Assertion failure crash with shadowed functools module: assert isinstance(singledispatch_obj, Instance)

[correctmost]

Crash Report

I noticed this crash when running mypy on the cpython codebase.

Traceback

Traceback (most recent call last):
  File ".venv-mypy/bin/mypy", line 7, in <module>
    sys.exit(console_entry())
  File "mypy/mypy/__main__.py", line 15, in console_entry
    main()
  File "mypy/mypy/main.py", line 127, in main
    res, messages, blockers = run_build(sources, options, fscache, t0, stdout, stderr)
  File "mypy/mypy/main.py", line 211, in run_build
    res = build.build(sources, options, None, flush_errors, fscache, stdout, stderr)
  File "mypy/mypy/build.py", line 223, in build
    result = _build(
  File "mypy/mypy/build.py", line 302, in _build
    graph = dispatch(sources, manager, stdout)
  File "mypy/mypy/build.py", line 2953, in dispatch
    process_graph(graph, manager)
  File "mypy/mypy/build.py", line 3344, in process_graph
    done, still_working = manager.wait_for_done(graph)
  File "mypy/mypy/build.py", line 926, in wait_for_done
    process_stale_scc(graph, next_scc, self)
  File "mypy/mypy/build.py", line 3482, in process_stale_scc
    graph[id].type_check_first_pass()
  File "mypy/mypy/build.py", line 2341, in type_check_first_pass
    self.type_checker().check_first_pass()
  File "mypy/mypy/checker.py", line 530, in check_first_pass
    self.accept(d)
  File "mypy/mypy/checker.py", line 641, in accept
    stmt.accept(self)
    ~~~~~~~~~~~^^^^^^
  File "mypy/mypy/nodes.py", line 1164, in accept
    return visitor.visit_decorator(self)
           ~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "mypy/mypy/checker.py", line 5466, in visit_decorator
    self.visit_decorator_inner(e)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~^^^
  File "mypy/mypy/checker.py", line 5502, in visit_decorator_inner
    sig, t2 = self.expr_checker.check_call(
              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
        dec, [temp], [nodes.ARG_POS], e, callable_name=fullname, object_type=object_type
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "mypy/mypy/checkexpr.py", line 1589, in check_call
    return self.check_callable_call(
           ~~~~~~~~~~~~~~~~~~~~~~~~^
        callee,
        ^^^^^^^
    ...<6 lines>...
        object_type,
        ^^^^^^^^^^^^
    )
    ^
  File "mypy/mypy/checkexpr.py", line 1832, in check_callable_call
    new_ret_type = self.apply_function_plugin(
        callee,
    ...<7 lines>...
        context,
    )
  File "mypy/mypy/checkexpr.py", line 1270, in apply_function_plugin
    return callback(
        FunctionContext(
    ...<8 lines>...
        )
    )
  File "mypy/mypy/plugins/singledispatch.py", line 109, in create_singledispatch_function_callback
    assert isinstance(singledispatch_obj, Instance)
           ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: 
functools.py:4: : note: use --pdb to drop into pdb

To Reproduce

Save the following snippet as functools.py and run mypy on it:

def singledispatch(func):
    pass

@singledispatch
def f(fn):
    pass

Your Environment

• Mypy version used: 67df116
• Mypy command-line flags: N/A
• Mypy configuration options from mypy.ini (and other config files): N/A
• Python version used: 3.13.7
• Operating system and version: Arch Linux

[hauntsaninja]

A simple fix would be to return ctx.default_return_type, but we'd maybe want a test that looks at real typeshed instead of fixtures, so that we correctly fail if something relevant in typeshed changes