IndexError when unpacking type

[correctmost]

Crash Report

The snippet below crashes mypy 1.19.1 and master (f363f35).

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 15, in console_entry
    main()
  File "mypy/mypy/main.py", line 130, in main
    res, messages, blockers = run_build(sources, options, fscache, t0, stdout, stderr)
  File "mypy/mypy/main.py", line 214, in run_build
    res = build.build(sources, options, None, flush_errors, fscache, stdout, stderr)
  File "mypy/mypy/build.py", line 378, in build
    result = build_inner(
  File "mypy/mypy/build.py", line 478, in build_inner
    graph = dispatch(sources, manager, stdout)
  File "mypy/mypy/build.py", line 3652, in dispatch
    process_graph(graph, manager)
  File "mypy/mypy/build.py", line 4105, in process_graph
    done, still_working, results = manager.wait_for_done(graph)
  File "mypy/mypy/build.py", line 1173, in wait_for_done
    process_stale_scc(graph, next_scc, self)
  File "mypy/mypy/build.py", line 4268, in process_stale_scc
    mypy.semanal_main.semantic_analysis_for_scc(graph, scc, manager.errors)
  File "mypy/mypy/semanal_main.py", line 92, in semantic_analysis_for_scc
    process_top_levels(graph, scc, patches)
  File "mypy/mypy/semanal_main.py", line 230, in process_top_levels
    deferred, incomplete, progress = semantic_analyze_target(
  File "mypy/mypy/semanal_main.py", line 407, 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 719, in refresh_top_level
    self.accept(d)
  File "mypy/mypy/semanal.py", line 7627, in accept
    node.accept(self)
    ~~~~~~~~~~~^^^^^^
  File "mypy/mypy/nodes.py", line 1752, in accept
    return visitor.visit_assignment_stmt(self)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "mypy/mypy/semanal.py", line 3345, in visit_assignment_stmt
    self.process_type_annotation(s)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^
  File "mypy/mypy/semanal.py", line 3883, in process_type_annotation
    if analyzed is None or has_placeholder(analyzed):
                           ~~~~~~~~~~~~~~~^^^^^^^^^^
  File "mypy/mypy/semanal_shared.py", line 377, in has_placeholder
    return typ.accept(HasPlaceholders())
           ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^
  File "mypy/mypy/types.py", line 418, in accept
    return visitor.visit_type_alias_type(self)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "mypy/mypy/type_visitor.py", line 598, in visit_type_alias_type
    return get_proper_type(t).accept(self)
           ~~~~~~~~~~~~~~~^^^
  File "mypy/mypy/types.py", line 3648, in get_proper_type
    typ = typ._expand_once()
  File "mypy/mypy/types.py", line 382, in _expand_once
    return self.alias.target.accept(InstantiateAliasVisitor(mapping))
           ~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "mypy/mypy/types.py", line 1672, in accept
    return visitor.visit_instance(self)
           ~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "mypy/mypy/expandtype.py", line 215, in visit_instance
    args = self.expand_type_tuple_with_unpack(t.args)
  File "mypy/mypy/expandtype.py", line 533, in expand_type_tuple_with_unpack
    items.append(item.accept(self))
                 ~~~~~~~~~~~^^^^^^
  File "mypy/mypy/types.py", line 1236, in accept
    return visitor.visit_unpack_type(self)
           ~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "mypy/mypy/expandtype.py", line 387, in visit_unpack_type
    return UnpackType(t.type.accept(self))
                      ~~~~~~~~~~~~~^^^^^^
  File "mypy/mypy/types.py", line 1672, in accept
    return visitor.visit_instance(self)
           ~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "mypy/mypy/expandtype.py", line 228, in visit_instance
    arg = args[0]
          ~~~~^^^
IndexError: list index out of range

To Reproduce

from typing import Unpack

class C[*Ts]:
    pass

type T[T, *Ts] = C[*Ts]

x: T[bool, Unpack[tuple[()]]]

Your Environment

• Mypy version used: 1.19.1 and f363f35
• Mypy command-line flags: N/A
• Mypy configuration options from mypy.ini (and other config files): N/A
• Python version used: 3.14.3
• Operating system and version: Arch Linux