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[v1.5 regression] Repeating a type guard condition causes the guard to be lost #5546
Description
Describe the Bug
Reported impact: 2 false-positive diagnostics in the attrs library
from typing import Any, TypeGuard
class C(Any):
pass
def has(cls: type) -> TypeGuard[type[C]]:
return hasattr(cls, "tag")
def f(t: type[object]) -> None:
if has(t) or has(t):
reveal_type(t)
t.tag # object has no class attribute `tag`Sandbox Link
(Only applicable for extension issues) IDE Information
No response
Activity
Divyansh151005 commentedon Nov 12, 2026
#claim
Root cause: #5256 switched
NarrowOp::OrfromJoinStyle::SimpleMergetoJoinStyle::NarrowOf(type_info). Besides collapsing a gradual base back toAny(what #5256 needed), theNarrowOfjoin also returns the base type wheneverbase <: union(branches). A class with anAnybase is assignable from anything, sotype[object] <: type[C]holds and the join gives backtype[object], even with two different guards (has(t) or has2(t)). 1.4.0 revealstype[C]for both.Plan: keep the
NarrowOfjoin for gradual bases and for facets, but use the plain union of the branches for the root type of a non-gradual base, as before #5256.github-actions commentedon Nov 12, 2026
@Divyansh151005 you've claimed this issue — it's now assigned to you. Thanks for picking it up! 🎉