Make paragraph/filling functions in texinfo-mode work better

* lisp/textmodes/texinfo.el (texinfo-mode): Make paragraph
definitions more traditional.  This makes (forward-paragraph 1)
behave better.
(texinfo-mode): Set a new filling function.
(texinfo--fill-paragraph): New filling function.
(texinfo-fillable-commands): New variable (bug#49558).

lisp/textmodes/texinfo.el

@@ -411,11 +411,9 @@ value of `texinfo-mode-hook'."
 		      "\\)\\>"))
   (setq-local require-final-newline mode-require-final-newline)
   (setq-local indent-tabs-mode nil)
-  (setq-local paragraph-separate
-	      (concat "@[a-zA-Z]*[ \n]\\|"
-		      paragraph-separate))
   (setq-local paragraph-start (concat "@[a-zA-Z]*[ \n]\\|"
 				      paragraph-start))
+  (setq-local fill-paragraph-function 'texinfo--fill-paragraph)
   (setq-local sentence-end-base "\\(@\\(end\\)?dots{}\\|[.?!]\\)[]\"'”)}]*")
   (setq-local fill-column 70)
   (setq-local comment-start "@c ")
@@ -457,6 +455,44 @@ value of `texinfo-mode-hook'."
 		    prevent-filling
 		  (concat auto-fill-inhibit-regexp "\\|" prevent-filling)))))
 
+(defvar texinfo-fillable-commands '("@noindent")
+  "A list of commands that can be filled.")
+
+(defun texinfo--fill-paragraph (justify)
+  "Function to fill a paragraph in `texinfo-mode'."
+  (let ((command-re "\\(@[a-zA-Z]+\\)[ \t\n]"))
+    (catch 'no-fill
+      (save-restriction
+        ;; First check whether we're on a command line that can be
+        ;; filled by itself.
+        (or
+         (save-excursion
+           (beginning-of-line)
+           (when (looking-at command-re)
+             (let ((command (match-string 1)))
+               (if (member command texinfo-fillable-commands)
+                   (progn
+                     (narrow-to-region (point) (progn (forward-line 1) (point)))
+                     t)
+                 (throw 'no-fill nil)))))
+         ;; We're not on such a line, so fill the region.
+         (save-excursion
+           (let ((regexp (concat command-re "\\|^[ \t]*$\\|\f")))
+             (narrow-to-region
+              (if (re-search-backward regexp nil t)
+                  (progn
+                    (forward-line 1)
+                    (point))
+                (point-min))
+              (if (re-search-forward regexp nil t)
+                  (match-beginning 0)
+                (point-max)))
+             (goto-char (point-min)))))
+        ;; We've now narrowed to the region we want to fill.
+        (let ((fill-paragraph-function nil)
+              (adaptive-fill-mode nil))
+          (fill-paragraph justify))))
+    t))
 
 
 ;;; Insert string commands

test/lisp/textmodes/texinfo-resources/fill.erts

@@ -0,0 +1,70 @@
+Code:
+  (lambda ()
+    (texinfo-mode)
+    (fill-paragraph))
+
+Name: fill1
+Point-Char: |
+
+=-=
+@noindent Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed.
+=-=
+@noindent Everyone is permitted to copy and distribute verbatim copies
+of this license document, but changing it is not allowed.
+=-=-=
+
+Name: fill2
+Point-Char: |
+
+=-=
+@cindex relative| remapping, faces
+@cindex base remapping, faces
+  The following functions implement a higher-level interface to @code{face-remapping-alist}.
+=-=-=
+
+
+Name: fill3
+Point-Char: |
+
+=-=
+@cindex relative remapping, faces
+@cindex base remapping, faces|
+  The following functions implement a higher-level interface to @code{face-remapping-alist}.
+=-=-=
+
+Name: fill4
+Point-Char: |
+
+=-=
+@cindex relative remapping, faces
+@cindex base remapping, faces
+  The following functions| implement a higher-level interface to @code{face-remapping-alist}.
+=-=
+@cindex relative remapping, faces
+@cindex base remapping, faces
+  The following functions| implement a higher-level interface to
+@code{face-remapping-alist}.
+=-=-=
+
+Name: fill5
+Point-Char: |
+
+=-=
+@defun face-remap-add-relative face &rest specs
+|This function adds the face spec in @var{specs} as relative
+remappings for face @var{face} in the current buffer.  The remaining
+arguments, @var{specs}, should form either a list of face names, or a
+property list of attribute/value pairs.
+=-=
+@defun face-remap-add-relative face &rest specs
+This function adds the face spec in @var{specs} as relative remappings
+for face @var{face} in the current buffer.  The remaining arguments,
+@var{specs}, should form either a list of face names, or a property
+list of attribute/value pairs.
+=-=-=
+
+Name: fill6
+
+=-=
+@subsection This is a very very very very very very very very very very long subsection name
+=-=-=

test/lisp/textmodes/texinfo-tests.el

@@ -0,0 +1,33 @@
+;;; texinfo-tests.el --- Tests for texinfo.el  -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2021 Free Software Foundation, Inc.
+
+;; This file is part of GNU Emacs.
+
+;; GNU Emacs is free software: you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; GNU Emacs is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;;
+
+;;; Code:
+
+(require 'texinfo)
+(require 'ert)
+(require 'ert-x)
+
+(ert-deftest test-filling ()
+  (ert-test-erts-file (ert-resource-file "fill.erts")))
+
+;;; texinfo-tests.el ends here