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fix: prevent false unreachable error when comparing generic callables #21188

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fix: prevent false unreachable error when comparing generic callables #21188

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f0c1fdc
fix: prevent false unreachable error when comparing generic callables
Bahtya Apr 8, 2026
07b6987
[pre-commit.ci] auto fixes from pre-commit.com hooks
pre-commit-ci[bot] Apr 8, 2026
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7 changes: 5 additions & 2 deletions mypy/checker.py
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Original file line number Diff line number Diff line change
Expand Up @@ -8478,13 +8478,16 @@ def conditional_types(
if from_equality:
# We erase generic args because values with different generic types can compare equal
# For instance, cast(list[str], []) and cast(list[int], [])
# We also erase the current type for the overlap check, to correctly handle
# generic callables with different type variables (see mypy#21182).
erased_current = shallow_erase_type_for_equality(current_type)
proposed_type = shallow_erase_type_for_equality(proposed_type)
if not is_overlapping_types(current_type, proposed_type, ignore_promotions=False):
if not is_overlapping_types(erased_current, proposed_type, ignore_promotions=False):
# Equality narrowing is one of the places at runtime where subtyping with promotion
# does happen to match runtime semantics
# Expression is never of any type in proposed_type_ranges
return UninhabitedType(), default
if not is_overlapping_types(current_type, proposed_type, ignore_promotions=True):
if not is_overlapping_types(erased_current, proposed_type, ignore_promotions=True):
return default, default
else:
if not is_overlapping_types(current_type, proposed_type, ignore_promotions=True):
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20 changes: 19 additions & 1 deletion mypy/erasetype.py
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Expand Up @@ -288,7 +288,13 @@ def visit_union_type(self, t: UnionType) -> Type:


def shallow_erase_type_for_equality(typ: Type) -> ProperType:
"""Erase type variables from Instance's"""
"""Erase type variables from types for equality narrowing.

At runtime, generic type parameters are erased, so values with different
generic types can compare equal (e.g. ``cast(list[str], []) == cast(list[int], [])``).
This function erases generic type information so that equality-based type
narrowing does not incorrectly conclude that two values can never be equal.
"""
p_typ = get_proper_type(typ)
if isinstance(p_typ, Instance):
if not p_typ.args:
Expand All @@ -298,4 +304,16 @@ def shallow_erase_type_for_equality(typ: Type) -> ProperType:
if isinstance(p_typ, UnionType):
items = [shallow_erase_type_for_equality(item) for item in p_typ.items]
return UnionType.make_union(items)
if isinstance(p_typ, CallableType):
# Erase generic type variables from non-type-object callables.
# Type objects (classes like TupleLike) keep their type identity,
# but regular generic functions have their type vars erased to Any
# since at runtime, generic functions with different type args can
# be the same object (e.g. the identity function).
if not p_typ.variables or p_typ.is_type_obj():
return p_typ
any_type = AnyType(TypeOfAny.special_form)
return p_typ.copy_modified(
arg_types=[any_type for _ in p_typ.arg_types], ret_type=any_type, variables=()
)
return p_typ
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