[1.20 regression] incorrect reachability for comparisons between set and frozenset

def foo(x: frozenset[str], y: set[str]):
    if x == y:
        reveal_type(x)

With --warn-unreachable the reveal_type is marked as unreachable. This relates to new narrowing logic in mypy 1.20. An easy fix could be to mark frozenset as having a custom __eq__.

@hauntsaninja

@hauntsaninja I think a proper fix would be to re-use all the special-casing from dangerous_comparison(). There is a whole bunch of special-casing for builtin/stdlib collections that reflects runtime behavior. Also keeping these two in sync would be generally beneficial.

Fix reachability for frozenset and dict view narrowing

Fix reachability for frozenset and dict view narrowing (#21151)

Exclude mypy 1.20 which has several regressions.

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[1.20 regression] incorrect reachability for comparisons between set and frozenset #21143

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[1.20 regression] incorrect reachability for comparisons between set and frozenset #21143

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ilevkivskyi commented on Apr 1, 2026

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