compat-31: Copy {incf,decf} definitions from emacs.git

As the new tests show, the previous implementations were not robust if
PLACE causes side effects.

2 files changed, 12 insertions, 4 deletions

diff --git a/compat-31.el b/compat-31.el
index bb87724..84ff7bf 100644
--- a/compat-31.el
+++ b/compat-31.el
@@ -52,7 +52,8 @@ The DELTA is first added to PLACE, and then stored in PLACE.
 Return the incremented value of PLACE.
 
 See also `decf'."
-  `(setf ,place (+ ,place (or ,delta 1))))
+  (gv-letplace (getter setter) place
+    (funcall setter `(+ ,getter ,(or delta 1)))))
 
 (compat-defmacro decf (place &optional delta) ;; <compat-tests:decf>
   "Decrement PLACE by DELTA (default to 1).
@@ -61,7 +62,8 @@ The DELTA is first subtracted from PLACE, and then stored in PLACE.
 Return the decremented value of PLACE.
 
 See also `incf'."
-  `(setf ,place (- ,place (or ,delta 1))))
+  (gv-letplace (getter setter) place
+    (funcall setter `(- ,getter ,(or delta 1)))))
 
 ;;;; Defined in color.el
 
diff --git a/compat-tests.el b/compat-tests.el
index 802b2c8..7c67dc0 100644
--- a/compat-tests.el
+++ b/compat-tests.el
@@ -3357,7 +3357,10 @@
   (let ((x 0))
     (should-error (eval '(incf x 'symb) t))
     (should-error (eval '(incf x [a b c]) t))
-    (ignore x)))
+    (ignore x))
+  (let ((vec (vector 1 2 3)) (i 0))
+    (incf (aref vec (incf i)))
+    (should (equal [1 3 3] vec))))
 
 (ert-deftest compat-decf ()
   (let ((x 3))
@@ -3385,7 +3388,10 @@
   (let ((x 0))
     (should-error (eval '(decf x 'symb) t))
     (should-error (eval '(decf x [a b c]) t))
-    (ignore x)))
+    (ignore x))
+  (let ((vec (vector 1 2 3)) (i 2))
+    (decf (aref vec (decf i)))
+    (should (equal [1 1 3] vec))))
 
 (ert-deftest compat-color-blend ()
   ;; example from the docstring