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

Commencez par exécuter l’exemple d’overload fourni avec mypy 0.910 et confirmez les types révélés ainsi que le diagnostic manquant. Suivez les vérifications de compatibilité de la signature d’implémentation ; le travail est terminé lorsque les overloads incompatibles, comme le cas false-flag renvoyant str, produisent une erreur sans rejeter les overloads compatibles.