Shared via mypy Playground

main.py

import typing as t

@t.overload
def stuff(isnone: t.Literal[True] = ...) -> None:
  ...

@t.overload
def stuff(isnone: t.Literal[False] = ...) -> str:
  ...


def stuff(isnone: bool = False) -> str | None:
  if isnone:
    return None
  else:
    return 'ᕕ( ᐛ )ᕗ'
    

a = stuff(True)
t.reveal_type(a) # expected None -> ok

b = stuff(False)
t.reveal_type(b) # expected builtins.str -> ok

c = stuff() # default is False
t.reveal_type(c) # expected builtins.str -> nope... got None v_v