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Retain None as constraints bottom if no bottoms were provided #19485

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Merged
ilevkivskyi merged 3 commits into from
Jul 21, 2025
Merged

Retain None as constraints bottom if no bottoms were provided #19485

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1da86bb
Retain `None` as constraints bottom if no bottoms were provided
sterliakov Jul 21, 2025
2ca095d
Add testcase
sterliakov Jul 21, 2025
82f3c74
Stop testing iterables for truthiness!
sterliakov Jul 21, 2025
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Stop testing iterables for truthiness!
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@sterliakov
sterliakov committed Jul 21, 2025
commit 82f3c7421fdd11422885698a33af2a64f63084ba
3 changes: 2 additions & 1 deletion mypy/solve.py
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Expand Up @@ -270,6 +270,7 @@ def solve_one(lowers: Iterable[Type], uppers: Iterable[Type]) -> Type | None:
uppers = new_uppers

# ...unless this is the only information we have, then we just pass it on.
lowers = list(lowers)
if not uppers and not lowers:
candidate = UninhabitedType()
candidate.ambiguous = True
Expand All @@ -285,7 +286,7 @@ def solve_one(lowers: Iterable[Type], uppers: Iterable[Type]) -> Type | None:
# This deviates from the general mypy semantics because
# recursive types are union-heavy in 95% of cases.
# Retain `None` when no bottoms were provided to avoid bogus `Never` inference.
bottom = UnionType.make_union(list(lowers))
bottom = UnionType.make_union(lowers)
else:
# The order of lowers is non-deterministic.
# We attempt to sort lowers because joins are non-associative. For instance:
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