[2.0 regression] INTERNAL ERROR: AssertionError in visit_decorator_inner on @types.coroutine generator function #21426

Bug Report

mypy 2.0.0 crashes with an AssertionError (assert isinstance(defn.type, CallableType)) when checking a generator function decorated with @types.coroutine. This is a regression: mypy 1.20.2 accepts the same code without issue. The bug also reproduces on master as of commit 11f945b.

Originally encountered while running mypy --ignore-missing-imports supervisor on the Home Assistant Supervisor project; the crash occurs inside the aiodocker dependency at aiodocker/jsonstream.py:21.

To Reproduce

import types

@types.coroutine
def f():
    yield
    return 1

Run: mypy tiny.py

Expected Behavior

No errors. @types.coroutine is a stdlib decorator that turns a generator function into a coroutine; the same file checks cleanly with mypy 1.x.

Actual Behavior

tiny.py:3: error: INTERNAL ERROR -- Please try using mypy master on GitHub:
https://mypy.readthedocs.io/en/stable/common_issues.html#using-a-development-mypy-build
Please report a bug at https://github.com/python/mypy/issues
version: 2.0.0
tiny.py:3: note: use --pdb to drop into pdb
Traceback (most recent call last):
  File "mypy/checker.py", line 765, in accept
  File "mypy/nodes.py", line 1352, in accept
  File "mypy/checker.py", line 5716, in visit_decorator
    self.visit_decorator_inner(e)
  File "mypy/checker.py", line 5724, in visit_decorator_inner
    assert isinstance(defn.type, CallableType)
AssertionError

Your Environment

• Mypy version used: 2.0.0 (compiled: yes); confirmed working on 1.20.2
• Mypy command-line flags: none (also reproduces with --ignore-missing-imports and --show-traceback)
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.14.4