Inferred attribute type feeds back as call context, rejecting its own defining assignment

Bug Report

Assigning a generic call returning K | None to a previously undeclared
self. attribute spuriously fails [arg-type] on that same assignment. The
attribute's inferred type (str | None) apparently comes back as call context
on a later pass, solving K = str | None; the invariant Mapping key then
rejects dict[str, str].

The same expression passes with no context, an annotated local, an annotated
attribute, or a reassigned local (whose second assignment is likewise checked
against its own inferred str | None) — the optional-stripping special case
seems skipped on the inferred-attribute path alone. A sync version (no
async/await) also passes, pointing at the deferred re-check await
triggers.

To Reproduce

from collections.abc import Mapping


async def select_one[K](options: Mapping[K, str]) -> K | None: ...


class ViaInferredAttr:
    async def m(self) -> None:
        cases = {"a": "x"}
        self._done = await select_one(cases)  # error (spurious)


async def via_local() -> None:
    cases = {"a": "x"}
    done = await select_one(cases)  # ok
    done = await select_one(cases)  # ok (context = inferred str | None)
    done2: str | None = await select_one(cases)  # ok


class ViaAnnotatedAttr:
    _done: str | None

    async def m(self) -> None:
        cases = {"a": "x"}
        self._done = await select_one(cases)  # ok

Expected Behavior

All call sites typecheck, with K = str.

Actual Behavior

repro.py:10: error: Argument 1 to "select_one" has incompatible type "dict[str, str]"; expected "Mapping[str | None, str]"  [arg-type]

The str | None key shows the return's | None leaked into K.

Your Environment

• mypy 1.20.2 (compiled), no flags
• Python 3.13

Related: #5874 (over-eager outer context with unions), #19304 (assignment target
changes results). New here: the context is self-inferred — no annotation exists
anywhere — and explicit spellings of the same type behave differently.

Beginne mit dem Reproducer im Issue und verfolge die verzögerte asynchrone Überprüfung für eine Zuweisung an ein nicht deklariertes self-Attribut. Vergleiche diesen Pfad mit den erfolgreichen Fällen für lokale und annotierte Attribute und konzentriere dich darauf, wie der inferierte Rückgabekontext zur Inferenz generischer Argumente gelangt. Erledigt ist dies, wenn der Reproducer mit als str inferiertem K erfolgreich durchläuft und ein Regressionstest das Verhalten abdeckt.