle.  And `customize-set-variable' should NOT just toggle
the current value anyway.  It should always prompt you for the new
value, which may or may not be already the current value.  The command
needs to be consistent.

Maybe the `boolean' widget should be changed to use `widge-prompt-value,
not `widget-boolean-prompt-value' or something?  That it inherits from
the `toggle' widget is fine - that's for the Toggle button in the
Customize UI.  But `customize-set-variable' needs a better prompt
(better behavior).

In GNU Emacs 27.1 (build 1, x86_64-w64-mingw32)
 of 2020-08-12 built on CIRROCUMULUS
Repository revision: 86d8d76aa36037184db0b2897c434cdaab1a9ae8
Repository branch: HEAD
Windowing system distributor 'Microsoft Corp.', version 10.0.18362
System Description: Microsoft Windows 10 Pro (v10.0.1903.18362.1082)