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Flatten union before contracting literals when checking subtyping #18898

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Merged
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Apr 9, 2025
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Flatten union before contracting literals when checking subtyping #18898

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5 changes: 4 additions & 1 deletion mypy/subtypes.py
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Original file line number Diff line number Diff line change
Expand Up @@ -67,6 +67,7 @@
UnionType,
UnpackType,
find_unpack_in_list,
flatten_nested_unions,
get_proper_type,
is_named_instance,
split_with_prefix_and_suffix,
Expand Down Expand Up @@ -327,7 +328,9 @@ def _is_subtype(
and isinstance(left, Instance)
and (left.type.is_enum or left.type.fullname == "builtins.bool")
):
right = UnionType(mypy.typeops.try_contracting_literals_in_union(right.items))
right = UnionType(
mypy.typeops.try_contracting_literals_in_union(flatten_nested_unions(right.items))
)
if proper_subtype:
is_subtype_of_item = any(
is_proper_subtype(orig_left, item, subtype_context=subtype_context)
Expand Down
2 changes: 2 additions & 0 deletions mypy/typeops.py
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Expand Up @@ -1069,6 +1069,8 @@ class Status(Enum):
def try_contracting_literals_in_union(types: Sequence[Type]) -> list[ProperType]:
"""Contracts any literal types back into a sum type if possible.

Requires a flattened union and does not descend into children.
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Maybe it's better to add an assert or transparently handle this correctly? Both usages of this in mypy seem fine but I think preventing misuse is good.

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I'd rather not - this function is called twice in the whole codebase, one of them in a rather hot function (subtyping check). While failure to pass a flattened union may result in some false positives, it isn't dangerous and can't cause any errors down the road.


Will replace the first instance of the literal with the sum type and
remove all others.

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22 changes: 22 additions & 0 deletions test-data/unit/check-literal.test
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Expand Up @@ -2765,6 +2765,28 @@ reveal_type(x) # N: Revealed type is "Literal[__main__.Foo.A]"
reveal_type(y) # N: Revealed type is "Literal[__main__.Foo.A]"
[builtins fixtures/tuple.pyi]

[case testLiteralUnionEnumAliasAssignable]
from enum import Enum
from typing import Literal, Union

class E(Enum):
A = 'a'
B = 'b'
C = 'c'

A = Literal[E.A]
B = Literal[E.B, E.C]

def f(x: Union[A, B]) -> None: ...
def f2(x: Union[A, Literal[E.B, E.C]]) -> None: ...
def f3(x: Union[Literal[E.A], B]) -> None: ...

def main(x: E) -> None:
f(x)
f2(x)
f3(x)
[builtins fixtures/tuple.pyi]

[case testStrictEqualityLiteralTrueVsFalse]
# mypy: strict-equality

Expand Down