reduce this in block scopes (#2526)

fixes #2455

lib/ast.js

@@ -313,9 +313,9 @@ var AST_Scope = DEFNODE("Scope", "variables functions uses_with uses_eval parent
         enclosed: "[SymbolDef*/S] a list of all symbol definitions that are accessed from this scope or any subscopes",
         cname: "[integer/S] current index for mangling variables (used internally by the mangler)",
     },
-    get_defun_scope: function () {
+    get_defun_scope: function() {
         var self = this;
-        while (self.is_block_scope() && self.parent_scope) {
+        while (self.is_block_scope()) {
             self = self.parent_scope;
         }
         return self;

lib/compress.js

@@ -4645,8 +4645,10 @@ merge(Compressor.prototype, {
                 var init;
                 if (fixed instanceof AST_This) {
                     if (!(d.orig[0] instanceof AST_SymbolFunarg)
-                        && all(d.references, function(ref) {
+                        && all(d.references, d.scope.is_block_scope() ? function(ref) {
                             return d.scope === ref.scope;
+                        } : function(ref) {
+                            return d.scope === ref.scope.get_defun_scope();
                         })) {
                         init = fixed;
                     }

lib/scope.js

@@ -183,11 +183,7 @@ AST_Toplevel.DEFMETHOD("figure_out_scope", function(options){
             // scope when we encounter the AST_Defun node (which is
             // instanceof AST_Scope) but we get to the symbol a bit
             // later.
-            var parent_lambda = defun.parent_scope;
+            mark_export((node.scope = defun.parent_scope.get_defun_scope()).def_function(node), 1);
-            while (parent_lambda.is_block_scope()) {
-                parent_lambda = parent_lambda.parent_scope;
-            }
-            mark_export((node.scope = parent_lambda).def_function(node), 1);
         }
         else if (node instanceof AST_SymbolClass) {
             mark_export(defun.def_variable(node), 1);

test/compress/reduce_vars.js

@@ -4887,3 +4887,21 @@ issue_2416: {
     expect_stdout: "Foo"
     node_version: ">=6"
 }
+
+issue_2455: {
+    options = {
+        reduce_vars: true,
+        unused: true,
+    }
+    input: {
+        function foo() {
+            var that = this;
+            for (;;) that.bar();
+        }
+    }
+    expect: {
+        function foo() {
+            for (;;) this.bar();
+        }
+    }
+}