Incompatible override when overriding non-overload with overload

from typing import overload, Union

class A:
    def f(self, x: Union[int, str]) -> None: ...
    
class B(A):
    @overload
    def f(self, x: int) -> None: ...
    @overload
    def f(self, x: str) -> None: ...
    def f(self, x: Union[int, str]) -> None: ...

https://mypy-play.net/?mypy=latest&python=3.10&gist=37c2291afd7ddd170576b1de208ba77c

produces:

main.py:7: error: Signature of "f" incompatible with supertype "A"
main.py:7: note:      Superclass:
main.py:7: note:          def f(self, x: Union[int, str]) -> None
main.py:7: note:      Subclass:
main.py:7: note:          @overload
main.py:7: note:          def f(self, x: int) -> None
main.py:7: note:          @overload
main.py:7: note:          def f(self, x: str) -> None
Found 1 error in 1 file (checked 1 source file)

But as far as I can see this is safe. Mypy accepts a union as an argument to B.f:

arg: Union[int, str]
B().f(arg)

Of course, the overloads are useless in this toy example, but the error also comes up if you vary the return type in the overloads. This came up in python/typeshed#7510.

Start by reproducing the example in the linked mypy-play case and compare the diagnostics for A.f and B.f. Trace the override compatibility check for an overloaded subclass method versus a non-overloaded superclass method. Done means this safe override is accepted while incompatible overloads remain rejected.