pyright doesn't understand collection is not None (hence iterable) after condition check

[Mike]

Describe the bug

pyright doesn't understand collection is not None (hence iterable) after condition check

Code or Screenshots

def demo(x: list[int] | list[str] | None):
    val = x[0] if x else None
    if val is None:
        return

    return [item for item in x]

Yields the error:

Object of type "None" cannot be used as iterable value on: return [item for item in x]

VS Code extension or command-line
Command line: pyright 1.1.362

[Eric Traut]

Pyright is working as designed here, so I don't consider this a bug.

Your code isn't using a supported type guard check for expression x. For a complete list of type guard patterns supported by pyright, refer to this documentation.

You can fix this in your code by switching to the check if x is None.

Incidentally, mypy generates the same type error as pyright in this code sample.