Naively implemented "Addable" Protocol does not throw Error for Tuples

[notatallshaw]

Bug Report

In attempting to create an "Addable Protocol" I messed up and it does not work for Tuples, but mypy does not give me an error where as pyright does.

To Reproduce

Create the Python file:

from typing import Protocol, TypeVar, Any

T = TypeVar("T")

class Addable(Protocol):
    def __add__(self: T, other: T, /) -> T:
        ...

x1: Addable = (1,)  # Expected No Error
x2: Addable = (1, 2)  # Expected Error

And run mypy --strict on this file.

Expected Behavior

Should throw an error because in some sense the return type of adding the tuple x2 will not be the same type when you add it, as the Addable Protocol would indicate. pyright gives the following error:

Expression of type "tuple[Literal[1], Literal[2]]" cannot be assigned to declared type "Addable"
  "tuple[Literal[1], Literal[2]]" is incompatible with protocol "Addable"
    "__add__" is an incompatible type
      No overloaded function matches type "(other: tuple[Literal[1], Literal[2]], /) -> tuple[Literal[1], Literal[2]]"
        Type "(__x: tuple[Literal[1, 2], ...], /) -> tuple[Literal[1, 2], ...]" cannot be assigned to type "(other: tuple[Literal[1], Literal[2]], /) -> tuple[Literal[1], Literal[2]]"
          Function return type "tuple[Literal[1, 2], ...]" is incompatible with type "tuple[Literal[1], Literal[2]]"
        Type "(__x: tuple[_T@__add__, ...], /) -> tuple[_T@__add__ | Literal[1, 2], ...]" cannot be assigned to type "(other: tuple[Literal[1], Literal[2]], /) -> tuple[Literal[1], Literal[2]]"
          Function return type "tuple[Literal[1, 2], ...]" is incompatible with type "tuple[Literal[1], 

One of the pyright maintainers kindly gave a detailed explanation here: microsoft/pyright#4613 (comment)

Actual Behavior

mypy outputs that everything is fine:

Success: no issues found in 1 source file

Your Environment

• Mypy version used: 1.0.0
• Mypy command-line flags: --strict
• Mypy configuration options from mypy.ini (and other config files): N/A
• Python version used: 3.11.1

[JelleZijlstra]

I don't think this is incorrect; it just depends on how the type of these tuple is inferred. Pyright infers the type as tuple[Literal[1], Literal[2]], which is valid, but another valid type for these tuples is tuple[int, ...], which does indeed follow the Addable protocol.

[notatallshaw]

I don't think this is incorrect; it just depends on how the type of these tuple is inferred. Pyright infers the type as tuple[Literal[1], Literal[2]], which is valid, but another valid type for these tuples is tuple[int, ...], which does indeed follow the Addable protocol.

Yup, I'm better understanding the design choices here of the different type checkers. Thanks for your time replying.