Adding a private field to an object is side-effectful and should block instant-evaluation.

Assuming you have already executed this in the console:

class Base {
  constructor(o) { return o; } 
}; 

class A extends Base { 
   #x 
} 
var obj = {}; 

Then typing new A(obj) should not be evaluated, as it would mutate obj. Then subsequently, if you hit enter, it would issue an error as the object would already have the field, which is a TypeError with private Fields.

Note: Until private fields are enabled by default, this test case will take the
early return. However, I have tested locally that it fails without the fix, and
passes with the fix.

https://hg.mozilla.org/mozilla-central/rev/5db8d39515b3