stream: fix Writable instanceof for subclasses

The current custom instanceof for `Writable` subclasses previously
returned false positives for instances of *other* subclasses of
`Writable` because it was inherited by these subclasses.

Fixes: #14943
PR-URL: #14945
Reviewed-By: Benjamin Gruenbaum <[email protected]>
Reviewed-By: Refael Ackermann <[email protected]>
Reviewed-By: Timothy Gu <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Ruben Bridgewater <[email protected]>
Reviewed-By: Evan Lucas <[email protected]>
Reviewed-By: James M Snell <[email protected]>

Author: addaleax

lib/_stream_writable.js

@@ -144,6 +144,8 @@ if (typeof Symbol === 'function' && Symbol.hasInstance) {
     value: function(object) {
       if (realHasInstance.call(this, object))
         return true;
+      if (this !== Writable)
+        return false;
 
       return object && object._writableState instanceof WritableState;
     }

test/parallel/test-stream-inheritance.js

@@ -49,3 +49,8 @@ Object.setPrototypeOf(CustomWritable.prototype, Writable.prototype);
 new CustomWritable();
 
 assert.throws(CustomWritable, /AssertionError: undefined does not inherit from CustomWritable/);
+
+class OtherCustomWritable extends Writable {}
+
+assert(!(new OtherCustomWritable() instanceof CustomWritable));
+assert(!(new CustomWritable() instanceof OtherCustomWritable));