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fix(checker): avoid false overload-cannot-match with ParamSpec args #21194

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fix(checker): avoid false overload-cannot-match with ParamSpec args #21194

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fix(checker): avoid false overload-cannot-match with ParamSpec args
Bahtya Apr 9, 2026
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[pre-commit.ci] auto fixes from pre-commit.com hooks
pre-commit-ci[bot] Apr 9, 2026
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fix(checker): avoid false overload-cannot-match with ParamSpec args 
When an overload uses ParamSpec-flavored *args (P.args) or **kwargs
(P.kwargs), erasing the ParamSpec to Any makes the signature appear to
accept all arguments. This causes a false 'overload will never be
matched' error when combined with a second overload that has explicit
keyword-only parameters.

Skip the can-never-match check when the first overload has
ParamSpec-flavored variadic arguments, since we cannot reliably
determine overlap after erasure.

Fixes #21171

Signed-off-by: bahtya <[email protected]>
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@Bahtya
Bahtya committed Apr 9, 2026
commit d37f5ab73662efcdeb127200f94645fa15c7df63
11 changes: 11 additions & 0 deletions mypy/checker.py
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Original file line number Diff line number Diff line change
Expand Up @@ -228,6 +228,7 @@
TypeVarId,
TypeVarLikeType,
TypeVarTupleType,
ParamSpecType,
TypeVarType,
UnboundType,
UninhabitedType,
Expand Down Expand Up @@ -8954,6 +8955,16 @@ def overload_can_never_match(signature: CallableType, other: CallableType) -> bo

Assumes that both signatures have overlapping argument counts.
"""
# If the signature uses ParamSpec-flavored *args or **kwargs, we cannot
# reliably determine overlap. Erasing a ParamSpec to Any makes
# P.args/P.kwargs look like *Any/**Any, which appears to accept all
# arguments — but in reality the ParamSpec is constrained to the
# wrapped function's parameters. This leads to false positives where
# we incorrectly conclude that the other overload can never match.
for arg_type in signature.arg_types:
if isinstance(arg_type, ParamSpecType) and arg_type.flavor != 0: # BARE = 0
return False

# The extra erasure is needed to prevent spurious errors
# in situations where an `Any` overload is used as a fallback
# for an overload with type variables. The spurious error appears
Expand Down
16 changes: 16 additions & 0 deletions test-data/unit/check-parameter-specification.test
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Expand Up @@ -2756,3 +2756,19 @@ reveal_type(Sneaky(f8, 1, y='').kwargs) # N: Revealed type is "builtins.dict[bu
reveal_type(Sneaky(f9, 1, y=0).kwargs) # N: Revealed type is "TypedDict('builtins.dict', {'x'?: builtins.int, 'y': builtins.int, 'z'?: builtins.str})"
reveal_type(Sneaky(f9, 1, y=0, z='').kwargs) # N: Revealed type is "TypedDict('builtins.dict', {'x'?: builtins.int, 'y': builtins.int, 'z'?: builtins.str})"
[builtins fixtures/paramspec.pyi]

[case testOverloadParamSpecNoFalsePositiveCannotMatch]
# Test that overloads using P.args/P.kwargs don't trigger false
# "overload will never be matched" errors when combined with
# overloads using explicit keyword-only parameters.
from typing import Any, overload, ParamSpec, TypeVar, Callable

P = ParamSpec("P")
T = TypeVar("T")

@overload
def bar(f: Callable[P, T], *a: P.args, **k: P.kwargs) -> T: ...
@overload
def bar(f: Callable[..., T], *a: Any, baz: int, **k: Any) -> T: ...
def bar(f, *a, **k): ...
[builtins fixtures/paramspec.pyi]
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