HasProperty + Get combo doesn't seem right for option bags

[domenic]

https://tc39.es/ecma402/#sec-getoption is what Ecma 402 does: only a single Get(), with a default if the result is undefined. https://heycam.github.io/webidl/#es-dictionary similarly for the web platform.

The dual step means you'll trigger double the meta-object protocol traps. This is mostly a theoretical concern since if you have a weird proxy where [[GetOwnProperty]] and [[Get]] disagree, you're screwed anyway. But it feels inelegant.

However that doesn't seem to account for your desired semantics of distinguishing present vs. undefined-but-not-present. I argue in #35 this is bad semantics, so I think that'd be the best way to fix this :). I'm not sure if there are any others.

[bakkot]

See prior discussion here and here.

To summarize, I think that just doing Get is reasonable for most options bags, because undefined is not a sensible value for the option and so can be treated equivalently to the option being unspecified, but if we are distinguishing between new Error('foo') and new Error('foo, { cause: e }) in general (cf #35) then we should still do so when e happens to be undefined.

[legendecas]

Closing as the proposal has reached consensus to stage 4.