Narrow individual items when matching a tuple to a sequence pattern #16905
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Fixes #12364
When matching a tuple to a sequence pattern, this change narrows the type of tuple items inside the matched case:
This also works with nested tuples, recursively:
This only partially fixes issue #12364; see my comment there for more context.
This is my first contribution to mypy, so I may miss some context or conventions; I'm eager for any feedback!