Make (defmain ...) a clean macro

hy/core/macros.hy

@@ -200,16 +200,14 @@
 
 (defmacro defmain [args &rest body]
   "Write a function named \"main\" and do the if __main__ dance"
-  (let [[retval (gensym)]]
-    `(do
-      (defn main [~@args]
-        ~@body)
-
-      (when (= --name-- "__main__")
-        (import sys)
-        (setv ~retval (apply main sys.argv))
-        (if (integer? ~retval)
-          (sys.exit ~retval))))))
+  (let [[retval (gensym)]
+        [mainfn `(fn [~@args]
+                   ~@body)]]
+    `(when (= --name-- "__main__")
+       (import sys)
+       (setv ~retval (apply ~mainfn sys.argv))
+       (if (integer? ~retval)
+         (sys.exit ~retval)))))
 
 
 (defmacro-alias [defn-alias defun-alias] [names lambda-list &rest body]

tests/native_tests/native_macros.hy

@@ -259,3 +259,19 @@
 
 (defn test-botsbuildbots []
   (assert (> (len (first (Botsbuildbots))) 50)))
+
+
+(defn test-defmain []
+  "NATIVE: make sure defmain is clean"
+  (global --name--)
+  (setv oldname --name--)
+  (setv --name-- "__main__")
+  (defn main []
+    (print 'Hy)
+    42)
+  (try
+    (defmain [&rest args]
+      (main))
+    (except [e SystemExit]
+      (assert (= (str e) "42"))))
+  (setv --name-- oldname))