The issues with private state (either WeakMaps or private symbols) and Proxies seems to largely boil down to the lack of the ability to bind |this| in forwarding method invocations to the target. This is the same limitation that prevents Proxies from being able to fully replace __noSuchMethod__. In either case, the internal state of the object is |this| sensitive and methods called on the Proxy can either malfunction or, in the case of WeakMap stored state, are completely unable to work.
A solution to this would be to add an [[Invoke]] internal method and a corresponding "invoke" Proxy trap. awb has referred to this as [[CallProperty]](property), but I think it might actually be better to use [[Invoke]](method) so that super calls can be trapped. The difference between the two is visible to a Proxy: [[Invoke]] would be "get" (usually) and then "invoke", [[CallProperty]] would be "callProperty" and then potentially a "get" (when forwarded). [[Invoke]] would also allow .call and .apply to be trapped as "invoke" instead of not trapped at all. Outside of Proxies, this change wouldn't be observable.
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