IndexError: tuple index out of range in analyze_unbound_tvar #21467

Traceback (most recent call last):
  File ".venv-mypy/bin/mypy", line 6, in <module>
    sys.exit(console_entry())
  File "mypy/mypy/__main__.py", line 16, in console_entry
    main()
  File "mypy/mypy/main.py", line 154, in main
    res, messages, blockers = run_build(sources, options, fscache, t0, stdout, stderr)
  File "mypy/mypy/main.py", line 244, in run_build
    res = build.build(sources, options, None, flush_errors, fscache, stdout, stderr)
  File "mypy/mypy/build.py", line 422, in build
    result = build_inner(
  File "mypy/mypy/build.py", line 537, in build_inner
    graph = dispatch(sources, manager, stdout, connect_threads)
  File "mypy/mypy/build.py", line 4150, in dispatch
    process_graph(graph, manager)
  File "mypy/mypy/build.py", line 4618, in process_graph
    done, still_working, results = manager.wait_for_done(graph)
  File "mypy/mypy/build.py", line 1492, in wait_for_done
    process_stale_scc(graph, next_scc, self)
  File "mypy/mypy/build.py", line 4785, in process_stale_scc
    mypy.semanal_main.semantic_analysis_for_scc(graph, scc, manager.errors)
  File "mypy/mypy/semanal_main.py", line 91, in semantic_analysis_for_scc
    process_top_levels(graph, scc, patches)
  File "mypy/mypy/semanal_main.py", line 200, in process_top_levels
    deferred, incomplete, progress = semantic_analyze_target(
  File "mypy/mypy/semanal_main.py", line 380, in semantic_analyze_target
    analyzer.refresh_partial(
  File "mypy/mypy/semanal.py", line 694, in refresh_partial
    self.refresh_top_level(node)
  File "mypy/mypy/semanal.py", line 721, in refresh_top_level
    self.accept(d)
  File "mypy/mypy/semanal.py", line 7695, in accept
    node.accept(self)
    ~~~~~~~~~~~^^^^^^
  File "mypy/mypy/nodes.py", line 1725, in accept
    return visitor.visit_class_def(self)
           ~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "mypy/mypy/semanal.py", line 1833, in visit_class_def
    self.analyze_class(defn)
    ~~~~~~~~~~~~~~~~~~^^^^^^
  File "mypy/mypy/semanal.py", line 1975, in analyze_class
    bases, tvar_defs, is_protocol = self.clean_up_bases_and_infer_type_variables(
                                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
        defn, bases, context=defn
        ^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "mypy/mypy/semanal.py", line 2307, in clean_up_bases_and_infer_type_variables
    result = self.analyze_class_typevar_declaration(base, has_type_var_tuple)
  File "mypy/mypy/semanal.py", line 2387, in analyze_class_typevar_declaration
    tvar = self.analyze_unbound_tvar(arg)
  File "mypy/mypy/semanal.py", line 2406, in analyze_unbound_tvar
    inner_t = t.args[0]
              ~~~~~~^^^
IndexError: tuple index out of range