from typing import *

def example_decorator(
    fn: Callable[[List[str]], List[str]]
) -> Callable[[List[str]], str]:
    def wrapper(data: List[str]) -> str:
        res = fn(data)
        return ', '.join(res)

    return wrapper


@example_decorator
def func(data: List[str]) -> List[str]:
    data.append('XYZ')
    return data


def test() -> str:
    result = func(['ABC', 'EFG'])
    print(type(result))  # <class 'str'>
    return result  # Incompatible return type [7]: Expected str but got typing.List[str].


test()