[1.20 regression] Mypy incorrectly marks a match case as unreachable

[rchen152]

I noticed this issue while updating the typing conformance tests. On this code snippet, which is taken from the tuples_type_compat conformance test, mypy 1.19.1 behaves correctly (errors on exactly one assert_type in each match case, showing that it infers one of the two acceptable types in each case), whereas mypy 1.20.0 incorrectly marks the code in the last match case as unreachable.

[hauntsaninja]

Thanks for the issue! I think #21192 should fix