From 5bd2c405cfbf0b7adff3dbd2316baec650271eea Mon Sep 17 00:00:00 2001
From: Daniel Mendler <mail@daniel-mendler.de>
Date: Sat, 11 Feb 2023 01:46:07 +0100
Subject: More robust method to require libraries conditionally

---
 compat-25.el   |  5 +++--
 compat-26.el   |  6 ++++--
 compat-27.el   |  8 +++++---
 compat-28.el   |  6 ++++--
 compat-29.el   | 12 +++++++-----
 compat-macs.el | 15 +++++++++------
 compat.el      | 10 ++++++++--
 7 files changed, 40 insertions(+), 22 deletions(-)

diff --git a/compat-25.el b/compat-25.el
index e710f86..f6ad2ad 100644
--- a/compat-25.el
+++ b/compat-25.el
@@ -21,8 +21,9 @@
 
 ;;; Code:
 
-(eval-when-compile (load "compat-macs.el" nil t t))
-(compat-declare-version "25.1")
+(eval-when-compile (require 'compat-macs))
+
+(compat-version "25.1")
 
 ;;;; Defined in alloc.c
 
diff --git a/compat-26.el b/compat-26.el
index 367b45e..956c4a7 100644
--- a/compat-26.el
+++ b/compat-26.el
@@ -21,8 +21,10 @@
 
 ;;; Code:
 
-(eval-when-compile (load "compat-macs.el" nil t t))
-(compat-declare-version "26.1")
+(eval-when-compile (require 'compat-macs))
+(compat-require compat-25 "25.1")
+
+(compat-version "26.1")
 
 ;;;; Defined in fns.c
 
diff --git a/compat-27.el b/compat-27.el
index 19db3a6..30aa97c 100644
--- a/compat-27.el
+++ b/compat-27.el
@@ -21,8 +21,10 @@
 
 ;;; Code:
 
-(eval-when-compile (load "compat-macs.el" nil t t))
-(compat-declare-version "27.1")
+(eval-when-compile (require 'compat-macs))
+(compat-require compat-26 "26.1")
+
+(compat-version "27.1")
 
 ;;;; Defined in fns.c
 
@@ -825,7 +827,7 @@ discarded."
 
 ;;;; Defined in map-ynp.el
 
-(compat-declare-version "26.2")
+(compat-version "26.2")
 
 (compat-defvar read-answer-short 'auto ;; <compat-tests:read-answer>
   "If non-nil, the `read-answer' function accepts single-character answers.
diff --git a/compat-28.el b/compat-28.el
index e3bde86..57a40f7 100644
--- a/compat-28.el
+++ b/compat-28.el
@@ -21,8 +21,10 @@
 
 ;;; Code:
 
-(eval-when-compile (load "compat-macs.el" nil t t))
-(compat-declare-version "28.1")
+(eval-when-compile (require 'compat-macs))
+(compat-require compat-27 "27.1")
+
+(compat-version "28.1")
 
 ;;;; Defined in fns.c
 
diff --git a/compat-29.el b/compat-29.el
index 11cccb6..9cf4a76 100644
--- a/compat-29.el
+++ b/compat-29.el
@@ -21,13 +21,15 @@
 
 ;;; Code:
 
-(eval-when-compile (load "compat-macs.el" nil t t))
-;; TODO Update to 29.1 as soon as the Emacs emacs-29 branch version bumped
-(compat-declare-version "29.0")
+(eval-when-compile (require 'compat-macs))
+(compat-require compat-28 "28.1")
 
-;;;; Preloaded in loadup.el
+;; Preloaded in loadup.el
+;; TODO Update to 29.1 as soon as the Emacs emacs-29 branch version bumped
+(compat-require seq "29.0") ;; <compat-tests:seq>
 
-(compat-guard t (require 'seq)) ;; <compat-tests:seq>
+;; TODO Update to 29.1 as soon as the Emacs emacs-29 branch version bumped
+(compat-version "29.0")
 
 ;;;; Defined in xdisp.c
 
diff --git a/compat-macs.el b/compat-macs.el
index 25b1088..9370b02 100644
--- a/compat-macs.el
+++ b/compat-macs.el
@@ -250,13 +250,16 @@ definition is generated.
         ,@(and local `((make-variable-buffer-local ',name)))
         ,@(and (eq local 'permanent) `((put ',name 'permanent-local t)))))))
 
-(defmacro compat-declare-version (version)
+(defmacro compat-version (version)
   "Set the Emacs version that is currently being handled to VERSION."
-  (prog1
-      (let ((before (1- (car (version-to-list version)))))
-        (when (and (< 24 before) (< emacs-major-version before))
-          `(require ',(require (intern (format "compat-%d" before))))))
-    (setq compat-macs--version version)))
+  (setq compat-macs--version version)
+  nil)
+
+(defmacro compat-require (feature version)
+  "Require FEATURE if the Emacs version is less than VERSION."
+  (when (version< emacs-version version)
+    (require feature)
+    `(require ',feature)))
 
 (provide 'compat-macs)
 ;;; compat-macs.el ends here
diff --git a/compat.el b/compat.el
index 56bed48..ed34965 100644
--- a/compat.el
+++ b/compat.el
@@ -46,8 +46,14 @@
 
 ;;; Code:
 
-(when (eval-when-compile (< emacs-major-version 29))
-  (require 'compat-29))
+;; Ensure that the newest compatibility layer is required at compile
+;; time and runtime, but only if needed.
+(eval-when-compile
+  (defmacro compat--maybe-require-29 ()
+    (when (< emacs-major-version 29)
+      (require 'compat-29)
+      '(require 'compat-29))))
+(compat--maybe-require-29)
 
 ;;;; Macros for extended compatibility function calls
 
-- 
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