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Avoid unnecessary sum type expansion when narrowing #22252

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Merged
hauntsaninja merged 1 commit into from
Oct 21, 2026
Merged

Avoid unnecessary sum type expansion when narrowing #22252

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9 changes: 2 additions & 7 deletions mypy/checker.py
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Original file line number Diff line number Diff line change
Expand Up @@ -7168,9 +7168,7 @@ def narrow_type_by_identity_equality(
if narrowable_expr_type is None:
if_type = else_type = ambiguous_expr_type
else:
narrowable_expr_type = try_expanding_sum_type_to_union(
coerce_to_literal(narrowable_expr_type), None
)
narrowable_expr_type = coerce_to_literal(narrowable_expr_type)
if_type, else_type = conditional_types(
narrowable_expr_type,
[TypeRange(target_type, is_upper_bound=False)],
Expand Down Expand Up @@ -7247,7 +7245,7 @@ def narrow_type_by_identity_equality(
# we narrow to in the if_map
or_if_maps.append({operands[i]: expr_type})

expr_type = coerce_to_literal(try_expanding_sum_type_to_union(expr_type, None))
expr_type = coerce_to_literal(expr_type)
for j in expr_indices:
if j in custom_eq_indices:
if i == j:
Expand All @@ -7273,9 +7271,6 @@ def narrow_type_by_identity_equality(
if narrowable_expr_type is None:
if_type = else_type = ambiguous_expr_type
else:
narrowable_expr_type = coerce_to_literal(
try_expanding_sum_type_to_union(narrowable_expr_type, None)
)
if_type, else_type = conditional_types(
narrowable_expr_type,
[TypeRange(target_type, is_upper_bound=False)],
Expand Down
28 changes: 17 additions & 11 deletions test-data/unit/check-narrowing.test
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Original file line number Diff line number Diff line change
Expand Up @@ -861,7 +861,7 @@ def f2(x: Union[Default, Literal[1], Literal[2]]):

[case testNarrowingEqualityCustomEqualityEnum]
# flags: --strict-equality --warn-unreachable
from typing import Literal, Union
from typing import Literal
from enum import Enum

class CustomEnum(Enum):
Expand All @@ -870,18 +870,24 @@ class CustomEnum(Enum):

def __eq__(self, other: object) -> bool: return True

x3: CustomEnum
key: Literal[CustomEnum.A]
if x3 == key:
reveal_type(x3) # N: Revealed type is "__main__.CustomEnum"
else:
reveal_type(x3) # N: Revealed type is "Literal[__main__.CustomEnum.B]"
def check_equality(x: CustomEnum, key: Literal[CustomEnum.A]) -> None:
if x == key:
reveal_type(x) # N: Revealed type is "__main__.CustomEnum"
else:
reveal_type(x) # N: Revealed type is "Literal[__main__.CustomEnum.B]"

# For comparison, this narrows since we bypass __eq__
if x3 is key:
reveal_type(x3) # N: Revealed type is "Literal[__main__.CustomEnum.A]"
else:
reveal_type(x3) # N: Revealed type is "Literal[__main__.CustomEnum.B]"
def check_identity(x: CustomEnum, key: Literal[CustomEnum.A]) -> None:
if x is key:
reveal_type(x) # N: Revealed type is "Literal[__main__.CustomEnum.A]"
else:
reveal_type(x) # N: Revealed type is "Literal[__main__.CustomEnum.B]"

def check_optional_equality(x: CustomEnum | None, key: Literal[CustomEnum.A]) -> None:
if x == key:
reveal_type(x) # N: Revealed type is "__main__.CustomEnum"
else:
reveal_type(x) # N: Revealed type is "Literal[__main__.CustomEnum.B] | None"
[builtins fixtures/primitives.pyi]

[case testNarrowingEqualityCustomEqualityChainedComparison]
Expand Down
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