fix corner case in unused (#5629)

fixes #5628

lib/compress.js

@@ -6905,7 +6905,7 @@ Compressor.prototype.compress = function(node) {
                     initializations.add(def.id, node);
                     if (tw.parent() instanceof AST_ExportDefault) {
                         export_defaults[def.id] = true;
-                        return;
+                        return scan_ref_scoped(node, descend, true);
                     }
                     if (drop && !(def.id in lambda_ids)) lambda_ids[def.id] = 1;
                     return true;

test/compress/exports.js

@@ -560,3 +560,23 @@ issue_5444: {
         export var a = console;
     }
 }
+
+issue_5628: {
+    options = {
+        unused: true,
+    }
+    input: {
+        var a;
+        export default function f() {
+            for (a in 42);
+        }
+        console.log(a);
+    }
+    expect: {
+        var a;
+        export default function f() {
+            for (a in 42);
+        }
+        console.log(a);
+    }
+}