Subexpression type narrowing for match statements

[erictraut]

Currently, pyright narrows only the subject expression of a match statement. However, there are cases where a subject expression contains a subexpression that could also be narrowed, at least in theory.

• An example is provided in this thread: Match not type guarding TypedDicts pylance-release#4248
• Another example: Narrowing and exhaustiveness errors when matching on a tuple of optionals python/mypy#14731
• Another example: mypy unable to narrow type of tuple elements in case clause in pattern matching python/mypy#12364

Likewise, expressions that utilize the subject as a subexpression could be narrowed, in theory.

• An example: Type narrowing on object attribute with match statement doesn't apply python/mypy#14911

I don't yet know if either of these can be implemented in a way that is computationally feasible, but it's worth investigating.

[erictraut]

I've addressed the first three cases where the subject expression contains a subexpression that can also be narrowed.

The last case is quite different, and I'm not convinced it's worth implementing at this time. It's something that we could consider in the future.

[erictraut]

This is addressed in pyright 1.1.304, which is now published. It will also be included in a future release of pylance.