Add the public API of Compat to the core

* lisp/emacs-lisp/compat.el: Add stub file with minimal definitions,
so that core packages, that haven't been installed from ELPA, can make
use of the public API and use more recent function signatures.
* lisp/progmodes/python.el (compat): Remove 'noerror flag, because
Compat can now be required without the real package being available.
* doc/lispref/package.texi (Forwards-Compatibility): Mention Compat
and link to the manual.
* etc/NEWS: Document change.  (Bug#66554)

doc/lispref/package.texi

@@ -28,6 +28,7 @@ these archives).
 * Multi-file Packages::     How to package multiple files.
 * Package Archives::        Maintaining package archives.
 * Archive Web Server::      Interfacing to an archive web server.
+* Forwards-Compatibility::  Supporting older versions of Emacs.
 @end menu
 
 @node Packaging Basics
@@ -399,3 +400,50 @@ Return the file. This will be the tarball for a multi-file
 package, or the single file for a simple package.
 
 @end table
+
+@node Forwards-Compatibility
+@section Supporting older versions of Emacs
+@cindex compatibility compat
+
+Packages that wish to support older releases of Emacs, without giving
+up on newer functionality from recent Emacs releases, one can make use
+of the Compat package on GNU ELPA.  By depending on the package, Emacs
+can provide compatibility definitions for missing functionality.
+
+The versioning of Compat follows that of Emacs, so next to the oldest
+version that a package relies on (via the @code{emacs}-package), one
+can also indicate what the newest version of Emacs is, that a package
+wishes to use definitions from:
+
+@example
+;; Package-Requires: ((emacs "27.2") (compat "29.1"))
+@end example
+
+Note that Compat provides replacement functions with extended
+functionality for functions that are already defined (@code{sort},
+@code{assoc}, @dots{}).  These functions may have changed their
+calling convention (additional optional arguments) or may have changed
+their behavior.  These functions must be looked up explicitly with
+@code{compat-function} or called explicitly with @code{compat-call}.
+We call them @dfn{Extended Definitions}.  In contrast, newly @dfn{Added
+Definitions} can be called as usual.
+
+@defmac compat-call fun &rest args
+This macro calls the compatibility function @var{fun} with @var{args}.
+Many functions provided by Compat can be called directly without this
+macro.  However in the case where Compat provides an alternative
+version of an existing function, the function call has to go through
+@code{compat-call}.
+@end defmac
+
+@defmac compat-function fun
+This macro returns the compatibility function symbol for @var{fun}.
+See @code{compat-call} for a more convenient macro to directly call
+compatibility functions.
+@end defmac
+
+For further details on how to make use of the package, see
+@ref{Usage,, Usage, compat, "Compat" Manual}.  In case you don't have
+the manual installed, you can also read the
+@url{https://elpa.gnu.org/packages/doc/compat.html#Usage, Online
+Compat manual}.

etc/NEWS

@@ -1396,6 +1396,13 @@ This minor mode generates the tags table automatically based on the
 current project configuration, and later updates it as you edit the
 files and save the changes.
 
++++
+** New package Compat
+Emacs now comes with a stub implementation of the
+forwards-compatibility Compat package from GNU ELPA.  This allows
+built-in packages to use the library more effectively, and helps
+preventing the installation of Compat if unnecessary.
+
 
 * Incompatible Lisp Changes in Emacs 30.1
 

lisp/emacs-lisp/compat.el

@@ -0,0 +1,92 @@
+;;; compat.el --- Stub of the Compatibility Library -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2021-2024 Free Software Foundation, Inc.
+
+;; Author:								\
+;;   Philip Kaludercic <philipk@posteo.net>,				\
+;;   Daniel Mendler <mail@daniel-mendler.de>
+;; Maintainer:								\
+;;   Daniel Mendler <mail@daniel-mendler.de>,				\
+;;   Compat Development <~pkal/compat-devel@lists.sr.ht>,
+;;   emacs-devel@gnu.org
+;; URL: https://github.com/emacs-compat/compat
+;; Keywords: lisp, maint
+
+;; This program 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.
+
+;; This program 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 this program.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;; The Compat package on ELPA provides forward-compatibility
+;; definitions for other packages.  While mostly transparent, a
+;; minimal API is necessary whenever core definitions change calling
+;; conventions (e.g. `plist-get' can be invoked with a predicate from
+;; Emacs 29.1 onward).  For core packages on ELPA to be able to take
+;; advantage of this functionality, the macros `compat-function' and
+;; `compat-call' have to be available in the core, usable even if
+;; users do not have the Compat package installed, which this file
+;; ensures.
+
+;; A basic introduction to Compat is given in the Info node `(elisp)
+;; Forwards Compatibility'.  Further details on Compat are documented
+;; in the Info node `(compat) Top' (installed along with the Compat
+;; package) or read the same manual online:
+;; https://elpa.gnu.org/packages/doc/compat.html.
+
+;;; Code:
+
+(defmacro compat-function (fun)
+  "Return compatibility function symbol for FUN.
+This is a pseudo-compatibility stub for core packages on ELPA,
+that depend on the Compat package, whenever the user doesn't have
+the package installed on their current system."
+  `#',fun)
+
+(defmacro compat-call (fun &rest args)
+  "Call compatibility function or macro FUN with ARGS.
+This is a pseudo-compatibility stub for core packages on ELPA,
+that depend on the Compat package, whenever the user doesn't have
+the package installed on their current system."
+  (cons fun args))
+
+;;;; Clever trick to avoid installing Compat if not necessary
+
+;; The versioning scheme of the Compat package follows that of Emacs,
+;; to indicate the version of Emacs, that functionality is being
+;; provided for.  For example, the Compat version number 29.2.3.9
+;; would attempt to provide compatibility definitions up to Emacs
+;; 29.2, while also designating that this is the third major release
+;; and ninth minor release of Compat, for the specific Emacs release.
+
+;; The package version of this file is specified programmatically,
+;; instead of giving a fixed version in the header of this file.  This
+;; is done to ensure that the version of compat.el provided by Emacs
+;; always corresponds to the current version of Emacs.  In addition to
+;; the major-minor version, a large "major release" makes sure that
+;; the built-in version of Compat is always preferred over an external
+;; installation.  This means that if a package specifies a dependency
+;; on Compat which matches the current or an older version of Emacs
+;; that is being used, no additional dependencies have to be
+;; downloaded.
+;;
+;; Further details and background on this file can be found in the
+;; bug#66554 discussion.
+
+;;;###autoload (push (list 'compat
+;;;###autoload             emacs-major-version
+;;;###autoload             emacs-minor-version
+;;;###autoload             9999)
+;;;###autoload       package--builtin-versions)
+
+(provide 'compat)
+;;; compat.el ends here

lisp/progmodes/python.el

@@ -273,7 +273,7 @@
 (eval-when-compile (require 'subr-x))   ;For `string-empty-p' and `string-join'.
 (require 'treesit)
 (require 'pcase)
-(require 'compat nil 'noerror)
+(require 'compat)
 (require 'project nil 'noerror)
 (require 'seq)