Running mypy --no-implicit-optional test.py with the code below gives the error Incompatible default for argument "default" (default has type "None", argument has type "D")

from typing import Sequence, TypeVar, Union

D = TypeVar("D")
E = TypeVar("E")


def get_first(values: Sequence[E], default: D = None) -> Union[E, D]:
    if len(values) == 0:
        return default
    else:
        return values[0]

I would expect mypy doesn't emit any error. The type of default should be resolved to None when the function is called without the optional argument, e.g. get_first([]), so that the return type is Union[E, None], which is equivalent to Optional[E].

I am using mypy 0.770 and python 3.7