Using Mypy stub file doesn't work as same as writing type hint in the definition of function

I try writing the type hint explicitly in the definition of the function

# test.py
def annotated(x: int, y: str) -> bool:
    return x < y

ran python3.5 -m mypy test.py and got following message

test.py:2: error: Unsupported operand types for > ("str" and "int")

It's correct. But when I try using stub file

# test.py
def annotated(x, y):
    return x < y

# test.pyi
def annotated(x: int, y: str) -> bool: ...

I got nothing. I saw this answer Using Mypy local stubs. So I try to import.

from test import annotated

annotated(2, 3)

It's ok. I got

error: Argument 2 to "annotated" has incompatible type "int"; expected "str"

But call annotated(2, 's') still got nothing.
So What should I do to check illegal operation with a stub file? Thanks