python / python/mypy

Assignability of overloaded implementation to `Proto[**P](Protocol)` is overload-order-sensitive

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bug topic-protocols
Dominant language
Python
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Description

Bug Report, To Reproduce & Actual Behaviour

When matching a method signature in a protocol with an overloaded implementation, the implementation's overload order appears to matter, even when only one of the overloads match.

In the following example, only the first overload of the implementation is tried for compatibility with the protocol (which looks similar to the issue in #14070). See mypy Playground:

# mypy: disable-error-code=no-overload-impl

from typing import *

_A_contra = TypeVar("_A_contra", bound=Literal["a", "b"], contravariant=True)
_P = ParamSpec("_P")

class Callback(Protocol[_A_contra, _P]):
    def method(self, a: _A_contra, *args: _P.args, **kwargs: _P.kwargs) -> None: ...
    
class Impl:
    @overload
    def method(self, a: Literal["a"], b: int) -> None: ...
    @overload
    def method(self, a: int, b: str) -> None: ...

class Impl2:  # Overload signature order is reversed compared with `Impl`
    @overload
    def method(self, a: int, b: str) -> None: ...
    @overload
    def method(self, a: Literal["a"], b: int) -> None: ...

def accepts_callback(cb: Callback[Literal["a"], _P], *args: _P.args, **kwargs: _P.kwargs) -> int: 
    return 1

a = accepts_callback(Impl(), 1)
b = accepts_callback(Impl2(), 1)  # Error
reveal_type(a)
reveal_type(b)

Expected Behavior

No errors

Your Environment

  • Mypy version used: 1.16.1, master
  • Python version used: 3.12

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start with the provided Python 3.12 reproducer in the mypy Playground and compare how Impl and Impl2 are checked against Callback. Verify the expected result by running the example and checking that both calls produce no error and that the revealed types remain consistent.

Written by the indexing model from the issue text.

Assessment

Tech stack
python
Domain
compilers
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
35/100

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