Narrowing: Variable type gets changed in a loop where it is only read from

[kilroy42]

Describe the Bug

def mySplit(s: str) -> None:
    inQuote: bool | str = False

    res: list[str] = []
    for c in s:
        if c == inQuote:
            ...
        elif inQuote:
            inQuote = c
            res[-1] += c

ERROR 10:13-25: `+=` is not supported between `str` and `bool` [[unsupported-operation](https://pyrefly.org/en/docs/error-kinds/#unsupported-operation)]

Although c is pretty clearly a "str", and never modified except in the for c in s, the type of inQuote is copied into the type of c (bool | str).

I have reported similiar bugs, but they came with another exception, had a slightly different reason and this one is hopefully easier to debug, so I reported this, too.

Sandbox Link

https://pyrefly.org/sandbox/?code=CYUwZgBAtgngygBwDYEsAuAKAzgLgltAJwEoIBaAPggDkB7AOxBwCgI2IV6BFAV1rSYQARrVpIIAH3xEIAXggAxAIZIsIZq3aEQuCKgIBtAoQC6ciAZOa2YWoQgBjDvXwt27jpCez5nXvyZrD3cAOjCg9xBUSD8+ATdgj1iA8wcIxO0sAzIARjMAank0oA

(Only applicable for extension issues) IDE Information

No response

[connernilsen]

Hey @kilroy42, thanks for reporting this! Seems like a type checking bug. I'll forward this to the type checking folks for them to take a look, though it looks like @yangdanny97 has already checked it out.

[yangdanny97]

The c == inQuote is refining the type of c to be the same as inQuote, bool | str. Somehow this is persisted in the else branch

[kilroy42]

I don't get why this should "refine" the type of c.

When I compare a variable a of type T1 with a variable b of type T1 | T2, the type of variable a will and should not change - I don't see a reason why this should ever be so?

[yangdanny97]

Yes, it's probably a bug in the refinement logic given that c is a narrower type than inQuote

[rchen152]

This no longer repros in the sandbox.