Question about proxies and non-class inheritance with private fields

[email at trevormccauley.com]

Given "Private fields are designed to have semantics analogous to 
WeakMaps" 
(https://tc39.github.io/proposal-class-fields/#sec-private-names) and 
"All ECMAScript objects, including Proxies, and any user exotic object, 
should have a [[PrivateFieldValues]] internal slot iniitlaized to an 
empty List." 
(https://tc39.github.io/proposal-class-fields/#sec-objectcreate), does 
this mean the implementation of private fields will break uses of 
Object.create and Proxy usage as user-defined WeakMap implementations do 
now?

For example:

```js
const priv = new WeakMap()

class Foo {
   constructor () {
     priv.set(this, 'data')
   }
   logChar () {
     console.log(priv.get(this).charAt(0))
   }
}
```

new Foo().logChar() //-> 'd'
Object.create(new Foo()).logChar() // Err: no charAt in undefined
new Proxy(new Foo(), {}).logChar() // Err: no charAt in undefined

The problem being that `this` in `logChar` for the created object and 
proxy are different instances than the `this` associated with the 
private map's 'data'.  So the "private slots" of those instances do not 
match the objects they wrap, instead being empty/uninitialized.

I figured proxies would at least need to be able to handle situations 
like this. An example on MDN shows a proxy using methods like 
`setItem()` and `getItem()` from its target which, if implemented with 
private data, would could cause failures like the one seen above 
(https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy#A_complete_traps_list_example). 
I've seen mention of the "proxy problem" in various places with respect 
to private fields but wasn't sure if there was a solution in place for 
it - or even if it necessarily referred to what I'm describing here.  
But the observations above suggests that using proxies may not be safe - 
at least not without familiarity with the implementation of the object 
you're proxying. So I just want to make sure I understand it correctly.

Thanks,
Trevor