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

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Merged
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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6 changes: 4 additions & 2 deletions mypy/solve.py
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Original file line number Diff line number Diff line change
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 @@ -281,10 +282,11 @@ def solve_one(lowers: Iterable[Type], uppers: Iterable[Type]) -> Type | None:
# Process each bound separately, and calculate the lower and upper
# bounds based on constraints. Note that we assume that the constraint
# targets do not have constraint references.
if type_state.infer_unions:
if type_state.infer_unions and lowers:
# This deviates from the general mypy semantics because
# recursive types are union-heavy in 95% of cases.
bottom = UnionType.make_union(list(lowers))
# Retain `None` when no bottoms were provided to avoid bogus `Never` inference.
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:
Expand Down
4 changes: 2 additions & 2 deletions test-data/unit/check-recursive-types.test
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Expand Up @@ -54,7 +54,7 @@ reveal_type(flatten([1, [2, [3]]])) # N: Revealed type is "builtins.list[builti

class Bad: ...
x: Nested[int] = [1, [2, [3]]]
x = [1, [Bad()]] # E: List item 1 has incompatible type "list[Bad]"; expected "Union[int, Nested[int]]"
x = [1, [Bad()]] # E: List item 0 has incompatible type "Bad"; expected "Union[int, Nested[int]]"
[builtins fixtures/isinstancelist.pyi]

[case testRecursiveAliasGenericInferenceNested]
Expand Down Expand Up @@ -605,7 +605,7 @@ class NT(NamedTuple, Generic[T]):
class A: ...
class B(A): ...

nti: NT[int] = NT(key=0, value=NT(key=1, value=A())) # E: Argument "value" to "NT" has incompatible type "NT[A]"; expected "Union[int, NT[int]]"
nti: NT[int] = NT(key=0, value=NT(key=1, value=A())) # E: Argument "value" to "NT" has incompatible type "A"; expected "Union[int, NT[int]]"
reveal_type(nti) # N: Revealed type is "tuple[builtins.int, Union[builtins.int, ...], fallback=__main__.NT[builtins.int]]"

nta: NT[A]
Expand Down
18 changes: 18 additions & 0 deletions test-data/unit/check-typeddict.test
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Expand Up @@ -4271,3 +4271,21 @@ reveal_type(dicts.TF) # N: Revealed type is "def (*, user_id: builtins.int =) -
reveal_type(dicts.TotalFalse) # N: Revealed type is "def (*, user_id: builtins.int =) -> TypedDict('__main__.Dicts.TF', {'user_id'?: builtins.int})"
[builtins fixtures/dict.pyi]
[typing fixtures/typing-typeddict.pyi]

[case testRecursiveNestedTypedDictInference]
from typing import TypedDict, Sequence
from typing_extensions import NotRequired

class Component(TypedDict):
type: str
components: NotRequired[Sequence['Component']]

inputs: Sequence[Component] = [{
'type': 'tuple',
'components': [
{'type': 'uint256'},
{'type': 'address'},
]
}]
[builtins fixtures/dict.pyi]
[typing fixtures/typing-typeddict.pyi]