Assertion failure crash: assert not any(isinstance(t, Parameters) for t in arg_types)

Crash Report

mypy 1.20.2 and master (0ea16df0c) crash on the snippet below.

Traceback

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 701, in refresh_partial
    self.refresh_top_level(node)
  File "mypy/mypy/semanal.py", line 728, in refresh_top_level
    self.accept(d)
  File "mypy/mypy/semanal.py", line 7701, in accept
    node.accept(self)
    ~~~~~~~~~~~^^^^^^
  File "mypy/mypy/nodes.py", line 1903, in accept
    return visitor.visit_assignment_stmt(self)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "mypy/mypy/semanal.py", line 3366, in visit_assignment_stmt
    self.process_type_annotation(s)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^
  File "mypy/mypy/semanal.py", line 3901, in process_type_annotation
    analyzed = self.anal_type(s.type, allow_tuple_literal=allow_tuple_literal)
  File "mypy/mypy/semanal.py", line 7858, in anal_type
    typ = typ.accept(a)
  File "mypy/mypy/types.py", line 1094, in accept
    return visitor.visit_unbound_type(self)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "mypy/mypy/typeanal.py", line 281, in visit_unbound_type
    typ = self.visit_unbound_type_nonoptional(t, defining_literal)
  File "mypy/mypy/typeanal.py", line 461, in visit_unbound_type_nonoptional
    special = self.try_analyze_special_unbound_type(t, fullname)
  File "mypy/mypy/typeanal.py", line 655, in try_analyze_special_unbound_type
    return self.analyze_callable_type(t)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~^^^
  File "mypy/mypy/typeanal.py", line 1588, in analyze_callable_type
    ) or self.analyze_callable_args_for_concatenate(
         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
        callable_args, ret_type, fallback
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "mypy/mypy/typeanal.py", line 1509, in analyze_callable_args_for_concatenate
    tvar_def = self.anal_type(callable_args, allow_param_spec=True)
  File "mypy/mypy/typeanal.py", line 1923, in anal_type
    analyzed = t.accept(self)
  File "mypy/mypy/types.py", line 1094, in accept
    return visitor.visit_unbound_type(self)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "mypy/mypy/typeanal.py", line 281, in visit_unbound_type
    typ = self.visit_unbound_type_nonoptional(t, defining_literal)
  File "mypy/mypy/typeanal.py", line 514, in visit_unbound_type_nonoptional
    return self.apply_concatenate_operator(t)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^
  File "mypy/mypy/typeanal.py", line 588, in apply_concatenate_operator
    pre = Parameters(
        args + pre.arg_types,
    ...<3 lines>...
        column=t.column,
    )
  File "mypy/mypy/types.py", line 1946, in __init__
    assert not any(isinstance(t, Parameters) for t in arg_types)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError

To Reproduce

from typing import Callable, Concatenate

f: Callable[Concatenate[Concatenate[...], ...], ...]

Your Environment

• Mypy version used: 1.20.2 and 0ea16df0c
• Mypy command-line flags: N/A
• Mypy configuration options from mypy.ini (and other config files): N/A
• Python version used: 3.14.4
• Operating system and version: Arch Linux

Reproduce the crash with the provided Callable and Concatenate snippet, then inspect mypy/typeanal.py around apply_concatenate_operator and types.py around Parameters.init. Trace how the nested Concatenate reaches the assertion; done means the reproducer no longer crashes and the behavior is covered by a regression test.