Fix off-by-one regression in --iterate

* dash.el (--iterate): Evaluate FORM N-1, not N, times, fixing a
regression in Dash 2.18.0.  Don't evaluate INIT if N is zero, fixing
a bug since the introduction of the macro.

* dev/examples.el (-flatten-n): Add regression test.
(-iterate): Test for superfluous evaluations.

Fixes #373.

dash.el

@@ -685,12 +685,15 @@ Thus function FN should return a list."
 (defmacro --iterate (form init n)
   "Anaphoric version of `-iterate'."
   (declare (debug (form form form)))
-  (let ((res (make-symbol "result")))
-    `(let ((it ,init) ,res)
-       (dotimes (_ ,n)
-         (push it ,res)
-         (setq it ,form))
-       (nreverse ,res))))
+  (let ((res (make-symbol "result"))
+        (len (make-symbol "n")))
+    `(let ((,len ,n))
+       (when (> ,len 0)
+         (let* ((it ,init)
+                (,res (list it)))
+           (dotimes (_ (1- ,len))
+             (push (setq it ,form) ,res))
+           (nreverse ,res))))))
 
 (defun -iterate (fun init n)
   "Return a list of iterated applications of FUN to INIT.

dev/examples.el

@@ -343,7 +343,8 @@ new list."
     (-flatten-n 3 '((1 2) ((3 4) ((5 6))))) => '(1 2 3 4 5 6)
     (-flatten-n 0 '(3 4)) => '(3 4)
     (-flatten-n 0 '((1 2) (3 4))) => '((1 2) (3 4))
-    (-flatten-n 0 '(((1 2) (3 4)))) => '(((1 2) (3 4))))
+    (-flatten-n 0 '(((1 2) (3 4)))) => '(((1 2) (3 4)))
+    (-flatten-n 1 '(((1 . 2)) ((3 . 4)))) => '((1 . 2) (3 . 4)))
 
   (defexamples -replace
     (-replace 1 "1" '(1 2 3 4 3 2 1)) => '("1" 2 3 4 3 2 "1")
@@ -606,7 +607,11 @@ value rather than consuming a list to produce a single value."
     (--iterate nil nil 0) => ()
     (--iterate nil nil 1) => '(nil)
     (--iterate nil nil 2) => '(nil nil)
-    (--iterate (setq it -1) 1 3) => '(1 -1 -1))
+    (--iterate (setq it -1) 1 3) => '(1 -1 -1)
+    (let (l) (--iterate (push 1 l) (push 0 l) -1) l) => ()
+    (let (l) (--iterate (push 1 l) (push 0 l) 0) l) => ()
+    (let (l) (--iterate (push 1 l) (push 0 l) 1) l) => '(0)
+    (let (l) (--iterate (push 1 l) (push 0 l) 2) l) => '(1 0))
 
   (defexamples -unfold
     (-unfold (lambda (x) (unless (= x 0) (cons x (1- x)))) 10) => '(10 9 8 7 6 5 4 3 2 1)