fix reduce_vars on arrow functions with this (#2504)

fixes #2496

lib/compress.js

@@ -2390,6 +2390,10 @@ merge(Compressor.prototype, {
                     }
                     return true;
                 }
+                if (node instanceof AST_This && self instanceof AST_Arrow) {
+                    result = false;
+                    return true;
+                }
             }));
             return result;
         });
@@ -4582,7 +4586,7 @@ merge(Compressor.prototype, {
             if (fixed instanceof AST_Defun) {
                 d.fixed = fixed = make_node(AST_Function, fixed, fixed);
             }
-            if (d.single_use && fixed instanceof AST_Function) {
+            if (d.single_use && is_func_expr(fixed)) {
                 if (d.scope !== self.scope
                     && (!compressor.option("reduce_funcs")
                         || d.escaped
@@ -4595,7 +4599,7 @@ merge(Compressor.prototype, {
                     if (d.single_use == "f") {
                         var scope = self.scope;
                         do {
-                            if (scope instanceof AST_Defun || scope instanceof AST_Function) {
+                            if (scope instanceof AST_Defun || is_func_expr(scope)) {
                                 scope.inlined = true;
                             }
                         } while (scope = scope.parent_scope);

test/compress/reduce_vars.js

@@ -4819,3 +4819,54 @@ issue_2485: {
     }
     expect_stdout: "6"
 }
+
+issue_2496: {
+    options = {
+        passes: 2,
+        reduce_funcs: true,
+        reduce_vars: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        function execute(callback) {
+            callback();
+        }
+        class Foo {
+            constructor(message) {
+                this.message = message;
+            }
+            go() {
+                this.message = "PASS";
+                console.log(this.message);
+            }
+            run() {
+                execute(() => {
+                    this.go();
+                });
+            }
+        }
+        new Foo("FAIL").run();
+    }
+    expect: {
+        class Foo {
+            constructor(message) {
+                this.message = message;
+            }
+            go() {
+                this.message = "PASS";
+                console.log(this.message);
+            }
+            run() {
+                (function(callback) {
+                    callback();
+                })(() => {
+                    this.go();
+                });
+            }
+        }
+        new Foo("FAIL").run();
+    }
+    expect_stdout: "PASS"
+    node_version: ">=6"
+}