Union changes in Python 3.14 crash mypy

Crash Report

typeshed PR python/typeshed#13740, which adapts the changes to Union in Python 3.14 for typeshed crash mypy. (When running the tests with Python 3.14 or 3.15. Older Python versions are fine.)

The changes

Traceback

python tests/stubtest_stdlib.py
  shell: /usr/bin/bash -e {0}
  env:
    PIP_DISABLE_PIP_VERSION_CHECK: 1
    FORCE_COLOR: 1
    TERM: xterm-256color
    pythonLocation: /opt/hostedtoolcache/Python/3.15.0-beta.2/x64
    PKG_CONFIG_PATH: /opt/hostedtoolcache/Python/3.15.0-beta.2/x64/lib/pkgconfig
    Python_ROOT_DIR: /opt/hostedtoolcache/Python/3.15.0-beta.2/x64
    Python2_ROOT_DIR: /opt/hostedtoolcache/Python/3.15.0-beta.2/x64
    Python3_ROOT_DIR: /opt/hostedtoolcache/Python/3.15.0-beta.2/x64
    LD_LIBRARY_PATH: /opt/hostedtoolcache/Python/3.15.0-beta.2/x64/lib
/opt/hostedtoolcache/Python/3.15.0-beta.2/x64/bin/python -m mypy.stubtest --check-typeshed --show-traceback --strict-type-check-only --custom-typeshed-dir . --allowlist stdlib/@tests/stubtest_allowlists/common.txt --allowlist stdlib/@tests/stubtest_allowlists/linux.txt --allowlist stdlib/@tests/stubtest_allowlists/py315.txt --allowlist stdlib/@tests/stubtest_allowlists/linux-py315.txt
/home/runner/work/typeshed/typeshed/stdlib/typing.pyi:1002: error: INTERNAL ERROR -- Please try using mypy master on GitHub:
https://mypy.readthedocs.io/en/stable/common_issues.html#using-a-development-mypy-build
Please report a bug at https://github.com/python/mypy/issues
version: 2.1.0
/home/runner/work/typeshed/typeshed/stdlib/typing.pyi:1002: note: use --pdb to drop into pdb
stdlib/typing_extensions.pyi:287: error: Overloaded function signatures 1 and 3 overlap with incompatible return types  [overload-overlap]
Traceback (most recent call last):
  File "<frozen runpy>", line 201, in _run_module_as_main
  File "<frozen runpy>", line 87, in _run_code
  File "/opt/hostedtoolcache/Python/3.15.0-beta.2/x64/lib/python3.15/site-packages/mypy/stubtest.py", line 2593, in <module>
    sys.exit(main())
  File "/opt/hostedtoolcache/Python/3.15.0-beta.2/x64/lib/python3.15/site-packages/mypy/stubtest.py", line 2589, in main
    return test_stubs(parse_options(sys.argv[1:]))
  File "/opt/hostedtoolcache/Python/3.15.0-beta.2/x64/lib/python3.15/site-packages/mypy/stubtest.py", line 2419, in test_stubs
    modules = build_stubs(modules, options, find_submodules=not args.check_typeshed)
  File "/opt/hostedtoolcache/Python/3.15.0-beta.2/x64/lib/python3.15/site-packages/mypy/stubtest.py", line 2215, in build_stubs
    res = mypy.build.build(sources=sources, options=options)
  File "/opt/hostedtoolcache/Python/3.15.0-beta.2/x64/lib/python3.15/site-packages/mypy/build.py", line 422, in build
    result = build_inner(
  File "/opt/hostedtoolcache/Python/3.15.0-beta.2/x64/lib/python3.15/site-packages/mypy/build.py", line 537, in build_inner
    graph = dispatch(sources, manager, stdout, connect_threads)
  File "/opt/hostedtoolcache/Python/3.15.0-beta.2/x64/lib/python3.15/site-packages/mypy/build.py", line 4150, in dispatch
    process_graph(graph, manager)
  File "/opt/hostedtoolcache/Python/3.15.0-beta.2/x64/lib/python3.15/site-packages/mypy/build.py", line 4618, in process_graph
    done, still_working, results = manager.wait_for_done(graph)
  File "/opt/hostedtoolcache/Python/3.15.0-beta.2/x64/lib/python3.15/site-packages/mypy/build.py", line 1492, in wait_for_done
    process_stale_scc(graph, next_scc, self)
  File "/opt/hostedtoolcache/Python/3.15.0-beta.2/x64/lib/python3.15/site-packages/mypy/build.py", line 4792, in process_stale_scc
    graph[id].type_check_first_pass()
  File "/opt/hostedtoolcache/Python/3.15.0-beta.2/x64/lib/python3.15/site-packages/mypy/build.py", line 3414, in type_check_first_pass
    self.type_checker().check_first_pass(recurse_into_functions=recurse_into_functions)
  File "/opt/hostedtoolcache/Python/3.15.0-beta.2/x64/lib/python3.15/site-packages/mypy/checker.py", line 618, in check_first_pass
    self.accept(d)
  File "/opt/hostedtoolcache/Python/3.15.0-beta.2/x64/lib/python3.15/site-packages/mypy/checker.py", line 765, in accept
    stmt.accept(self)
  File "/opt/hostedtoolcache/Python/3.15.0-beta.2/x64/lib/python3.15/site-packages/mypy/nodes.py", line 1907, in accept
    return visitor.visit_assignment_stmt(self)
  File "/opt/hostedtoolcache/Python/3.15.0-beta.2/x64/lib/python3.15/site-packages/mypy/checker.py", line 3329, in visit_assignment_stmt
    self.check_type_alias_rvalue(s)
  File "/opt/hostedtoolcache/Python/3.15.0-beta.2/x64/lib/python3.15/site-packages/mypy/checker.py", line 3370, in check_type_alias_rvalue
    alias_type = self.expr_checker.accept(s.rvalue)
  File "/opt/hostedtoolcache/Python/3.15.0-beta.2/x64/lib/python3.15/site-packages/mypy/checkexpr.py", line 6176, in accept
    typ = self.accept_maybe_cache(node, type_context=type_context)
  File "/opt/hostedtoolcache/Python/3.15.0-beta.2/x64/lib/python3.15/site-packages/mypy/checkexpr.py", line 6211, in accept_maybe_cache
    typ = node.accept(self)
  File "/opt/hostedtoolcache/Python/3.15.0-beta.2/x64/lib/python3.15/site-packages/mypy/nodes.py", line 2697, in accept
    return visitor.visit_op_expr(self)
  File "/opt/hostedtoolcache/Python/3.15.0-beta.2/x64/lib/python3.15/site-packages/mypy/checkexpr.py", line 3559, in visit_op_expr
    return self.accept(e.analyzed)
  File "/opt/hostedtoolcache/Python/3.15.0-beta.2/x64/lib/python3.15/site-packages/mypy/checkexpr.py", line 6178, in accept
    typ = node.accept(self)  # r-value type, when interpreted as a value expression
  File "/opt/hostedtoolcache/Python/3.15.0-beta.2/x64/lib/python3.15/site-packages/mypy/nodes.py", line 3420, in accept
    return visitor.visit_type_alias_expr(self)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "/opt/hostedtoolcache/Python/3.15.0-beta.2/x64/lib/python3.15/site-packages/mypy/checkexpr.py", line 4967, in visit_type_alias_expr
    return self.alias_type_in_runtime_context(alias.node, ctx=alias, alias_definition=True)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/hostedtoolcache/Python/3.15.0-beta.2/x64/lib/python3.15/site-packages/mypy/checkexpr.py", line 5024, in alias_type_in_runtime_context
    return self.chk.named_generic_type("types.UnionType", item.items)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/hostedtoolcache/Python/3.15.0-beta.2/x64/lib/python3.15/site-packages/mypy/checker.py", line 7788, in named_generic_type
    info = self.lookup_typeinfo(name)
  File "/opt/hostedtoolcache/Python/3.15.0-beta.2/x64/lib/python3.15/site-packages/mypy/checker.py", line 7797, in lookup_typeinfo
    assert isinstance(node, TypeInfo), node
           ~~~~~~~~~~^^^^^^^^^^^^^^^^
AssertionError: TypeAlias:723(
  UnionType
  Never
  False)

Your Environment

Command run was: /opt/hostedtoolcache/Python/3.15.0-beta.2/x64/bin/python -m mypy.stubtest --check-typeshed --show-traceback --strict-type-check-only --custom-typeshed-dir . --allowlist stdlib/@tests/stubtest_allowlists/common.txt --allowlist stdlib/@tests/stubtest_allowlists/linux.txt --allowlist stdlib/@tests/stubtest_allowlists/py315.txt --allowlist stdlib/@tests/stubtest_allowlists/linux-py315.txt

• Mypy version used: 2.1.0
• Python version used: 3.14.5/3.15.0b2
• Operating system and version: Crashed on Ubuntu, Windows, MacOS

Start with alias_type_in_runtime_context in mypy/checkexpr.py and the lookup_typeinfo path in mypy/checker.py, using the traceback as the entry point. Reproduce with the provided python -m mypy.stubtest command on Python 3.14 or 3.15, then verify that the typeshed stubtest run completes without the internal error.