Handle empty defmain args

Closes hylang/hy#1707.

hy/core/macros.hy

@@ -223,11 +223,17 @@ Such 'o!' params are available within `body` as the equivalent 'g!' symbol."
 
 
 (defmacro defmain [args &rest body]
-  "Write a function named \"main\" and do the 'if __main__' dance"
-  (setv retval (gensym))
+  "Write a function named \"main\" and do the 'if __main__' dance.
+
+The symbols in `args` are bound to the elements in `sys.argv`, which means that
+the first symbol in `args` will always take the value of the script/executable
+name (i.e. `sys.argv[0]`).
+"
+  (setv retval (gensym)
+        restval (gensym))
   `(when (= --name-- "__main__")
      (import sys)
-     (setv ~retval ((fn [~@args] ~@body) #* sys.argv))
+     (setv ~retval ((fn [~@(or args `[&rest ~restval])] ~@body) #* sys.argv))
      (if (integer? ~retval)
        (sys.exit ~retval))))
 

tests/native_tests/native_macros.hy

@@ -325,14 +325,38 @@
   (global --name--)
   (setv oldname --name--)
   (setv --name-- "__main__")
-  (defn main []
-    (print 'Hy)
-    42)
+
+  (defn main [x]
+    (print (integer? x))
+    x)
+
   (try
     (defmain [&rest args]
-      (main))
+      (main 42))
+    (assert False)
     (except [e SystemExit]
       (assert (= (str e) "42"))))
+
+  ;; Try a `defmain` without args
+  (try
+    (defmain []
+      (main 42))
+    (assert False)
+    (except [e SystemExit]
+      (assert (= (str e) "42"))))
+
+  ;; Try a `defmain` with only one arg
+  (import sys)
+  (setv oldargv sys.argv)
+  (try
+    (setv sys.argv [1])
+    (defmain [x]
+      (main x))
+    (assert False)
+    (except [e SystemExit]
+      (assert (= (str e) "1"))))
+
+  (setv sys.argv oldargv)
   (setv --name-- oldname))
 
 (defn test-macro-namespace-resolution []