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Document undesirable generator expression behavior in test and fix one case #20594

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Document undesirable generator expression behavior in test and fix one case #20594

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2 changes: 1 addition & 1 deletion mypy/checkexpr.py
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Original file line number Diff line number Diff line change
Expand Up @@ -2918,7 +2918,7 @@ def infer_overload_return_type(

for typ in plausible_targets:
assert self.msg is self.chk.msg
with self.msg.filter_errors() as w:
with self.msg.filter_errors(filter_revealed_type=True) as w:
with self.chk.local_type_map as m:
ret_type, infer_type = self.check_call(
callee=typ,
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21 changes: 21 additions & 0 deletions test-data/unit/check-expressions.test
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Expand Up @@ -1456,6 +1456,27 @@ if int():
b = (x for x in a) # E: Generator has incompatible item type "Callable[[], str]"; expected "Callable[[], int]"
[builtins fixtures/list.pyi]

[case testGeneratorNoSpuriousError]
from typing import Iterable, overload

@overload
def take_iterable(iterable: Iterable[bool], /) -> None: ...
@overload
def take_iterable(iterable: Iterable[int], /) -> None: ...
def take_iterable(iterable): pass

take_iterable(1 for _ in [])
take_iterable(reveal_type(1 for _ in [])) # N: Revealed type is "typing.Generator[builtins.int, None, None]"

# NOTE: Type is revealed for every overload tried
# TODO: Overload shouldn't fail if expression contains an error that shouldn't affect the inferred type.
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@michaelm-openai michaelm-openai Jan 16, 2026 •
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I guess this is not strictly true, since it could be that the overload adds contextual information that causes the expression to have an error. Maybe a better behavior is if every overload contains errors, then pick the one with a valid inferred expression type.

take_iterable(reveal_type(1 if (-"") else 1 for _ in [])) # N: Revealed type is "typing.Generator[builtins.int, None, None]" \
# N: Revealed type is "typing.Generator[builtins.bool, None, None]" \
# E: Generator has incompatible item type "int"; expected "bool" \
# E: Unsupported operand type for unary - ("str")

[builtins fixtures/for.pyi]

-- Conditional expressions
-- -----------------------

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