convert void 0 to a new var

fixes mishoo#2585

lib/compress.js

@@ -90,6 +90,8 @@ function Compressor(options, false_by_default) {
         unsafe_regexp : false,
         unsafe_undefined: false,
         unused        : !false_by_default,
+        // TODO Is "void" a good name? "void_to_var"?
+        void          : false,
         warnings      : false,
     }, true);
     var global_defs = this.options["global_defs"];
@@ -199,6 +201,13 @@ merge(Compressor.prototype, {
         if (this.option("expression")) {
             node.process_expression(false);
         }
+        if (this.option("void")) {
+            try {
+                node.process_void();
+            } catch (e) {
+                console.warn(e);
+            }
+        }
         return node;
     },
     info: function() {
@@ -315,6 +324,75 @@ merge(Compressor.prototype, {
         self.transform(tt);
     });
 
+    AST_Scope.DEFMETHOD("process_void", function (compressor) {
+        var lambdas_with_var = [];
+        var tw = new TreeWalker(function(node) {
+            if (node instanceof AST_Var) {
+                // TODO Find parent efficiently
+                // TODO Should AST_Scope be used instead?
+                var lambda = tw.find_parent(AST_Lambda);
+                if (lambda && !lambda._first_var) {
+                    lambda._first_var = node;
+                    lambdas_with_var.push(lambda);
+                }
+            } else if (node instanceof AST_UnaryPrefix
+                && node.operator == "void"
+                && node.expression instanceof AST_Number) {
+                // TODO Find parent efficiently
+                // TODO Should AST_Scope be used instead?
+                var lambda = tw.find_parent(AST_Lambda);
+                if (lambda) {
+                    if (!lambda._void_uses) lambda._void_uses = [];
+                    // TODO Simpler way?
+                    node._parent = tw.parent();
+                    lambda._void_uses.push(node);
+                }
+            } else if (node instanceof AST_Catch
+                && node.argname.name.indexOf("undefined") == 0) {
+                // TODO Is there a cleaner way to do this?
+                return true;
+            }
+        });
+
+        this.walk(tw);
+
+        // TODO Is there a better way to replace a node?
+        var replacer = new TreeTransformer(function(node) {
+            if (node._replacement) {
+                return node._replacement;
+            }
+        });
+
+        lambdas_with_var.forEach(function(lambda) {
+            if (lambda._processed_void || !lambda._void_uses) return;
+
+            // TODO This is similar to make_sym() below. Should they be combined?
+            var new_var = make_node(AST_SymbolVar, null, {
+                name: lambda.make_var_name("undefined"),
+                scope: lambda,
+            });
+
+            var def = lambda.def_variable(new_var);
+            lambda.enclosed.push(def);
+
+            var new_var_def = make_node(AST_VarDef, lambda._first_var, {
+                name: new_var,
+                value: null,
+            });
+
+            lambda._first_var.definitions.push(new_var_def);
+
+
+            lambda._void_uses.forEach(function(void_use) {
+                // TODO Is there a better way to replace a node?
+                void_use._replacement = make_node(AST_SymbolRef, void_use, new_var);
+                void_use._parent.transform(replacer);
+            });
+
+            lambda._processed_void = true;
+        });
+    });
+
     (function(def){
         def(AST_Node, noop);
 

test/compress/void.js

@@ -0,0 +1,125 @@
+void_1: {
+    options = {
+        void: true,
+    }
+    input: {
+        var a = 0;
+        x = void 0;
+        if (void 0 === b)
+            c = void 0;
+
+        function f1() {
+            var a = 1;
+            console.log(void 0);
+        }
+
+        function f2(undefined) {
+            var a = 2;
+            console.log(void 0);
+        }
+
+        function f3() {
+            var undefined = 3;
+            console.log(void 0);
+        }
+
+        function f4() {
+            console.log(void 0);
+            for (var a = 4;;);
+            var b = 4;
+
+            function f5() {
+                var c = 5;
+                var d = 5;
+                console.log(void 0);
+            }
+        }
+
+        function f6() {
+            try {
+                var a = 6;
+                console.log(void 0);
+            } catch (e) {
+                console.log(void 0);
+            }
+        }
+    }
+    expect: {
+        var a = 0;
+        x = void 0;
+        if (void 0 === b)
+            c = void 0;
+
+        function f1() {
+            var a = 1, undefined;
+            console.log(undefined)
+        }
+
+        function f2(undefined) {
+            var a = 2, undefined$0;
+            console.log(undefined$0)
+        }
+
+        function f3() {
+            var undefined = 3, undefined$0;
+            console.log(undefined$0)
+        }
+
+        function f4() {
+            console.log(undefined);
+            for (var a = 4, undefined;;);
+            var b = 4;
+
+            function f5() {
+                var c = 5, undefined;
+                var d = 5;
+                console.log(undefined)
+            }
+        }
+
+        function f6() {
+            try {
+                var a = 6, undefined;
+                console.log(undefined)
+            } catch (e) {
+                console.log(undefined)
+            }
+        }
+    }
+}
+
+void_2: {
+    options = {
+        void: true,
+    }
+    input: {
+        f();
+        function f() {
+            var a = 1;
+            console.log(void 0);
+
+            try {
+                throw "FAIL";
+            } catch (undefined) {
+                console.log(void 0);
+            }
+        }
+    }
+    expect: {
+        f();
+        function f() {
+            var a = 1, undefined;
+            console.log(undefined);
+            try {
+                throw "FAIL"
+            } catch (undefined) {
+                console.log(void 0);
+            }
+        }
+
+    }
+    expect_stdout: [
+        "undefined",
+        "undefined",
+    ]
+}