fix narrowing on dict (and typed dict) .get

[asukaminato0721]

fix #238

Taught the narrowing logic to treat dict.get("literal") calls as key facets, so truthiness and is not None checks now refine the corresponding dictionary entry

Added regression coverage for both explicit is not None and plain truthiness checks on literal-key .get calls to confirm the dictionary entry narrows as expected

[yangdanny97]

Awesome, thanks!

[yangdanny97]

The concern I have here is that this would apply to get methods on other types as well, not just dict and TypedDict. We might need a different type of Facet or Subject that somehow checks if we're calling a method on dict when we solve the narrow - if it's not a dict or TypedDict then we should not apply any narrowing.

[yangdanny97]

Back to you with comments, I think this is a great start, just needs one more piece to make sure it doesn't have any unintended effects

[asukaminato0721]

Introduced FacetOrigin/FacetSubject to track whether facet chains came from direct subscripts or .get calls, and only reuse them when the base is a dict-like source

Added runtime checks so .get-derived narrows only fire when the underlying type is a builtin dict or (partial) TypedDict

[yangdanny97]

Thanks!

[meta-codesync]

@yangdanny97 has imported this pull request. If you are a Meta employee, you can view this in D85959099.

[stroxler]

Review automatically exported from Phabricator review in Meta.