Mypy should ensure overloads are compatible with the implementation

Bug Report

Mypy should ensure overloads are compatible with the implementation

To Reproduce

from typing import Union, overload
from typing_extensions import Literal

@overload
def f(*, flag: Literal[True]) -> float: ...
@overload
def f(*, flag: Literal[False] = False) -> str: ...
def f(*, flag: bool = False) -> Union[str, float]:
    return 5

reveal_type(f(flag=True))
reveal_type(f(flag=False))
reveal_type(f())

Expected Behavior

an error along the lines of "signature 2 incompatible with implementation" because it cannot return a str when flag=False

Actual Behavior

$ mypy test.py 
test.py:11:13: note: Revealed type is "builtins.float"
test.py:12:13: note: Revealed type is "builtins.str"
test.py:13:13: note: Revealed type is "builtins.str"

Your Environment

• Mypy version used:

$ mypy --version
mypy 0.910

Start by running the provided overload example with mypy 0.910 and confirm the revealed types and missing diagnostic. Trace the overload compatibility checks for the implementation signature; done means incompatible overloads, such as the false-flag case returning str, produce an error without rejecting compatible overloads.