mypy incorrectly reports incompatible return types for overloaded methods

[Prometheus3375]

Bug Report

@overload
def foo(cls: type[dict], mapping: SupportsKeysAndGetItem[K, V], /) -> dict[K, V]: ...
@overload
def foo(cls: type[dict], mapping: SupportsKeysAndGetItem[str, V], /, **kwargs: V) -> dict[str, V]: ...

Return types are compatible.

class FrozendictBase(Generic[K, V]):
    @overload
    def __new__(
            cls,
            mapping: SupportsKeysAndGetItem[K, V],
            /,
            ) -> 'FrozendictBase[K, V]': ...

    @overload
    def __new__(
            cls,
            mapping: SupportsKeysAndGetItem[str, V],
            /,
            **kwargs: V,
            ) -> 'FrozendictBase[str, V]': ...

Return types are also compatible, but mypy reports an error.

To Reproduce

1. Download file.txt and save as sandbox.py.
2. Run mypy sandbox.py.

Expected Behavior

sandbox.py:8: error: Invariant type variable "V" used in protocol where covariant one is expected

This is due to #5775.

Actual Behavior

sandbox.py:8: error: Invariant type variable "V" used in protocol where covariant one is expected
sandbox.py:39: error: Overloaded function signatures 3 and 4 overlap with incompatible return types
sandbox.py:54: error: Overloaded function signatures 5 and 6 overlap with incompatible return types

Your Environment

• Mypy version used: mypy 0.950
• Mypy command-line flags:
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: Python 3.10.4
• Operating system and version: Windows 10

[hauntsaninja]

I thiiiiink mypy is correct here, although it's certainly not intuitive.

The tricky thing is dict is invariant in its key type. So if you pass SupportsKeysAndGetItem[StrSubclass, int] and no extra kwargs, both overloads would match, but overload 1 returns you a dict[StrSubclass, int] and overload 2 returns you a dict[str, int] and those are incompatible.

[Prometheus3375]

@hauntsaninja

1. mypy reports errors only in FrozendictBase methods.
2. SupportsKeysAndGetItem is also invariant in both key and values in this example, so SupportsKeysAndGetItem[StrSubclass, int] is not subclass of SupportsKeysAndGetItem[str, int]; thus, overload 2 is not macthed.