Keys and values could be narrower for closed TypedDicts

I was very excited to try out mypy 2.2 with the initial support for the TypedDict closed keyword.
But in our code base, where we often iterate over TypedDicts, it did not help much because the keys and values are still inferred as str and object, respectively.

from typing_extensions import TypedDict, reveal_type


class Closed(TypedDict, closed=True, total=False):
    a: int
    b: str

c = Closed(a=1, b='a')
reveal_type(c.items())

for key, val in c.items():
    c[key]

$ mypy --pretty test.py
test.py:9: note: Revealed type is "_collections_abc.dict_items[str, object]"
test.py:12: error: TypedDict key must be a string literal; expected one of ("a", "b")  [literal-required]
        c[key]
          ^~~
Found 1 error in 1 file (checked 1 source file)

ty narrows both keys and values here

$ ty check test.py
ty check test.py 
info[revealed-type]: Revealed type
 --> test.py:9:13
  |
9 | reveal_type(c.items())
  |             ^^^^^^^^^^ `dict_items[Literal["a", "b"], int | str]`
  |

Found 1 diagnostic

まず、mypy --pretty を使って issue の test.py の例を再現し、TypedDict.items() とそれに続く c[key] アクセスに焦点を当てます。これらの推論された型を担う TypedDict の型チェック経路を追跡します。完了条件は、閉じた TypedDict の反復でキーのリテラルユニオンと対応する値の型が明らかになり、反復されたキーによるインデックスアクセスが受け入れられることです。