Unable to specify a default value for a generic parameter

from typing import TypeVar

_T = TypeVar('_T')
def foo(a: _T = 42) -> _T:  # E: Incompatible types in assignment (expression has type "int", variable has type "_T")
    return a

_T = TypeVar('_T')
def noop_parser(x: str) -> str:
    return x

def foo(value: str, parser: Callable[[str], _T] = noop_parser) -> _T:
    return parser(value)