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Be strict about arg names in protocol compatibility #21016

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Be strict about arg names in protocol compatibility
hauntsaninja Mar 13, 2026
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2 changes: 1 addition & 1 deletion mypy/messages.py
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Original file line number Diff line number Diff line change
Expand Up @@ -3113,7 +3113,7 @@ def get_conflict_protocol_types(
subtype = mypy.typeops.get_protocol_member(left, member, class_obj)
if not subtype:
continue
is_compat = is_subtype(subtype, supertype, ignore_pos_arg_names=True, options=options)
is_compat = is_subtype(subtype, supertype, options=options)
if not is_compat:
conflicts.append((member, subtype, supertype, False))
superflags = get_member_flags(member, right)
Expand Down
13 changes: 2 additions & 11 deletions mypy/subtypes.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1218,7 +1218,6 @@ def f(self) -> A: ...
for member in right.type.protocol_members:
if member in members_not_to_check:
continue
ignore_names = member != "__call__" # __call__ can be passed kwargs
# The third argument below indicates to what self type is bound.
# We always bind self to the subtype. (Similarly to nominal types).
supertype = find_member(member, right, left)
Expand All @@ -1234,9 +1233,7 @@ def f(self) -> A: ...
# Nominal check currently ignores arg names
# NOTE: If we ever change this, be sure to also change the call to
# SubtypeVisitor.build_subtype_kind(...) down below.
is_compat = is_subtype(
subtype, supertype, ignore_pos_arg_names=ignore_names, options=options
)
is_compat = is_subtype(subtype, supertype, options=options)
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@ilevkivskyi ilevkivskyi Mar 13, 2026

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You will need to also remove it from SubtypeContext() ~50 lines below.

else:
is_compat = is_proper_subtype(subtype, supertype)
if not is_compat:
Expand Down Expand Up @@ -1280,14 +1277,8 @@ def f(self) -> A: ...
if IS_CLASS_OR_STATIC in superflags and IS_CLASS_OR_STATIC not in subflags:
return False

if not proper_subtype:
# Nominal check currently ignores arg names, but __call__ is special for protocols
ignore_names = right.type.protocol_members != ["__call__"]
else:
ignore_names = False
subtype_kind = SubtypeVisitor.build_subtype_kind(
subtype_context=SubtypeContext(ignore_pos_arg_names=ignore_names),
proper_subtype=proper_subtype,
subtype_context=SubtypeContext(), proper_subtype=proper_subtype
)
type_state.record_subtype_cache_entry(subtype_kind, left, right)
return True
Expand Down
82 changes: 63 additions & 19 deletions test-data/unit/check-protocols.test
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Expand Up @@ -2640,8 +2640,13 @@ b: Bad

func(a)
func(b) # E: Argument 1 to "func" has incompatible type "Bad"; expected "One" \
# N: Following member(s) of "Bad" have conflicts: \
# N: Expected: \
# N: def __call__(self, x: str) -> None \
# N: Got: \
# N: def __call__(self, zzz: str) -> None \
# N: "One.__call__" has type "def __call__(self, x: str) -> None"
[out]


[case testJoinProtocolCallback]
from typing import Protocol, Callable
Expand Down Expand Up @@ -3391,7 +3396,7 @@ test(D) # OK
from typing import Any, Protocol

class P(Protocol):
def foo(self, obj: Any) -> int: ...
def foo(self, obj: Any, /) -> int: ...

class B:
def foo(self) -> int: ...
Expand All @@ -3403,15 +3408,15 @@ test(B) # OK
test(C) # E: Argument 1 to "test" has incompatible type "type[C]"; expected "P" \
# N: Following member(s) of "C" have conflicts: \
# N: Expected: \
# N: def foo(obj: Any) -> int \
# N: def foo(Any, /) -> int \
# N: Got: \
# N: def foo(self: C) -> str

[case testProtocolClassObjectInstanceMethodArg]
from typing import Any, Protocol

class P(Protocol):
def foo(self, obj: B) -> int: ...
def foo(self, obj: B, /) -> int: ...

class B:
def foo(self) -> int: ...
Expand All @@ -3423,7 +3428,7 @@ test(B) # OK
test(C) # E: Argument 1 to "test" has incompatible type "type[C]"; expected "P" \
# N: Following member(s) of "C" have conflicts: \
# N: Expected: \
# N: def foo(obj: B) -> int \
# N: def foo(B, /) -> int \
# N: Got: \
# N: def foo(self: C) -> int

Expand All @@ -3432,9 +3437,9 @@ from typing import Any, Protocol, overload

class P(Protocol):
@overload
def foo(self, obj: Any, arg: int) -> int: ...
def foo(self, obj: Any, /, arg: int) -> int: ...
@overload
def foo(self, obj: Any, arg: str) -> str: ...
def foo(self, obj: Any, /, arg: str) -> str: ...

class B:
@overload
Expand All @@ -3458,9 +3463,9 @@ test(C) # E: Argument 1 to "test" has incompatible type "type[C]"; expected "P"
# N: Following member(s) of "C" have conflicts: \
# N: Expected: \
# N: @overload \
# N: def foo(obj: Any, arg: int) -> int \
# N: def foo(Any, /, arg: int) -> int \
# N: @overload \
# N: def foo(obj: Any, arg: str) -> str \
# N: def foo(Any, /, arg: str) -> str \
# N: Got: \
# N: @overload \
# N: def foo(self: C, arg: int) -> int \
Expand Down Expand Up @@ -3517,7 +3522,7 @@ test(C) # E: Argument 1 to "test" has incompatible type "type[C]"; expected "P"
from typing import Any, Protocol, Generic, List, TypeVar

class P(Protocol):
def foo(self, obj: Any) -> List[int]: ...
def foo(self, obj: Any, /) -> List[int]: ...

T = TypeVar("T")
class A(Generic[T]):
Expand All @@ -3532,7 +3537,7 @@ test(B) # OK
test(C) # E: Argument 1 to "test" has incompatible type "type[C]"; expected "P" \
# N: Following member(s) of "C" have conflicts: \
# N: Expected: \
# N: def foo(obj: Any) -> list[int] \
# N: def foo(Any, /) -> list[int] \
# N: Got: \
# N: def foo(self: A[list[str]]) -> list[str]
[builtins fixtures/list.pyi]
Expand Down Expand Up @@ -3567,7 +3572,7 @@ from typing import Protocol, TypeVar, Union

T = TypeVar("T")
class P(Protocol):
def foo(self, arg: T) -> T: ...
def foo(self, arg: T, /) -> T: ...

class B:
def foo(self: T) -> T: ...
Expand All @@ -3579,7 +3584,7 @@ test(B) # OK
test(C) # E: Argument 1 to "test" has incompatible type "type[C]"; expected "P" \
# N: Following member(s) of "C" have conflicts: \
# N: Expected: \
# N: def [T] foo(arg: T) -> T \
# N: def [T] foo(T, /) -> T \
# N: Got: \
# N: def [T] foo(self: T) -> T | int

Expand Down Expand Up @@ -3711,7 +3716,7 @@ test(d) # E: Argument 1 to "test" has incompatible type "type[D]"; expected "P"
from typing import Any, Protocol, Type

class P(Protocol):
def foo(self, cls: Any) -> int: ...
def foo(self, cls: Any, /) -> int: ...

class B:
def foo(self) -> int: ...
Expand All @@ -3725,7 +3730,7 @@ test(b) # OK
test(c) # E: Argument 1 to "test" has incompatible type "type[C]"; expected "P" \
# N: Following member(s) of "C" have conflicts: \
# N: Expected: \
# N: def foo(cls: Any) -> int \
# N: def foo(Any, /) -> int \
# N: Got: \
# N: def foo(self: C) -> str

Expand Down Expand Up @@ -3759,7 +3764,7 @@ from typing import Protocol, Type, TypeVar, Union

T = TypeVar("T")
class P(Protocol):
def foo(self, arg: T) -> T: ...
def foo(self, arg: T, /) -> T: ...

class B:
def foo(self: T) -> T: ...
Expand All @@ -3773,7 +3778,7 @@ test(b) # OK
test(c) # E: Argument 1 to "test" has incompatible type "type[C]"; expected "P" \
# N: Following member(s) of "C" have conflicts: \
# N: Expected: \
# N: def [T] foo(arg: T) -> T \
# N: def [T] foo(T, /) -> T \
# N: Got: \
# N: def [T] foo(self: T) -> T | int

Expand All @@ -3782,7 +3787,7 @@ from typing import Any, Protocol, TypeVar

T = TypeVar("T", contravariant=True)
class P(Protocol[T]):
def foo(self, obj: T) -> int: ...
def foo(self, obj: T, /) -> int: ...

class B:
def foo(self) -> int: ...
Expand All @@ -3796,7 +3801,7 @@ from typing import Any, Protocol, TypeVar, Type

T = TypeVar("T", contravariant=True)
class P(Protocol[T]):
def foo(self, obj: T) -> int: ...
def foo(self, obj: T, /) -> int: ...

class B:
def foo(self) -> int: ...
Expand All @@ -3806,6 +3811,7 @@ def test(arg: P[S]) -> S: ...
b: Type[B]
reveal_type(test(b)) # N: Revealed type is "__main__.B"


[case testTypeAliasInProtocolBody]
from typing import Protocol, List

Expand Down Expand Up @@ -4746,3 +4752,41 @@ tmp/a.py:8: note: Expected:
tmp/a.py:8: note: def f(self) -> PNested
tmp/a.py:8: note: Got:
tmp/a.py:8: note: def f(self) -> CNested

[case testProtocolArgNames]
from typing import Protocol

class P1(Protocol):
def foo(self, a: int) -> None: ...

class C1:
def foo(self, b: int) -> None: pass

x1: P1 = C1() # E: Incompatible types in assignment (expression has type "C1", variable has type "P1") \
# N: Following member(s) of "C1" have conflicts: \
# N: Expected: \
# N: def foo(self, a: int) -> None \
# N: Got: \
# N: def foo(self, b: int) -> None

class P2(Protocol):
def foo(self, a: int) -> None: ...

class C2:
def foo(self, *args: int) -> None: pass

x2: P2 = C2() # E: Incompatible types in assignment (expression has type "C2", variable has type "P2") \
# N: Following member(s) of "C2" have conflicts: \
# N: Expected: \
# N: def foo(self, a: int) -> None \
# N: Got: \
# N: def foo(self, *args: int) -> None

class P3(Protocol):
def foo(self, a: int, /) -> None: ...

class C3:
def foo(self, *args: int) -> None: pass

okay3: P3 = C3()
[builtins fixtures/tuple.pyi]
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