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Don't cache partial protocol checks that skip __call__ #22153

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Don't cache partial protocol checks that skip __call__ #22153

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4aff6a2
Don't cache partial protocol checks that skip __call__ (fixes #22151)
PGrayCS Oct 9, 2026
05916e7
[pre-commit.ci] auto fixes from pre-commit.com hooks
pre-commit-ci[bot] Oct 9, 2026
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3 changes: 3 additions & 0 deletions mypy/subtypes.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1381,6 +1381,9 @@ def f(self) -> A: ...
if IS_CLASS_OR_STATIC in superflags and IS_CLASS_OR_STATIC not in subflags:
return False

if skip:
# The result is only valid for the partial check, don't pollute the subtype cache.
return True
if not proper_subtype:
# Nominal check currently ignores arg names, but __call__ is special for protocols
ignore_names = right.type.protocol_members != ["__call__"]
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22 changes: 22 additions & 0 deletions test-data/unit/check-protocols.test
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Expand Up @@ -4819,3 +4819,25 @@ class Bad(Generic[T]):
bad: Bad[str] = Bad()
fail: SupportsFoo = bad # E: Incompatible types in assignment (expression has type "Bad[str]", variable has type "SupportsFoo")
[builtins fixtures/tuple.pyi]

[case testCallbackProtocolUnionInferenceOrderIndependent]
from typing import Protocol, TypeVar

T = TypeVar("T")
PT = TypeVar("PT", contravariant=True)

class OneArg(Protocol[PT]):
def __call__(self, x: PT, /) -> None: ...

class TwoArgs(Protocol[PT]):
def __call__(self, x: PT, y: int, /) -> None: ...

def first_arg(fn: OneArg[T] | TwoArgs[T]) -> T: ...

def one(x: int) -> None: ...
def two(x: str, y: int) -> None: ...

reveal_type(first_arg(one)) # N: Revealed type is "builtins.int"
reveal_type(first_arg(two)) # N: Revealed type is "builtins.str"
reveal_type(first_arg(one)) # N: Revealed type is "builtins.int"
[builtins fixtures/tuple.pyi]
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