bug#38384: (next|previous)-buffer silent about not switching

[Juanma Barranquero <lekktu@gmail.com>]

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Package: emacs
Severity: minor
Tags: patch
X-Debbugs-Cc: rudalics@gmx.at

In some circumstances, next-buffer and previous-buffer do not have any
buffer to switch to, and in that case they silently do nothing.  (Strictly
speaking, they call switch-to-(next|prev)-buffer, which do nothing and
return nil).

I think they should throw a user-error, to inform the user about the
situation.

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diff --git i/lisp/window.el w/lisp/window.el
index 49fad75d3c..c8a5816b8c 100644
--- i/lisp/window.el
+++ w/lisp/window.el
@@ -4850,7 +4850,8 @@ next-buffer
     (user-error "Window is strongly dedicated to its buffer"))
    (t
     (dotimes (_ (or arg 1))
-      (switch-to-next-buffer)))))
+      (unless (switch-to-next-buffer)
+        (user-error "No next buffer"))))))
 
 (defun previous-buffer (&optional arg)
   "In selected window switch to ARGth previous buffer.
@@ -4864,7 +4865,8 @@ previous-buffer
     (user-error "Window is strongly dedicated to its buffer"))
    (t
     (dotimes (_ (or arg 1))
-      (switch-to-prev-buffer)))))
+      (unless (switch-to-prev-buffer)
+        (user-error "No previous buffer"))))))
 
 (defun delete-windows-on (&optional buffer-or-name frame)
   "Delete all windows showing BUFFER-OR-NAME.