return value of sorted() cannot be upcast

Bug Report

Mypy allows the return value of list() to be upcasted. eg. list[int] -> list[object] (or list[int | None]). Mypy does not permit the same behaviour with the return value of sorted(). Like list(), sorted() is a builtin, creates a new list, and does not retain a reference to the returned value. Mypy should allow the return value of sorted() to be upcast, because it is safe to do so.

To Reproduce

# file: foo.py
ints: list[int] = [1,2,3]
objects: list[object] = sorted(ints) # error!

Command line:

python -m mypy foo.py

Expected Behavior

That no error should be reported, much like if list() were used instead of sorted(). ie:

ints: list[int] = [1, 2, 3]
objects: list[object] = list(ints)

# undesirable work around 
objects2: list[object] = list(sorted(ints))

Actual Behavior

foo.py:11: error: Incompatible types in assignment (expression has type "list[int]", variable has type "list[object]")  [assignment]
foo.py:11: note: "List" is invariant -- see https://mypy.readthedocs.io/en/stable/common_issues.html#variance
foo.py:11: note: Consider using "Sequence" instead, which is covariant
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 1.10.0
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.12.0

まず、issue にある再現例で python -m mypy foo.py を実行し、次に mypy が list() と sorted() の戻り値の型をどのように扱うかを調べます。組み込み型とジェネリックリストの型チェックに関係する経路を追跡します。完了条件は、list(sorted(ints)) の回避策を必要とせず、示されている sorted(ints) の代入でエラーが発生しないことです。