fix corner cases in reduce_vars (#1524)

Avoid variable substitution in the following cases:
- use of variable before declaration
- declaration within conditional code blocks
- declaration within loop body

fixes #1518
fixes #1525

lib/compress.js

@@ -179,38 +179,105 @@ merge(Compressor.prototype, {
 
     AST_Node.DEFMETHOD("reset_opt_flags", function(compressor, rescan){
         var reduce_vars = rescan && compressor.option("reduce_vars");
-        var unsafe = compressor.option("unsafe");
+        var safe_ids = [];
+        push();
         var tw = new TreeWalker(function(node){
+            if (!(node instanceof AST_Directive || node instanceof AST_Constant)) {
+                node._squeezed = false;
+                node._optimized = false;
+            }
             if (reduce_vars) {
                 if (node instanceof AST_Toplevel) node.globals.each(reset_def);
                 if (node instanceof AST_Scope) node.variables.each(reset_def);
                 if (node instanceof AST_SymbolRef) {
                     var d = node.definition();
                     d.references.push(node);
-                    if (d.fixed && (d.orig.length > 1 || isModified(node, 0))) {
+                    if (!d.fixed || isModified(node, 0) || !is_safe(d)) {
                         d.fixed = false;
                     }
                 }
-                if (node instanceof AST_Call && node.expression instanceof AST_Function) {
-                    node.expression.argnames.forEach(function(arg, i) {
-                        arg.definition().init = node.args[i] || make_node(AST_Undefined, node);
+                if (node instanceof AST_VarDef) {
+                    var d = node.name.definition();
+                    if (d.fixed === undefined) {
+                        d.fixed = node.value || make_node(AST_Undefined, node);
+                        mark_as_safe(d);
+                    } else {
+                        d.fixed = false;
+                    }
+                }
+                var iife;
+                if (node instanceof AST_Function
+                    && (iife = tw.parent()) instanceof AST_Call
+                    && iife.expression === node) {
+                    node.argnames.forEach(function(arg, i) {
+                        var d = arg.definition();
+                        d.fixed = iife.args[i] || make_node(AST_Undefined, iife);
+                        mark_as_safe(d);
                     });
                 }
-            }
-            if (!(node instanceof AST_Directive || node instanceof AST_Constant)) {
-                node._squeezed = false;
-                node._optimized = false;
+                if (node instanceof AST_If || node instanceof AST_DWLoop) {
+                    node.condition.walk(tw);
+                    push();
+                    node.body.walk(tw);
+                    pop();
+                    if (node.alternative) {
+                        push();
+                        node.alternative.walk(tw);
+                        pop();
+                    }
+                    return true;
+                }
+                if (node instanceof AST_LabeledStatement) {
+                    push();
+                    node.body.walk(tw);
+                    pop();
+                    return true;
+                }
+                if (node instanceof AST_For) {
+                    if (node.init) node.init.walk(tw);
+                    push();
+                    if (node.condition) node.condition.walk(tw);
+                    node.body.walk(tw);
+                    if (node.step) node.step.walk(tw);
+                    pop();
+                    return true;
+                }
+                if (node instanceof AST_ForIn) {
+                    if (node.init instanceof AST_SymbolRef) {
+                        node.init.definition().fixed = false;
+                    }
+                    node.object.walk(tw);
+                    push();
+                    node.body.walk(tw);
+                    pop();
+                    return true;
+                }
             }
         });
         this.walk(tw);
 
+        function mark_as_safe(def) {
+            safe_ids[safe_ids.length - 1][def.id] = true;
+        }
+
+        function is_safe(def) {
+            for (var i = safe_ids.length, id = def.id; --i >= 0;) {
+                if (safe_ids[i][id]) return true;
+            }
+        }
+
+        function push() {
+            safe_ids.push(Object.create(null));
+        }
+
+        function pop() {
+            safe_ids.pop();
+        }
+
         function reset_def(def) {
-            def.fixed = true;
+            def.fixed = undefined;
             def.references = [];
             def.should_replace = undefined;
-            if (unsafe && def.init) {
-                def.init._evaluated = undefined;
-            }
         }
 
         function isModified(node, level) {
@@ -1263,14 +1330,14 @@ merge(Compressor.prototype, {
             this._evaluating = true;
             try {
                 var d = this.definition();
-                if (compressor.option("reduce_vars") && d.fixed && d.init) {
+                if (compressor.option("reduce_vars") && d.fixed) {
                     if (compressor.option("unsafe")) {
-                        if (d.init._evaluated === undefined) {
-                            d.init._evaluated = ev(d.init, compressor);
+                        if (!HOP(d.fixed, "_evaluated")) {
+                            d.fixed._evaluated = ev(d.fixed, compressor);
                         }
-                        return d.init._evaluated;
+                        return d.fixed._evaluated;
                     }
-                    return ev(d.init, compressor);
+                    return ev(d.fixed, compressor);
                 }
             } finally {
                 this._evaluating = false;
@@ -3046,9 +3113,9 @@ merge(Compressor.prototype, {
         }
         if (compressor.option("evaluate") && compressor.option("reduce_vars")) {
             var d = self.definition();
-            if (d.fixed && d.init) {
+            if (d.fixed) {
                 if (d.should_replace === undefined) {
-                    var init = d.init.evaluate(compressor);
+                    var init = d.fixed.evaluate(compressor);
                     if (init.length > 1) {
                         var value = init[0].print_to_string().length;
                         var name = d.name.length;