functools.update_wrapper currently copies over every attribute listed in the "assigned" parameter, which defaults to WRAPPER_ASSIGNMENTS, which means it copies the wrapped function's __annotations__ to the wrapper.  This is slightly wrong if the wrapper occludes an annotated parameter:

    def foo(a: int, b: str, c: float):
        print(a, b, c)

    import functools

    foo_a = functools.partial(foo, 3)
    functools.update_wrapper(foo_a, foo)

    print(foo_a.__annotations__)

In this case, foo_a.__annotations__ contains an annotation for a parameter named "a", even though foo_a doesn't have a parameter named "a".

This problem occurred to me just after I filed #46846; the two issues are definitely related.