lineStart: int | None = None
lineno: int = 0

def needsInt(i: int):
    ...

for part in ['a', 'b', 'c', 'd']:
    if part == 'a':
        ...     # removing this "fixes" the error
    elif part == 'b':
        lineno = lineStart if lineStart is not None else 0      # same happens with "lineno = lineStart or 0"
    elif part == 'c':
        needsInt(lineno)        # somehow lineno is detected as "int | None" <-- this is the bug in line 13
    elif part == 'd':
        lineStart = lineno      # this seems to trigger the bug