Hello,

Gnus that comes with GNU Emacs 27.2 fails to display an iCalendar event
if it sets no "charset" parameter.  Strictly speaking, this does not
seem to be a Gnus' fault as section 3.1.4 of RFC5545 requires a
"charset" Content-Type parameter in MIME transports.  On the other hand,
there may be a buggy application that generates iCalendar streams in a
non-UTF-8 charset or does not specify the "charset" parameter.  The
attached patch would let Gnus tolerate these two types of bugs.

I hit the latter case where the "charset" property was missing, or more
precisely wrongly labelled "encoding":
======================================================================
Debugger entered--Lisp error: (wrong-type-argument char-or-string-p nil)
  gnus-icalendar-event-from-handle((#<buffer  *mm*-382724> ("text/calendar" (method . "REQUEST") (encoding . "utf-8") (filename . "event.ics")) nil nil nil nil nil nil) ("Jae-hyeon Park" "email@address\\.com"))
  gnus-icalendar-mm-inline((#<buffer  *mm*-382724> ("text/calendar" (method . "REQUEST") (encoding . "utf-8") (filename . "event.ics")) nil nil nil nil nil nil))
  mm-display-inline((#<buffer  *mm*-382724> ("text/calendar" (method . "REQUEST") (encoding . "utf-8") (filename . "event.ics")) nil nil nil nil nil nil))
  mm-display-part((#<buffer  *mm*-382724> ("text/calendar" (method . "REQUEST") (encoding . "utf-8") (filename . "event.ics")) nil nil nil nil nil nil) t)
  gnus-mime-display-single((#<buffer  *mm*-382724> ("text/calendar" (method . "REQUEST") (encoding . "utf-8") (filename . "event.ics")) nil nil nil nil nil nil))
  gnus-mime-display-part((#<buffer  *mm*-382724> ("text/calendar" (method . "REQUEST") (encoding . "utf-8") (filename . "event.ics")) nil nil nil nil nil nil))
  mapcar(gnus-mime-display-part ((#<buffer  *mm*-886772> ("text/plain" (charset . "utf-8")) 8bit (lambda nil (let ((inhibit-read-only t)) (delete-region #<marker (moves after insertion) at 7484 in *Article mail.list.physics.hep*> #<marker at 9377 in *Article mail.list.physics.hep*>))) ("inline") nil nil nil) (#<buffer  *mm*-382724> ("text/calendar" (method . "REQUEST") (encoding . "utf-8") (filename . "event.ics")) nil nil nil nil nil nil)))
  gnus-mime-display-mixed(((#<buffer  *mm*-886772> ("text/plain" (charset . "utf-8")) 8bit (lambda nil (let ((inhibit-read-only t)) (delete-region #<marker (moves after insertion) at 7484 in *Article mail.list.physics.hep*> #<marker at 9377 in *Article mail.list.physics.hep*>))) ("inline") nil nil nil) (#<buffer  *mm*-382724> ("text/calendar" (method . "REQUEST") (encoding . "utf-8") (filename . "event.ics")) nil nil nil nil nil nil)))
  gnus-mime-display-part((#("multipart/mixed" 0 15 (start nil from "seminar@organizer.org" buffer #<buffer  *mm*> boundary "===============5843370041342135681==")) (#<buffer  *mm*-886772> ("text/plain" (charset . "utf-8")) 8bit (lambda nil (let ((inhibit-read-only t)) (delete-region #<marker (moves after insertion) at 7484 in *Article mail.list.physics.hep*> #<marker at 9377 in *Article mail.list.physics.hep*>))) ("inline") nil nil nil) (#<buffer  *mm*-382724> ("text/calendar" (method . "REQUEST") (encoding . "utf-8") (filename . "event.ics")) nil nil nil nil nil nil)))
  gnus-display-mime()
  gnus-article-prepare-display()
  gnus-article-prepare(1787 nil)
  gnus-summary-display-article(1787)
  gnus-summary-next-page(nil)
  funcall-interactively(gnus-summary-next-page nil)
  call-interactively(gnus-summary-next-page nil nil)
  command-execute(gnus-summary-next-page)
======================================================================

Best regards,
Jae-hyeon