$ cat <<'# EOF' | tee >b006.py b006_preview.py
def f(x: list[int] = []) -> int:
    return sum(x)
# EOF

$ ruff --isolated check b006_preview.py --select B006,RUF013 --unsafe-fixes --fix --preview
Found 2 errors (2 fixed, 0 remaining).

$ cat b006_preview.py
def f(x: list[int] | None = None) -> int:
    if x is None:
        x: list[int] = []
    return sum(x)

$ mypy b006_preview.py
b006_preview.py:3: error: Name "x" already defined on line 1  [no-redef]
b006_preview.py:4: error: Argument 1 to "sum" has incompatible type "list[int] | None"; expected "Iterable[bool]"  [arg-type]
Found 2 errors in 1 file (checked 1 source file)

$ pyright b006_preview.py | sed 's:/.*\(b006_preview\.py\):\1:'
b006_preview.py
  b006_preview.py:1:7 - error: Parameter declaration "x" is obscured by a declaration of the same name (reportRedeclaration)
1 error, 0 warnings, 0 informations

$ ruff --isolated check b006.py --select B006,RUF013 --unsafe-fixes --fix
Found 2 errors (2 fixed, 0 remaining).

$ cat b006.py
def f(x: list[int] | None = None) -> int:
    if x is None:
        x = []
    return sum(x)

$ mypy b006.py
Success: no issues found in 1 source file

$ pyright b006.py
0 errors, 0 warnings, 0 informations