Crash on nested unpacking of next() call

[hamdanal]

Crash Report

Mypy crashes with the following traceback. Small reproducer below. Tested with version 0.991 (compiled) and mypy master (not compiled).

Traceback

$ mypy --show-traceback t.py
t.py:3: 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: 1.0.0+dev.b8c03ab6809aab56928f3cd865edb44944a600a2
Traceback (most recent call last):
  File "/tmp/venv/bin/mypy", line 8, in <module>
    sys.exit(console_entry())
  File "/tmp/venv/lib/python3.10/site-packages/mypy/__main__.py", line 15, in console_entry
    main()
  File "/tmp/venv/lib/python3.10/site-packages/mypy/main.py", line 95, in main
    res, messages, blockers = run_build(sources, options, fscache, t0, stdout, stderr)
  File "/tmp/venv/lib/python3.10/site-packages/mypy/main.py", line 174, in run_build
    res = build.build(sources, options, None, flush_errors, fscache, stdout, stderr)
  File "/tmp/venv/lib/python3.10/site-packages/mypy/build.py", line 194, in build
    result = _build(
  File "/tmp/venv/lib/python3.10/site-packages/mypy/build.py", line 277, in _build
    graph = dispatch(sources, manager, stdout)
  File "/tmp/venv/lib/python3.10/site-packages/mypy/build.py", line 2914, in dispatch
    process_graph(graph, manager)
  File "/tmp/venv/lib/python3.10/site-packages/mypy/build.py", line 3311, in process_graph
    process_stale_scc(graph, scc, manager)
  File "/tmp/venv/lib/python3.10/site-packages/mypy/build.py", line 3412, in process_stale_scc
    graph[id].type_check_first_pass()
  File "/tmp/venv/lib/python3.10/site-packages/mypy/build.py", line 2317, in type_check_first_pass
    self.type_checker().check_first_pass()
  File "/tmp/venv/lib/python3.10/site-packages/mypy/checker.py", line 471, in check_first_pass
    self.accept(d)
  File "/tmp/venv/lib/python3.10/site-packages/mypy/checker.py", line 579, in accept
    stmt.accept(self)
  File "/tmp/venv/lib/python3.10/site-packages/mypy/nodes.py", line 790, in accept
    return visitor.visit_func_def(self)
  File "/tmp/venv/lib/python3.10/site-packages/mypy/checker.py", line 956, in visit_func_def
    self._visit_func_def(defn)
  File "/tmp/venv/lib/python3.10/site-packages/mypy/checker.py", line 960, in _visit_func_def
    self.check_func_item(defn, name=defn.name)
  File "/tmp/venv/lib/python3.10/site-packages/mypy/checker.py", line 1032, in check_func_item
    self.check_func_def(defn, typ, name, allow_empty)
  File "/tmp/venv/lib/python3.10/site-packages/mypy/checker.py", line 1212, in check_func_def
    self.accept(item.body)
  File "/tmp/venv/lib/python3.10/site-packages/mypy/checker.py", line 579, in accept
    stmt.accept(self)
  File "/tmp/venv/lib/python3.10/site-packages/mypy/nodes.py", line 1202, in accept
    return visitor.visit_block(self)
  File "/tmp/venv/lib/python3.10/site-packages/mypy/checker.py", line 2592, in visit_block
    self.accept(s)
  File "/tmp/venv/lib/python3.10/site-packages/mypy/checker.py", line 579, in accept
    stmt.accept(self)
  File "/tmp/venv/lib/python3.10/site-packages/mypy/nodes.py", line 1289, in accept
    return visitor.visit_assignment_stmt(self)
  File "/tmp/venv/lib/python3.10/site-packages/mypy/checker.py", line 2640, in visit_assignment_stmt
    self.check_assignment(s.lvalues[-1], s.rvalue, s.type is None, s.new_syntax)
  File "/tmp/venv/lib/python3.10/site-packages/mypy/checker.py", line 2695, in check_assignment
    self.check_assignment_to_multiple_lvalues(
  File "/tmp/venv/lib/python3.10/site-packages/mypy/checker.py", line 3349, in check_assignment_to_multiple_lvalues    self.check_multi_assignment(lvalues, rvalue, context, infer_lvalue_type)
  File "/tmp/venv/lib/python3.10/site-packages/mypy/checker.py", line 3396, in check_multi_assignment
    self.check_multi_assignment_from_tuple(
  File "/tmp/venv/lib/python3.10/site-packages/mypy/checker.py", line 3511, in check_multi_assignment_from_tuple
    self.expr_checker.accept(rvalue, lvalue_type)
  File "/tmp/venv/lib/python3.10/site-packages/mypy/checkexpr.py", line 4768, in accept
    typ = node.accept(self)
  File "/tmp/venv/lib/python3.10/site-packages/mypy/nodes.py", line 1871, in accept
    return visitor.visit_call_expr(self)
  File "/tmp/venv/lib/python3.10/site-packages/mypy/checkexpr.py", line 424, in visit_call_expr
    return self.visit_call_expr_inner(e, allow_none_return=allow_none_return)
  File "/tmp/venv/lib/python3.10/site-packages/mypy/checkexpr.py", line 544, in visit_call_expr_inner
    ret_type = self.check_call_expr_with_callee_type(
  File "/tmp/venv/lib/python3.10/site-packages/mypy/checkexpr.py", line 1195, in check_call_expr_with_callee_type
    ret_type, callee_type = self.check_call(
  File "/tmp/venv/lib/python3.10/site-packages/mypy/checkexpr.py", line 1289, in check_call
    return self.check_overload_call(
  File "/tmp/venv/lib/python3.10/site-packages/mypy/checkexpr.py", line 2211, in check_overload_call
    inferred_result = self.infer_overload_return_type(
  File "/tmp/venv/lib/python3.10/site-packages/mypy/checkexpr.py", line 2365, in infer_overload_return_type
    ret_type, infer_type = self.check_call(
  File "/tmp/venv/lib/python3.10/site-packages/mypy/checkexpr.py", line 1278, in check_call
    return self.check_callable_call(
  File "/tmp/venv/lib/python3.10/site-packages/mypy/checkexpr.py", line 1419, in check_callable_call
    callee = self.infer_function_type_arguments_using_context(callee, context)
  File "/tmp/venv/lib/python3.10/site-packages/mypy/checkexpr.py", line 1648, in infer_function_type_arguments_using_context
    erased_ctx = replace_meta_vars(ctx, ErasedType())
  File "/tmp/venv/lib/python3.10/site-packages/mypy/erasetype.py", line 153, in replace_meta_vars
    return t.accept(TypeVarEraser(lambda id: id.is_meta_var(), target_type))
  File "/tmp/venv/lib/python3.10/site-packages/mypy/types.py", line 2205, in accept
    return visitor.visit_tuple_type(self)
  File "/tmp/venv/lib/python3.10/site-packages/mypy/type_visitor.py", line 249, in visit_tuple_type
    self.translate_types(t.items),
  File "/tmp/venv/lib/python3.10/site-packages/mypy/type_visitor.py", line 276, in translate_types
    return [t.accept(self) for t in types]
  File "/tmp/venv/lib/python3.10/site-packages/mypy/type_visitor.py", line 276, in <listcomp>
    return [t.accept(self) for t in types]
  File "/tmp/venv/lib/python3.10/site-packages/mypy/types.py", line 2205, in accept
    return visitor.visit_tuple_type(self)
  File "/tmp/venv/lib/python3.10/site-packages/mypy/type_visitor.py", line 249, in visit_tuple_type
    self.translate_types(t.items),
  File "/tmp/venv/lib/python3.10/site-packages/mypy/type_visitor.py", line 276, in translate_types
    return [t.accept(self) for t in types]
  File "/tmp/venv/lib/python3.10/site-packages/mypy/type_visitor.py", line 276, in <listcomp>
    return [t.accept(self) for t in types]
  File "/tmp/venv/lib/python3.10/site-packages/mypy/types.py", line 2566, in accept
    assert isinstance(visitor, SyntheticTypeVisitor)
AssertionError:
t.py:3: : note: use --pdb to drop into pdb

To Reproduce

$ cat t.py
def mypy_crash() -> None:
    d: dict[int, tuple[str, int, str]] = {}
    k, (v1, *_) = next(iter(d.items()))  # <-- mypy crashes here

Your Environment

• Mypy version used: mypy 1.0.0+dev.b8c03ab6809aab56928f3cd865edb44944a600a2 (compiled: no) (installed with pip install git+https://github.com/python/mypy#master)
• Mypy command-line flags: mypy t.py
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: Python 3.10.6 (main, Nov 2 2022, 18:53:38) [GCC 11.3.0]
• Operating system and version:
  • OS: Ubuntu 22.04.1 LTS on Windows 10 x86_64
  • Kernel: 5.15.74.2-microsoft-standard-WSL2
  • Also crashes on Debian Bullseye native (non WSL)

[AlexWaygood]

Looks like this has crashed since at least mypy 0.620! https://mypy-play.net/?mypy=0.620&python=3.11&flags=show-traceback&gist=a318677a6b8022dea43e5e2b95f70795

[hamdanal]

Note that if the same two lines are not in a function, the crash disappears and the file checks successfully

$ cat t.py
d: dict[int, tuple[str, int, str]] = {}
k, (v1, *_) = next(iter(d.items()))

$ mypy t.py
Success: no issues found in 1 source file