Fix TypedDict in narrowing wrongly eliminating subtype for extra_items keys

[Nilesh Patil (nileshpatil6)]

For a TypedDict with extra_items (PEP 728), a membership check on a key that is not a known item wrongly makes the negative branch unreachable:

class Movie(TypedDict, extra_items=bool):
    name: str

def func(movie: Movie) -> None:
    if "novel_adaptation" in movie:
        return
    reveal_type(movie)  # branch is wrongly marked unreachable today

The cause is in narrowTypeForTypedDictKey: the negative branch used knownItems.get(key) ?? extraItems, so a key that only matched the extra_items pseudo-entry was treated like a known provided item and eliminated the subtype. An extra item may or may not be present at runtime, so its absence can never rule the type out. Only a known item that is required or provided can do that.

The positive branch keeps its current behavior, it still uses the extra_items entry to mark the key as provided after the check.

Testing: added a case to typedDictClosed1.py where code after a negative in check on an extra key must stay reachable (it asserts an unrelated type error is still reported there, which the unreachability bug swallows). Without the fix that sample reports 7 errors instead of 8, with it all 10 TypedDictClosed tests pass and the full typeEvaluator suite is green: 1184 tests across 8 suites, 0 failures.

[Stella Huang (StellaHuang95)]

🔒 Automated review in progress — Stella Huang (@StellaHuang95) is auto-reviewing this PR.

[Stella Huang (StellaHuang95)]

Warning · Non-blocking recommendation

Please also cover a negative in check on a known required key (such as name) and assert that its following code remains unreachable. This would protect the existing narrowing behavior against an accidental over-relaxation of this change. [verified]

[Stella Huang (StellaHuang95)]

Approved via Review Center.

[Nilesh Patil (nileshpatil6)]

Good call. Added in f379f89: a negative in check on name (required known item) with an assignment in the branch that would be a type error if the branch were reachable. The sample's expected error count is unchanged, so the test fails if that branch ever becomes reachable.

Also sanity checked it guards what you had in mind: if I over-relax the fix to return the subtype unconditionally in the negative branch, TypedDictClosed1 fails on exactly this case; with the fix as written, all 10 TypedDictClosed tests pass.

[Stella Huang (StellaHuang95)]

Approved via Review Center.

[Rich Chiodo (rchiodo)]

Approved via Review Center.

[Rich Chiodo (rchiodo)]

Approved via Review Center.