improve code reuse (#3718)

lib/compress.js

@@ -3071,8 +3071,13 @@ merge(Compressor.prototype, {
         // If the node has been successfully reduced to a constant,
         // then its value is returned; otherwise the element itself
         // is returned.
+        //
         // They can be distinguished as constant value is never a
         // descendant of AST_Node.
+        //
+        // When `ignore_side_effects` is `true`, inspect the constant value
+        // produced without worrying about any side effects caused by said
+        // expression.
         AST_Node.DEFMETHOD("evaluate", function(compressor, ignore_side_effects) {
             if (!compressor.option("evaluate")) return this;
             var cached = [];
@@ -4766,21 +4771,17 @@ merge(Compressor.prototype, {
                     return exprs && make_sequence(this, exprs);
                 }
                 if (exp instanceof AST_Function && (!exp.name || !exp.name.definition().references.length)) {
-                    var node = this.clone();
                     exp.process_expression(false, function(node) {
                         var value = node.value && node.value.drop_side_effect_free(compressor, true);
                         return value ? make_node(AST_SimpleStatement, node, {
                             body: value
                         }) : make_node(AST_EmptyStatement, node);
                     });
-                    exp.walk(new TreeWalker(function(node) {
-                        if (node instanceof AST_Return && node.value) {
-                            node.value = node.value.drop_side_effect_free(compressor);
-                            return true;
-                        }
-                        if (node instanceof AST_Scope && node !== exp) return true;
-                    }));
-                    return node;
+                    scan_local_returns(exp, function(node) {
+                        if (node.value) node.value = node.value.drop_side_effect_free(compressor);
+                    });
+                    // always shallow clone to ensure stripping of negated IIFEs
+                    return this.clone();
                 }
                 return this;
             }