Overlapping overload detection fails when signature contains TypeVar

Mypy detects and reports overlapping function overloads that differ in return type.

In the example below, it correctly reports that the second overload of func1 overlaps the first overload. However, when the concrete type str is replaced by a generic type _T, it fails to report the error.

@overload
def func1(
    a: str,
    b: Literal[False] = ...,
) -> List[str]: ...
@overload
def func1(
    a: str,
    b: bool = ...,
) -> List[Union[str, int]]: ...

_T = TypeVar("_T")
@overload
def func2(
    a: _T,
    b: Literal[False] = ...,
) -> List[_T]: ...
@overload
def func2(
    a: _T,
    b: bool = ...,
) -> List[Union[_T, int]]: ...

Start by running the two overload examples from the issue through mypy and compare the diagnostics for func1 and func2. Then trace the overload-overlap detection path responsible for these diagnostics. Done means mypy reports the overlapping overloads in the TypeVar-based func2 example without regressing the concrete case.