(defconst bug-39884-orig-message "\
From: a <at> b.ce
To: c <at> d.de
Subject: Test
--text follows this line--
Test

<#mml type=message/rfc822 disposition=inline>
<#multipart type=alternative>
<#part type=text/plain charset=\"UTF-8\" disposition=inline nofile=yes>
Some text.
<#part type=text/html charset=\"UTF-8\" nofile=yes>
Some more text.
<#/multipart>
<#/mml>
")

(defconst bug-39884-correct-result "\
From: a <at> b.ce
To: c <at> d.de
Subject: Test
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary=\"=-=-=\"
--text follows this line--
--=-=-=
Content-Type: text/plain

Test


--=-=-=
Content-Type: message/rfc822
Content-Disposition: inline

--====-=-=
Content-Disposition: inline
MIME-Version: 1.0
Content-Type: multipart/alternative; boundary=\"====-=-=\"

Some text.

--====-=-=
Content-Type: text/html; charset=utf-8

Some more text.

--====-=-=--

--=-=-=--
")

(defconst 39884-incorrect-results-of-step-9
  "\
From: a <at> b.ce
To: c <at> d.de
Subject: Test
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary=\"=-=-=\"
--text follows this line--
--=-=-=
Content-Type: multipart/alternative; boundary=\"====-=-=\"
Content-Transfer-Encoding: base64

VGV4dAoK
--=-=-=
Content-Type: message/rfc822
Content-Disposition: inline

--====-=-=
Content-Disposition: inline
MIME-Version: 1.0
Content-Type: multipart/alternative; boundary=\"====-=-=\"

Some text.

--====-=-=
Content-Type: text/html; charset=utf-8

Some more text.

--====-=-=--

--=-=-=--
")

;; (defvar mml-text/plain-tag-literal-constant)

;; (defun check-setcdr (fun cell newcdr)
;;   (when (eq cell mml-text/plain-tag-literal-constant)
;;     (debug nil))
;;   (funcall fun cell newcdr))
;; (advice-add 'setcdr :around #'check-setcdr)
;; (load-library "mml.el") ;; `setcdr' has an opcode, so must run from source to advise it.

;; 2. M-x message-mode
(message-mode)
(setq buffer-file-name nil)
(setq buffer-auto-save-file-name nil)
;; 3. Clear scratch buffer, paste original message
(erase-buffer)
(insert bug-39884-orig-message)

;; 4. M-: (mml-to-mime) --> gives CORRECT result
(goto-char (point-min))
(mml-to-mime)
(cl-assert (equal (buffer-string) bug-39884-correct-result))

;; 5. Clear scratch buffer, paste original message
(erase-buffer)
(insert bug-39884-orig-message)

;; 6. Insert a new line "<#secure method=pgpmime mode=sign>" at start of 
;; message (after line "--text follows this line--"). This makes Emacs try 
;; to sign the mail.
(search-backward "--text follows this line--")
(forward-line)
(insert "<#secure method=pgpmime mode=sign>\n")

;; 7. M-: (mml-to-mime) --> throws (expected) signer name error
(cl-assert (condition-case ()
               (progn (goto-char (point-min))
                      (mml-to-mime)
                      nil)
             (error t)))

;; 8. Clear scratch buffer, paste original message
(erase-buffer)
(insert bug-39884-orig-message)

;; 9. M-: (mml-to-mime)
;;   --> broken result (first Content-Type after "text follows..." is 
;; wrong)
(goto-char (point-min))
(mml-to-mime)