`get_function_signature_hook` is not invoked on function decorators

使用 plug.py 中的插件对 foo.py 复现代码运行 mypy，并比较针对 foo.deco 的 get_function_hook 和 get_function_signature_hook 调用。跟踪装饰器调用的插件 dispatch 路径，并添加或更新覆盖率，使 signature hook 针对 foo.deco 被调用；当预期的 callback 出现且不破坏现有 hook 行为时即表示完成。

Bug Report

It seems to me there is no way to intercept and change the signature of a decorator during the invocation.

I see that get_function_hook has been called for foo.deco (eventually allowing me to change the return type of the decorator) but get_function_signature_hook is missing

To Reproduce

very basic example:

foo.py

from collections.abc import Callable
def deco(fun: Callable[..., None]) -> None: ...
@deco
def fun() -> None: ...

plug.py

from mypy.plugin import *
from mypy.types import *
from collections.abc import Callable

class CustomPlugin(Plugin):
    def get_function_hook(self, fullname: str) -> Callable[[FunctionContext], Type] | None:
        print(f'get_function_hook - {fullname=}')
        return None

    def get_function_signature_hook(self, fullname: str) -> Callable[[FunctionSigContext], FunctionLike] | None:
        print(f'get_function_signature_hook - {fullname=}')
        return None

def plugin(_version: str, /) -> type[Plugin]:
    return CustomPlugin

If I run mypy with the plugin and capture the stdout I get (avoiding the duplicates):

get_function_hook - fullname='enum.auto'
get_function_hook - fullname='foo.deco'
get_function_hook - fullname='<list>'
get_function_hook - fullname='typing_extensions.disjoint_base'
get_function_hook - fullname='typing_extensions.final'
get_function_hook - fullname='typing.final'
get_function_hook - fullname='typing.runtime_checkable'
get_function_hook - fullname='typing.type_check_only'
get_function_hook - fullname='warnings.deprecated'
get_function_signature_hook - fullname='enum.auto'
get_function_signature_hook - fullname='warnings.deprecate

Expected Behavior

get_function_signature_hook is called also for foo.deco

Actual Behavior

it's missing

Your Environment

mypy 2.3.1 (compiled: yes)
Python 3.14.0