take infinite loops into account in type narrowing

[carljm]

The fix for #62 will probably lead us to reject this program:

from typing import Optional

def f(x: Optional[str]) -> str:
    while True:
        if x is None:
            continue
        return x

print(f(None))

In order to fix this, we need to understand that the while loop does not terminate (unless there's a break) and narrow the types in the code following the loop accordingly.

[LuKuangChen]

Here's another program that might be affected by a fix

(The test is from Lib/test/test_compiler/test_static/tests.py)

    def test_assign_while_returns_but_assigns_first(self):
        codestr = """
            from typing import Optional
            def f(x: Optional[int]) -> int:
                y: Optional[int] = 1
                while x is None:
                    y = None
                    return 1
                return y
        """
        self.compile(codestr, modname="foo")

[carljm]

Thanks @LuKC1024 ! I think this program will be fine; it does not involve implicit-None-return anywhere so it should not be affected by the fix for #62, and it does not involve a definitely-infinite loop, so it should not be impacted by whatever is done for this issue.

Are you seeing an issue related to our handling of this program in your model?

[LuKuangChen]

Hi Carl. If it's not affected, that's great. The model has trouble with this program because the type-checker needs to know y = None won’t be effective after the while-loop exits (because the function has returned before that).

[carljm]

Right. The SP type checker does detect if a loop unconditionally terminates, and in that case ignores its local-type effects after the loop.

[carljm]

Fixed by 49e44ef93d03259e3a0c45b0a73894ff81a383c0