@runtime_checkable rejects ordinary protocol subclasses (is_protocol does not propagate through base classes) #21950

Bug Report

@runtime_checkable is only accepted on classes whose direct base is typing.Protocol. A subclass of a protocol class — an ordinary protocol in every other respect — is rejected, which is inconsistent with the runtime behavior and with other type checkers.

To Reproduce

from typing import Protocol, runtime_checkable

class Mid(Protocol): ...

@runtime_checkable
class NewProto(Mid): ...

Result (mypy 2.3.1):

main.py:7: error: @runtime_checkable can only be used with protocol classes  [misc]

• Pyright: no errors on the same file.
• CPython runtime: typing.runtime_checkable performs a duck-typed check (the protocol metaclass sets _is_protocol), so decorating NewProto works fine at runtime.

Expected Behavior

NewProto is a protocol class: it inherits the protocol metaclass and the protocol machinery from Mid. @runtime_checkable should be accepted, consistent with the runtime and with Pyright.

Actual Results

mypy rejects it with misc: @runtime_checkable can only be used with protocol classes.

Additional context

In mypy/semanal.py, is_protocol is only set when a direct base expression's fully-qualified name is in PROTOCOL_NAMES (the "also remove bare 'Protocol' bases" branch in analyze_class), and defn.info.is_protocol = is_protocol does not propagate the flag from protocol base classes.

This came up in the discussion on #21940, where the hardcoding of PROTOCOL_NAMES was raised. Unlike removing that hardcoding (which would conflict with mypy's name-based identification of typing features), propagating is_protocol from protocol base classes looks like a well-scoped improvement. Please let me know if the current behavior is intentional.