bug#28280: Maximally unhelpful diagnostic message.

[Paul Eggert <eggert@cs.ucla.edu>]

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> GM> A few seconds grep suggests this is due to the code in newline, added ~ 3
> GM> years ago in 62ee8b1 ("Add assertions to try and help catch bug#18913").
> GM> The complete lack of progress in that report since then suggests it may be
> GM> time to remove/downgrade those assertions.
> 
> Agreed.

Make sense, so I installed the attached into emacs-26.

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From 81b3261b0324137b7c8c05353227673f18264849 Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert@cs.ucla.edu>
Date: Sun, 17 Sep 2017 22:32:31 -0700
Subject: [PATCH] Remove old cl-assert calls in 'newline'

* lisp/simple.el (newline): Remove cl-assert calls
that didn't seem to be helping us debug Bug#18913,
and that caused problems as reported in Bug#28280.
Suggested by Glenn Morris (Bug#28280#8).
---
 lisp/simple.el | 39 ++++++++++++++++-----------------------
 1 file changed, 16 insertions(+), 23 deletions(-)

diff --git a/lisp/simple.el b/lisp/simple.el
index 1ffe181..4e42fd5 100644
--- a/lisp/simple.el
+++ b/lisp/simple.el
@@ -434,10 +434,6 @@ newline
           ;; Do the rest in post-self-insert-hook, because we want to do it
           ;; *before* other functions on that hook.
           (lambda ()
-            ;; We are not going to insert any newlines if arg is
-            ;; non-positive.
-            (or (and (numberp arg) (<= arg 0))
-                (cl-assert (eq ?\n (char-before))))
             ;; Mark the newline(s) `hard'.
             (if use-hard-newlines
                 (set-hard-newline-properties
@@ -456,25 +452,22 @@ newline
             ;; starts a page.
             (or was-page-start
                 (move-to-left-margin nil t)))))
-    (unwind-protect
-        (if (not interactive)
-            ;; FIXME: For non-interactive uses, many calls actually
-            ;; just want (insert "\n"), so maybe we should do just
-            ;; that, so as to avoid the risk of filling or running
-            ;; abbrevs unexpectedly.
-            (let ((post-self-insert-hook (list postproc)))
-              (self-insert-command arg))
-          (unwind-protect
-              (progn
-                (add-hook 'post-self-insert-hook postproc nil t)
-                (self-insert-command arg))
-            ;; We first used let-binding to protect the hook, but that
-            ;; was naive since add-hook affects the symbol-default
-            ;; value of the variable, whereas the let-binding might
-            ;; only protect the buffer-local value.
-            (remove-hook 'post-self-insert-hook postproc t)))
-      (cl-assert (not (member postproc post-self-insert-hook)))
-      (cl-assert (not (member postproc (default-value 'post-self-insert-hook))))))
+    (if (not interactive)
+	;; FIXME: For non-interactive uses, many calls actually
+	;; just want (insert "\n"), so maybe we should do just
+	;; that, so as to avoid the risk of filling or running
+	;; abbrevs unexpectedly.
+	(let ((post-self-insert-hook (list postproc)))
+	  (self-insert-command arg))
+      (unwind-protect
+	  (progn
+	    (add-hook 'post-self-insert-hook postproc nil t)
+	    (self-insert-command arg))
+	;; We first used let-binding to protect the hook, but that
+	;; was naive since add-hook affects the symbol-default
+	;; value of the variable, whereas the let-binding might
+	;; only protect the buffer-local value.
+	(remove-hook 'post-self-insert-hook postproc t))))
   nil)
 
 (defun set-hard-newline-properties (from to)
-- 
2.7.4