calls_argument_type: literal None rejected for a `X | None` parameter

[MelbourneDeveloper]

Version

basilisk 0.35.0 (also present on 0.39.0).

What happens

Passing the literal None to a parameter declared dict[str, Any] | None is
reported as an error:

warning[calls_argument_type]: Argument `body` of `_probe` expects `dict[str, Any] | None`
but received a scalar literal where `dict[str, Any] | None` is required
— no type-variable assignment makes it valid

Minimal reproduction

from typing import Any


def probe(body: dict[str, Any] | None) -> None: ...


probe(None)  # warning[calls_argument_type]

Expected

No diagnostic. None is a member of dict[str, Any] | None by definition, so
the call is well-typed.

Notes

The message calls None "a scalar literal", which suggests the argument check
is comparing the literal against the dict[str, Any] member of the union only
and never trying the None member. A non-literal None-typed variable is
accepted at the same call site, so it looks specific to the literal path.