Overload ambiguity is ignored in `self`-annotated methods

[BvB93]

Bug Report

Per the title: mypy does not check for overload ambiguity in self-annotated methods.
Mypy will instead, effectively, ignore self and then return the first matching overload,
thus circumventing the usual Any that is returned when overload ambiguity is involved.
This has lead to a number of false positives in numpy/numpy#20099 and (in part) numpy/numpy#20072.

The conditions for triggering this bug are as following:

• The callable is a method (things work fine for functions).
• The method is overloaded.
• self must be annotated with a parametrized generic.

To Reproduce

from typing import Any, Generic, overload, TypeVar, TYPE_CHECKING
import numpy as np

_CharType = TypeVar("_CharType", np.str_, np.bytes_)


class CharArray(Generic[_CharType]):
    @overload
    def strip(self: CharArray[np.str_], chars: str = ...) -> CharArray[np.str_]: ...
    @overload
    def strip(self: CharArray[np.bytes_], chars: bytes = ...) -> CharArray[np.bytes_]: ...


ar_any: CharArray[Any]
ar_str: CharArray[np.str_]
ar_bytes: CharArray[np.bytes_]

if TYPE_CHECKING:
    # The good
    reveal_type(ar_str.strip())  # E: Revealed type is "__main__.CharArray[numpy.str_]"
    reveal_type(ar_bytes.strip())  # E: Revealed type is "__main__.CharArray[numpy.bytes_]"

    # The bad
    # "__main__.CharArray[Any]" would be expected here due to overload ambiguity
    reveal_type(ar_any.strip())  # E: Revealed type is "__main__.CharArray[numpy.str_]"

    # Calling the method directly from the class does do the trick
    reveal_type(CharArray.strip(ar_any))  # E: Revealed type is "test.CharArray[Any]"

Your Environment

• Mypy version used: 0.910
• Mypy command-line flags: n.a.
• Mypy configuration options from mypy.ini (and other config files): n.a.
• Python version used: 3.9.6
• Operating system and version: macOS 11.4

[sobolevn]

Simplified reproduction

Without self annotation

Let's see what will happen when we use regular @overload with Any

from typing import Generic, TypeVar, overload, Any

T = TypeVar('T')

class Some:
    @overload
    def method(self, arg: int) -> bool: ...
    @overload
    def method(self, arg: str) -> float: ...
    def method(self): ...

# ok
s1: Some
s2: Some
reveal_type(s1.method(1))  # N: Revealed type is "builtins.bool"
reveal_type(s2.method('a')) # N: Revealed type is "builtins.float"

# also ok
s3: Some
a: Any
reveal_type(s3.method(a))  # N: Revealed type is "Any"

With self annotation

from typing import Generic, TypeVar, overload, Any

T = TypeVar('T')

class Some(Generic[T]):
    @overload
    def method(self: Some[int]) -> bool: ...
    @overload
    def method(self: Some[str]) -> float: ...
    def method(self): ...

# ok
s1: Some[int]
s2: Some[str]
reveal_type(s1.method())  # N: Revealed type is "builtins.bool"
reveal_type(s2.method())  # N: Revealed type is "builtins.float"

# error
s3: Some[Any]
reveal_type(s3.method()) # N: Revealed type is "builtins.bool"
# Should be: Revealed type is "Any"

Proposed solution

Wrong @overload cases should also be Any even with self annotations.

I will send a PR with the fix.

[sobolevn]

On the second thought: why s3.method() should return Any? While we can be sure that method returns Union[bool, float].

s3: Some[Any]
reveal_type(s3.method()) # N: Revealed type is "builtins.bool"
# Should be: Revealed type is "Union[bool, float]"

[BvB93]

On the second thought: why s3.method() should return Any? While we can be sure that method returns Union[bool, float].

I imagine this would be a great addition if we had a dedicated unsafe-union-type (e.g. python/typing#566),
both for methods as well as functions in general. Without it, you're very quickly going to run into the typical
issues associated with unsafe unions (#1693).

[sobolevn]

Yeap, @BvB93! I've decided that this is out of scope 👍

[vnmabus]

BTW: this issue is why the solution #10207 (comment) does not work in Mypy.