`NotImplementedError` crash on mutually recursive `TypeVarTuple` type aliases

[MeGaGiGaGon]

Crash Report

I wanted to make an infinitely expanding tuple, ran into the error: Invalid recursive alias: a tuple item of itself safeguard, and tried to get around it with mutual recursion.

Traceback

Collapsed for readability

Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "Scripts\mypy.exe\__main__.py", line 10, in <module>
    sys.exit(console_entry())
  File "Lib\site-packages\mypy\__main__.py", line 15, in console_entry
    main()
  File "Lib\site-packages\mypy\main.py", line 141, in main
    res, messages, blockers = run_build(sources, options, fscache, t0, stdout, stderr)
  File "Lib\site-packages\mypy\main.py", line 225, in run_build
    res = build.build(sources, options, None, flush_errors, fscache, stdout, stderr)
  File "Lib\site-packages\mypy\build.py", line 391, in build
    result = build_inner(
  File "Lib\site-packages\mypy\build.py", line 488, in build_inner
    graph = dispatch(sources, manager, stdout)
  File "Lib\site-packages\mypy\build.py", line 3754, in dispatch
    process_graph(graph, manager)
  File "Lib\site-packages\mypy\build.py", line 4206, in process_graph
    done, still_working, results = manager.wait_for_done(graph)
  File "Lib\site-packages\mypy\build.py", line 1208, in wait_for_done
    process_stale_scc(graph, next_scc, self)
  File "Lib\site-packages\mypy\build.py", line 4371, in process_stale_scc
    mypy.semanal_main.semantic_analysis_for_scc(graph, scc, manager.errors)
  File "Lib\site-packages\mypy\semanal_main.py", line 91, in semantic_analysis_for_scc
    process_top_levels(graph, scc, patches)
  File "Lib\site-packages\mypy\semanal_main.py", line 200, in process_top_levels
    deferred, incomplete, progress = semantic_analyze_target(
  File "Lib\site-packages\mypy\semanal_main.py", line 380, in semantic_analyze_target
    analyzer.refresh_partial(
  File "Lib\site-packages\mypy\semanal.py", line 701, in refresh_partial
    self.refresh_top_level(node)
  File "Lib\site-packages\mypy\semanal.py", line 730, in refresh_top_level
    self.accept(d)
  File "Lib\site-packages\mypy\semanal.py", line 7710, in accept
    node.accept(self)
    ~~~~~~~~~~~^^^^^^
  File "Lib\site-packages\mypy\nodes.py", line 2162, in accept
    return visitor.visit_type_alias_stmt(self)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "Lib\site-packages\mypy\semanal.py", line 5695, in visit_type_alias_stmt
    res, alias_tvars, depends_on, indexed = self.analyze_alias(
                                            ~~~~~~~~~~~~~~~~~~^
        s.name.name,
        ^^^^^^^^^^^^
    ...<4 lines>...
        python_3_12_type_alias=True,
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "Lib\site-packages\mypy\semanal.py", line 4017, in analyze_alias
    rvalue.accept(self)
    ~~~~~~~~~~~~~^^^^^^
  File "Lib\site-packages\mypy\nodes.py", line 2533, in accept
    return visitor.visit_index_expr(self)
           ~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "Lib\site-packages\mypy\semanal.py", line 6216, in visit_index_expr
    self.analyze_type_application(expr)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "Lib\site-packages\mypy\semanal.py", line 6222, in analyze_type_application
    types = self.analyze_type_application_args(expr)
  File "Lib\site-packages\mypy\semanal.py", line 6301, in analyze_type_application_args
    flattened_items = flatten_nested_tuples(types)
  File "Lib\site-packages\mypy\types.py", line 4211, in flatten_nested_tuples
    p_type = get_proper_type(typ.type)
  File "Lib\site-packages\mypy\types.py", line 3649, in get_proper_type
    typ = typ._expand_once()
  File "Lib\site-packages\mypy\types.py", line 382, in _expand_once
    return self.alias.target.accept(InstantiateAliasVisitor(mapping))
           ~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "Lib\site-packages\mypy\types.py", line 2818, in accept
    return visitor.visit_tuple_type(self)
           ~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "Lib\site-packages\mypy\expandtype.py", line 537, in visit_tuple_type
    items = self.expand_type_list_with_unpack(t.items)
  File "Lib\site-packages\mypy\expandtype.py", line 522, in expand_type_list_with_unpack
    items.append(item.accept(self))
                 ~~~~~~~~~~~^^^^^^
  File "Lib\site-packages\mypy\types.py", line 1237, in accept
    return visitor.visit_unpack_type(self)
           ~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "Lib\site-packages\mypy\expandtype.py", line 387, in visit_unpack_type
    return UnpackType(t.type.accept(self))
                      ~~~~~~~~~~~~~^^^^^^
  File "Lib\site-packages\mypy\types.py", line 418, in accept
    return visitor.visit_type_alias_type(self)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "Lib\site-packages\mypy\expandtype.py", line 609, in visit_type_alias_type
    args = self.expand_type_list_with_unpack(t.args)
  File "Lib\site-packages\mypy\expandtype.py", line 522, in expand_type_list_with_unpack
    items.append(item.accept(self))
                 ~~~~~~~~~~~^^^^^^
  File "Lib\site-packages\mypy\types.py", line 998, in accept
    return visitor.visit_type_var_tuple(self)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "Lib\site-packages\mypy\expandtype.py", line 375, in visit_type_var_tuple
    raise NotImplementedError
NotImplementedError:

To Reproduce

https://mypy-play.net/?gist=d6bb4e3d5ec8b07b5441407ee428204d

type A[*TS] = tuple[*B[TS]]
type B[*TS] = tuple[*A[TS]]

Powershell command used

PS ~\Downloads> uvx mypy@https://github.com/python/mypy.git --show-traceback -c @'
type A[*TS] = tuple[*B[TS]]
type B[*TS] = tuple[*A[TS]]
'@ > output.txt

Your Environment

• Mypy version used: Current master
• Mypy command-line flags: --show-traceback -c "code"
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.14? Whatever uvx does.
• Operating system and version: Windows 11