update if-let

Fixes #1189

src/boot/boot.janet

@@ -674,7 +674,11 @@
       (do
         (def bl (in bindings i))
         (def br (in bindings (+ 1 i)))
-        (tuple 'if (tuple 'def bl br) (aux (+ 2 i)) fal))))
+        (if (symbol? bl)
+          (tuple 'if (tuple 'def bl br) (aux (+ 2 i)) fal)
+          (tuple 'if (tuple 'def (def sym (gensym)) br)
+                 (tuple 'do (tuple 'def bl sym) (aux (+ 2 i)))
+                 fal)))))
   (aux 0))
 
 (defmacro when-let

test/suite-boot.janet

@@ -129,6 +129,8 @@
 (assert (= (if-let [a my-array k (next a 5)] :t :f) :f) "if-let 4")
 (assert (= (if-let [[a b] my-array] a) 1) "if-let 5")
 (assert (= (if-let [{:a a :b b} {:a 1 :b 2}] b) 2) "if-let 6")
+(assert (= (if-let [[a b] nil] :t :f) :f) "if-let 7")
+(assert (= (if-let [a true b false] b a) true) "if-let 8")
 
 (assert (= 14 (sum (map inc @[1 2 3 4]))) "sum map")
 (def myfun (juxt + - * /))