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Title: [1.20 regression] Mypy incorrectly marks a match case as unreachable · Issue #21189 · python/mypy · GitHub

Open Graph Title: [1.20 regression] Mypy incorrectly marks a match case as unreachable · Issue #21189 · python/mypy

X Title: [1.20 regression] Mypy incorrectly marks a match case as unreachable · Issue #21189 · python/mypy

Description: 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, sho...

Open Graph Description: 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...

X Description: 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...

Opengraph URL: https://github.com/python/mypy/issues/21189

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#21192https://github.com/python/mypy/pull/21192
[1.20 regression] Mypy incorrectly marks a match case as unreachablehttps://github.com/python/mypy/issues/21189#top
#21192https://github.com/python/mypy/pull/21192
bugmypy got something wronghttps://github.com/python/mypy/issues?q=state%3Aopen%20label%3A%22bug%22
https://github.com/rchen152
rchen152https://github.com/rchen152
on Apr 8, 2026https://github.com/python/mypy/issues/21189#issue-4227810083
this code snippethttps://mypy-play.net/?mypy=1.20.0&python=3.12&flags=warn-unreachable&gist=6492575ead11bc9aff0002d70956c862
tuples_type_compathttps://github.com/python/typing/blob/main/conformance/tests/tuples_type_compat.py
bugmypy got something wronghttps://github.com/python/mypy/issues?q=state%3Aopen%20label%3A%22bug%22
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