improve literal return optimization (#1860)

lib/compress.js

@@ -1780,6 +1780,35 @@ merge(Compressor.prototype, {
         node.DEFMETHOD("has_side_effects", func);
     });
 
+    // determine if expression is constant
+    (function(def){
+        function all(list) {
+            for (var i = list.length; --i >= 0;)
+                if (!list[i].is_constant_expression())
+                    return false;
+            return true;
+        }
+        def(AST_Node, return_false);
+        def(AST_Constant, return_true);
+        def(AST_Unary, function(){
+            return this.expression.is_constant_expression();
+        });
+        def(AST_Binary, function(){
+            return this.left.is_constant_expression() && this.right.is_constant_expression();
+        });
+        def(AST_Array, function(){
+            return all(this.elements);
+        });
+        def(AST_Object, function(){
+            return all(this.properties);
+        });
+        def(AST_ObjectProperty, function(){
+            return this.value.is_constant_expression();
+        });
+    })(function(node, func){
+        node.DEFMETHOD("is_constant_expression", func);
+    });
+
     // tell me if a statement aborts
     function aborts(thing) {
         return thing && thing.aborts();
@@ -3004,7 +3033,7 @@ merge(Compressor.prototype, {
         if (exp instanceof AST_Function) {
             if (exp.body[0] instanceof AST_Return) {
                 var value = exp.body[0].value;
-                if (!value || value.is_constant()) {
+                if (!value || value.is_constant_expression()) {
                     var args = self.args.concat(value || make_node(AST_Undefined, self));
                     return make_sequence(self, args).transform(compressor);
                 }