No error for overlapping overloads that vary in parameters with default values

Bug Report

Mypy doesn't complain that the following overloads overlap with incompatible return types, but probably should:

from typing import  overload

@overload
def foo(*, a: str = ...) -> str: ...
@overload
def foo(*, a: int | str = ...) -> int: ...

For comparison, pyright emits a diagnostic here (playground). Mypy does emit an error if a default value for a is omitted in either overload.

Reproduce the reported overload example in mypy and compare it with the existing behavior when a default value is omitted. Trace the overload-overlap checking entry point and add coverage for keyword-only parameters with default values; done means mypy reports the incompatible return types in the shown case without regressing the existing diagnostic.