from __future__ import annotations turns type annotations into strings that can later be evaluated at runtime via typing.get_type_hints see PEP 563. This is problematic because TestSlide uses the following pattern to find the expected type:

    try:
        argspec = inspect.getfullargspec(template)
    except TypeError:
        return
    expected_type = argspec.annotations.get("return")

In this scenario, expected_type is a string when the expected type is Type (see _validate_argument_type() which only works if expected_type is a Type).

It seems like a potential solution is:

    try:
        hints = get_type_hints(template)
        expected_type = hints.get("return")
    except TypeError:
        return

Will continue testing (I am on python 3.8.6).