compat-30: Add static-if

3 files changed, 45 insertions, 2 deletions

diff --git a/compat-30.el b/compat-30.el
index 72110f2..3aaf997 100644
--- a/compat-30.el
+++ b/compat-30.el
@@ -22,8 +22,7 @@
 ;;; 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-require compat-29 "29.0.90")
+(compat-require compat-29 "29.1")
 
 ;; TODO Update to 30.1 as soon as the Emacs emacs-30 branch version bumped
 (compat-version "30.0.50")
@@ -56,5 +55,15 @@
           tree)
       tree)))
 
+(compat-defmacro static-if (condition then-form &rest else-forms) ;; <compat-tests:static-if>
+  "A conditional compilation macro.
+Evaluate CONDITION at macro-expansion time.  If it is non-nil,
+expand the macro to THEN-FORM.  Otherwise expand it to ELSE-FORMS
+enclosed in a `progn' form.  ELSE-FORMS may be empty."
+  (declare (indent 2) (debug (sexp sexp &rest sexp)))
+  (if (eval condition lexical-binding)
+      then-form
+    (cons 'progn else-forms)))
+
 (provide 'compat-30)
 ;;; compat-30.el ends here
diff --git a/compat-tests.el b/compat-tests.el
index 1bb1f3e..98bb0d9 100644
--- a/compat-tests.el
+++ b/compat-tests.el
@@ -75,6 +75,7 @@
 (defmacro should-equal (a b)
   `(should (equal ,a ,b)))
 
+;; TODO replace `compat-tests--if' with `static-if'
 (defmacro compat-tests--if (cond then &rest else)
   (declare (indent 2))
   (if (eval cond t) then (macroexp-progn else)))
@@ -3049,5 +3050,13 @@
                            "(a (b ((c) . d) e) (f))"
                            "(a (b ((c) . d) e) (f))"))))))
 
+(ert-deftest compat-static-if ()
+  ;; TODO enable if CI Emacs 30 snapshot has been updated
+  (compat-tests--if (< emacs-major-version 30)
+    (progn
+      (should-equal "true" (static-if t "true"))
+      (should-not (static-if nil "true"))
+      (should-equal "else2" (static-if nil "true" "else1" "else2")))))
+
 (provide 'compat-tests)
 ;;; compat-tests.el ends here
diff --git a/compat.texi b/compat.texi
index 178106c..a6ea20b 100644
--- a/compat.texi
+++ b/compat.texi
@@ -3348,6 +3348,31 @@ older than 30.1.  Note that due to upstream changes, it might happen
 that there will be the need for changes, so use these functions with
 care.
 
+@defmac static-if condition then-form else-forms...
+Test @var{condition} at macro-expansion time.  If its value is
+non-@code{nil}, expand the macro to @var{then-form}, otherwise expand
+it to @var{else-forms} enclosed in a @code{progn}.  @var{else-forms}
+may be empty.
+
+Here is an example of its use from CC Mode, which prevents a
+@code{defadvice} form being compiled in newer versions of Emacs:
+@example
+@group
+(static-if (boundp 'comment-line-break-function)
+    (progn)
+  (defvar c-inside-line-break-advice nil)
+  (defadvice indent-new-comment-line (around c-line-break-advice
+                                             activate preactivate)
+    "Call `c-indent-new-comment-line' if in CC Mode."
+    (if (or c-inside-line-break-advice
+            (not c-buffer-is-cc-mode))
+        ad-do-it
+      (let ((c-inside-line-break-advice t))
+        (c-indent-new-comment-line (ad-get-arg 0))))))
+@end group
+@end example
+@end defmac
+
 @subsection Extended Definitions
 These functions must be called explicitly via @code{compat-call},
 since their calling convention or behavior was extended in Emacs 30.1: