λ cat e.py
def merge_debugs(debugs: list[int | None]) -> None:
    metadata = None
    for debug in debugs:
        if debug is not None:
            if metadata is None:
                metadata = {}

λ mypy  e.py --warn-unreachable
e.py:6: error: Need type annotation for "metadata" (hint: "metadata: dict[<type>, <type>] = ...")  [var-annotated]
e.py:6: error: Statement is unreachable  [unreachable]
Found 2 errors in 1 file (checked 1 source file)