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Remember the pair in is_overlapping_types if at least one of them is an alias #20127

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Merged
merged 3 commits into from
Oct 28, 2025
Merged

Remember the pair in is_overlapping_types if at least one of them is an alias #20127

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95eb26c
One alias is enough to trigger infinite recursion
sterliakov Oct 27, 2025
f914bef
Only do that for recursive aliases?
sterliakov Oct 27, 2025
d3b662e
Reuse is_recursive_pair
sterliakov Oct 27, 2025
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2 changes: 1 addition & 1 deletion mypy/meet.py
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Original file line number Diff line number Diff line change
Expand Up @@ -346,7 +346,7 @@ def is_overlapping_types(
seen_types = set()
elif (left, right) in seen_types:
return True
if isinstance(left, TypeAliasType) and isinstance(right, TypeAliasType):
if is_recursive_pair(left, right):
seen_types.add((left, right))

left, right = get_proper_types((left, right))
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16 changes: 16 additions & 0 deletions test-data/unit/check-recursive-types.test
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Expand Up @@ -1024,3 +1024,19 @@ L = list[T]

A = L[A]
a: A = 1 # E: Incompatible types in assignment (expression has type "int", variable has type "A")

[case testRecursiveAliasInstanceOverlapCheck]
# flags: --warn-unreachable
from typing_extensions import TypeAlias

OneClass: TypeAlias = 'list[OneClass]'

class TwoClass(list['TwoClass']):
pass

def f(obj: OneClass) -> None:
if isinstance(obj, TwoClass):
reveal_type(obj) # N: Revealed type is "__main__.TwoClass"
else:
reveal_type(obj) # N: Revealed type is "builtins.list[...]"
[builtins fixtures/isinstancelist.pyi]