bug#39076: 26.3; rmail-summary-mark-deleted error with no arguments (or nil)

[Ed Sabol]

rmail-summary-mark-deleted is defined like so:

(defun rmail-summary-mark-deleted (&optional n undel) ...)

As you can see, n and undel are supposed to be optional arguments. However,
if I call

rmail-summary-mark-deleted with no arguments (or a nil argument), I get the
following lisp error:

(wrong-type-argument number-or-marker-p nil)

as shown in this backtrace:

Debugger entered--Lisp error: (wrong-type-argument number-or-marker-p nil)
  rmail-summary-mark-deleted(nil)
  rmail-summary-delete-forward(-1)
  rmail-summary-delete-backward(1)
  funcall-interactively(rmail-summary-delete-backward 1)
  call-interactively(rmail-summary-delete-backward nil nil)
  command-execute(rmail-summary-delete-backward)

The issue is that rmail-summary-mark-deleted evaluates (1- n) which causes
the error when n is nil.

However, this is really a symptom of another issue I'm seeing. Immediately
after I hit "g" to get new mail in some rmail-summary buffer,
rmail-current-message is sometimes nil until I move to another message, but I
haven't tracked down yet why that's happening and I haven't been able to
reproduce it with `emacs -Q'....

That said, I *can* reproduce the error with (rmail-summary-mark-deleted) with
`emacs -Q'. It is easiest to do with an email whose "Subject:" header is
"(rmail-summary-mark-deleted)". Then, in the rmail-summary buffer, go to the
message with that subject, move point to just after the close paren in
"(rmail-summary-mark-deleted)", and hit C-x C-e to evaluate the expression in
the rmail-summary buffer.


In GNU Emacs 26.3 (build 2, x86_64-pc-linux-gnu, X toolkit)
 of 2019-11-19 built on heasarcdev
Windowing system distributor 'Scientific Linux', version 11.0.11704000
System Description:     Scientific Linux release 6.10 (Carbon)

Recent messages:
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/software/usr/contrib/share/emacs/26.3/lisp/mail/rmail.el.gz and 
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Mark saved where search started [2 times]
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