util: improve Symbol.toStringTag handling

Only special handle `Symbol.toStringTag` if the property is not
enumerable or not the own property of the inspected object.

PR-URL: #27342
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

lib/internal/util/inspect.js

@@ -574,8 +574,15 @@ function formatRaw(ctx, value, recurseTimes, typedArray) {
 
   const constructor = getConstructorName(value, ctx);
   let tag = value[Symbol.toStringTag];
-  if (typeof tag !== 'string')
+  // Only list the tag in case it's non-enumerable / not an own property.
+  // Otherwise we'd print this twice.
+  if (typeof tag !== 'string' ||
+      tag !== '' &&
+      (ctx.showHidden ? hasOwnProperty : propertyIsEnumerable)(
+        value, Symbol.toStringTag
+      )) {
     tag = '';
+  }
   let base = '';
   let formatter = getEmptyFormatArray;
   let braces;

test/parallel/test-util-inspect.js

@@ -1267,8 +1267,20 @@ util.inspect(process);
 
 {
   // @@toStringTag
-  assert.strictEqual(util.inspect({ [Symbol.toStringTag]: 'a' }),
-                     "Object [a] { [Symbol(Symbol.toStringTag)]: 'a' }");
+  const obj = { [Symbol.toStringTag]: 'a' };
+  assert.strictEqual(
+    util.inspect(obj),
+    "{ [Symbol(Symbol.toStringTag)]: 'a' }"
+  );
+  Object.defineProperty(obj, Symbol.toStringTag, {
+    value: 'a',
+    enumerable: false
+  });
+  assert.strictEqual(util.inspect(obj), 'Object [a] {}');
+  assert.strictEqual(
+    util.inspect(obj, { showHidden: true }),
+    "{ [Symbol(Symbol.toStringTag)]: 'a' }"
+  );
 
   class Foo {
     constructor() {