when descendents of a common base implement a shared protocol, a ternary returning either cannot be assigned to the protocol

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Bug Report

When descendents of a common base implement a shared protocol, a ternary returning either cannot be assigned to the protocol.

To Reproduce

from typing import Protocol


class O:
    def y(self) -> None:
        ...


class A(O):
    def x(self) -> None:
        ...


class B(O):
    def x(self) -> None:
        ...


class P(Protocol):
    def x(self) -> None:
        ...


def choice() -> bool:
    return True


p: P = B() if choice() else A()

ab: A | B = B() if choice() else A()

p2: P = ab

https://mypy-play.net/?mypy=latest&python=3.11&gist=8e6bdde0ef585f354461226647d2b66f

Expected Behavior

This should not produce errors.

Actual Behavior

main.py:28: error: Incompatible types in assignment (expression has type "O", variable has type "P")  [assignment]
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 1.6.0
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): non
• Python version used: 3.11

[erictraut]

This is another join-vs-union issue. See #5128, #12009, #12056, #15808, #16055, etc.

[AlexWaygood]

This is another join-vs-union issue. See #5128, #12009, #12056, #15808, #16055, etc.

Yeah, I think we have enough of these open. I'm going to close this as "duplicate of all of the above" :-)