False positive `statement is unreachable` in try-finally related to KeyboardInterrupt

[bbatliner]

Bug Report

A KeyboardInterrupt may interleave between the start of a try block and its contained statements. mypy seems to assume that the flow of execution will not be asynchronously interrupted.

def fn() -> None:
    i: Optional[int] = None
    try:
        i = 1
    finally:
        if i is None:
            return  # error: Statement is unreachable

Expected Behavior

It's trivial to show that this statement is reachable. Insert a sleep before the assignment:

def fn() -> None:
    i: Optional[int] = None
    try:
        time.sleep(10)
        i = 1
    finally:
        if i is None:
            print("reachable")
            return

And then Ctrl+C (KeyboardInterrupt) the function during its execution. The print will run.

Your Environment

• Mypy version used: 0.960
• Mypy command-line flags: --warn-unreachable seems to be the only flag necessary to reproduce this

[bbatliner]

Technically any exception could be raised before the statements inside of the try block execute, not just KeyboardInterrupt, via CPython's asynchronous exception state that each thread tracks: https://docs.python.org/3/c-api/init.html#c.PyThreadState_SetAsyncExc

[bbatliner]

What makes this extra interesting is that the unreachable block appears necessary to narrow the Optional and avoid an attribute error:

def fn() -> None:
    s: Optional[str] = None
    try:
        s = "hello, world"
    finally:
        if s is None:
            return  # error: Statement is unreachable
        print(s.upper())

def fn() -> None:
    s: Optional[str] = None
    try:
        s = "hello, world"
    finally:
        print(s.upper())  # Item "None" of "Optional[str]" has no attribute "upper"

whereas such narrowing is implicit without the try-finally:

def fn() -> None:
    s: Optional[str] = None
    s = "hello, world"
    print(s.upper())