`isinstance` check with nested `Protocol`.

isinstance(obj, ProtocolSubclass) only checks the the existence of ProtocolSubclass's methods on obj and not the type signature. To provide deeper checks, maybe isinstance could check attributes/methods on ProtocolSubclass that are themselves a Protocol.

A small example showing the change in behavior:

@runtime_checkable
class FooLike(Protocol):
    attr: str

@runtime_checkable
class BarLike(Protocol):
    attr: FooLike

@dataclass
class Foo:
    attr: str = "test"

@dataclass
class Bar:
    attr: Foo

foo = Foo()
bar = Bar(foo)

# No change in current behavior
assert isinstance(foo, FooLike)  # passes
assert isinstance(bar, BarLike)  # passes
assert isinstance(bar, FooLike)  # passes

# Change in behavior
assert not isinstance(foo, BarLike)  # NOT because `BarLike.attr` should be `FooLike` and `foo.attr` is not

レポート内の isinstance(obj, ProtocolSubclass) の実行時動作と @runtime_checkable Protocol の例から始めてください。現在の浅い属性存在チェックと、要求されているネストされた Protocol の動作を比較し、その後、例の assertions を使って完了条件を定義してください。