fix corner case in collapse_vars (#4686)

fixes #4685

lib/compress.js

@@ -2119,7 +2119,9 @@ merge(Compressor.prototype, {
                             }
                             arg = null;
                         }
-                        if (node instanceof AST_ObjectIdentity && (fn_strict || !tw.find_parent(AST_Scope))) {
+                        if (node instanceof AST_ObjectIdentity && (fn_strict
+                            || !tw.find_parent(AST_Scope)
+                            || is_arrow(arg) && iife instanceof AST_New)) {
                             arg = null;
                             return true;
                         }

test/compress/arrows.js

@@ -703,3 +703,24 @@ issue_4666: {
     expect_stdout: "true"
     node_version: ">=4"
 }
+
+issue_4685: {
+    options = {
+        collapse_vars: true,
+        unused: true,
+    }
+    input: {
+        new function(f) {
+            if (f() !== this)
+                console.log("PASS");
+        }(() => this);
+    }
+    expect: {
+        new function(f) {
+            if (f() !== this)
+                console.log("PASS");
+        }(() => this);
+    }
+    expect_stdout: "PASS"
+    node_version: ">=4"
+}