Crash on syntax error with unpacking

[remdragon]

Please provide more information to help us understand the issue:

• Are you reporting a bug, or opening a feature request?
  bug
• Please insert below the code you are checking with mypy,
  or a mock-up repro if the source is private. We would appreciate
  if you try to simplify your case to a minimal repro.

import sys
from types import TracebackType
from typing import Optional as Opt, Tuple, Type, Union

class Event:
	exc_info: Opt[Union[Tuple[Type[BaseException],BaseException,TracebackType],Tuple[None,None,None]]] = None

def foo ( event: Event ) -> None:
	try:
		assert False
	except Exception:
		event.exc_info = *sys.exc_info() # mypy crashes on this syntax error
	
event = Event()
foo ( event )
print ( repr ( event.exc_info ) )

• What is the actual behavior/output?
  see traceback below
• What is the behavior/output you expect?
  report the syntax error without crashing
• What are the versions of mypy and Python you are using?
  Do you see the same issue after installing mypy from Git master?
  Python 3.8.0 (tags/v3.8.0:fa919fd, Oct 14 2019, 19:21:23) [MSC v.1916 32 bit (Intel)] on win32
  mypy 0.782
  Haven't tried master, this bug isn't a show stopper
• What are the mypy flags you are using? (For example --strict-optional)
  --strict --implicit-reexport

boom.py:12: error: Can use starred expression only as assignment target
boom.py:12: error: INTERNAL ERROR -- Please try using mypy master on Github:
https://mypy.rtfd.io/en/latest/common_issues.html#using-a-development-mypy-build
Please report a bug at https://github.com/python/mypy/issues
version: 0.782
Traceback (most recent call last):
  File "c:\python38-32\lib\runpy.py", line 192, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "c:\python38-32\lib\runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "C:\Python38-32\Scripts\mypy.exe\__main__.py", line 7, in <module>
    sys.exit(console_entry())
  File "c:\python38-32\lib\site-packages\mypy\__main__.py", line 8, in console_entry
    main(None, sys.stdout, sys.stderr)
  File "c:\python38-32\lib\site-packages\mypy\main.py", line 89, in main
    res = build.build(sources, options, None, flush_errors, fscache, stdout, stderr)
  File "c:\python38-32\lib\site-packages\mypy\build.py", line 180, in build
    result = _build(
  File "c:\python38-32\lib\site-packages\mypy\build.py", line 252, in _build
    graph = dispatch(sources, manager, stdout)
  File "c:\python38-32\lib\site-packages\mypy\build.py", line 2626, in dispatch
    process_graph(graph, manager)
  File "c:\python38-32\lib\site-packages\mypy\build.py", line 2949, in process_graph
    process_stale_scc(graph, scc, manager)
  File "c:\python38-32\lib\site-packages\mypy\build.py", line 3047, in process_stale_scc
    graph[id].type_check_first_pass()
  File "c:\python38-32\lib\site-packages\mypy\build.py", line 2107, in type_check_first_pass
    self.type_checker().check_first_pass()
  File "c:\python38-32\lib\site-packages\mypy\checker.py", line 294, in check_first_pass
    self.accept(d)
  File "c:\python38-32\lib\site-packages\mypy\checker.py", line 401, in accept
    stmt.accept(self)
  File "c:\python38-32\lib\site-packages\mypy\nodes.py", line 676, in accept
    return visitor.visit_func_def(self)
  File "c:\python38-32\lib\site-packages\mypy\checker.py", line 726, in visit_func_def
    self._visit_func_def(defn)
  File "c:\python38-32\lib\site-packages\mypy\checker.py", line 730, in _visit_func_def
    self.check_func_item(defn, name=defn.name)
  File "c:\python38-32\lib\site-packages\mypy\checker.py", line 792, in check_func_item
    self.check_func_def(defn, typ, name)
  File "c:\python38-32\lib\site-packages\mypy\checker.py", line 975, in check_func_def
    self.accept(item.body)
  File "c:\python38-32\lib\site-packages\mypy\checker.py", line 401, in accept
    stmt.accept(self)
  File "c:\python38-32\lib\site-packages\mypy\nodes.py", line 1004, in accept
    return visitor.visit_block(self)
  File "c:\python38-32\lib\site-packages\mypy\checker.py", line 1973, in visit_block
    self.accept(s)
  File "c:\python38-32\lib\site-packages\mypy\checker.py", line 401, in accept
    stmt.accept(self)
  File "c:\python38-32\lib\site-packages\mypy\nodes.py", line 1234, in accept
    return visitor.visit_try_stmt(self)
  File "c:\python38-32\lib\site-packages\mypy\checker.py", line 3308, in visit_try_stmt
    self.visit_try_without_finally(s, try_frame=bool(s.finally_body))
  File "c:\python38-32\lib\site-packages\mypy\checker.py", line 3363, in visit_try_without_finally
    self.accept(s.handlers[i])
  File "c:\python38-32\lib\site-packages\mypy\checker.py", line 401, in accept
    stmt.accept(self)
  File "c:\python38-32\lib\site-packages\mypy\nodes.py", line 1004, in accept
    return visitor.visit_block(self)
  File "c:\python38-32\lib\site-packages\mypy\checker.py", line 1973, in visit_block
    self.accept(s)
  File "c:\python38-32\lib\site-packages\mypy\checker.py", line 401, in accept
    stmt.accept(self)
  File "c:\python38-32\lib\site-packages\mypy\nodes.py", line 1062, in accept
    return visitor.visit_assignment_stmt(self)
  File "c:\python38-32\lib\site-packages\mypy\checker.py", line 2011, in visit_assignment_stmt
    self.check_assignment(s.lvalues[-1], s.rvalue, s.type is None, s.new_syntax)
  File "c:\python38-32\lib\site-packages\mypy\checker.py", line 2119, in check_assignment
    rvalue_type, lvalue_type, infer_lvalue_type = self.check_member_assignment(
  File "c:\python38-32\lib\site-packages\mypy\checker.py", line 2974, in check_member_assignment
    rvalue_type = self.check_simple_assignment(attribute_type, rvalue, context,
  File "c:\python38-32\lib\site-packages\mypy\checker.py", line 2945, in check_simple_assignment
    self.check_subtype(rvalue_type, lvalue_type, context, msg,
  File "c:\python38-32\lib\site-packages\mypy\checker.py", line 4482, in check_subtype
    if is_subtype(subtype, supertype):
  File "c:\python38-32\lib\site-packages\mypy\subtypes.py", line 93, in is_subtype
    return _is_subtype(left, right,
  File "c:\python38-32\lib\site-packages\mypy\subtypes.py", line 118, in _is_subtype
    is_subtype_of_item = any(is_subtype(orig_left, item,
  File "c:\python38-32\lib\site-packages\mypy\subtypes.py", line 118, in <genexpr>
    is_subtype_of_item = any(is_subtype(orig_left, item,
  File "c:\python38-32\lib\site-packages\mypy\subtypes.py", line 93, in is_subtype
    return _is_subtype(left, right,
  File "c:\python38-32\lib\site-packages\mypy\subtypes.py", line 135, in _is_subtype
    return left.accept(SubtypeVisitor(orig_right,
  File "c:\python38-32\lib\site-packages\mypy\types.py", line 1676, in accept
    assert isinstance(visitor, SyntheticTypeVisitor)
AssertionError:
boom.py:12: : note: use --pdb to drop into pdb

[JukkaL]

I verified the crash. #8938 and #3825 seem related, but it's unclear if these are the exact same issue.

[progval]

A simpler example to reproduce this crash:

$ echo "*foo" > crashy.py 

$ mypy --show-traceback crashy.py 
crashy.py:1: error: Can use starred expression only as assignment target
crashy.py: 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: 0.820+dev.28718facbeea5b3c33ec96bc335bb018d3b0a395
Traceback (most recent call last):
  File "/home/dev/.local/bin/mypy", line 8, in <module>
    sys.exit(console_entry())
  File "/home/dev/.local/lib/python3.7/site-packages/mypy/__main__.py", line 11, in console_entry
    main(None, sys.stdout, sys.stderr)
  File "/home/dev/.local/lib/python3.7/site-packages/mypy/main.py", line 98, in main
    res = build.build(sources, options, None, flush_errors, fscache, stdout, stderr)
  File "/home/dev/.local/lib/python3.7/site-packages/mypy/build.py", line 180, in build
    sources, options, alt_lib_path, flush_errors, fscache, stdout, stderr, extra_plugins
  File "/home/dev/.local/lib/python3.7/site-packages/mypy/build.py", line 253, in _build
    graph = dispatch(sources, manager, stdout)
  File "/home/dev/.local/lib/python3.7/site-packages/mypy/build.py", line 2689, in dispatch
    process_graph(graph, manager)
  File "/home/dev/.local/lib/python3.7/site-packages/mypy/build.py", line 3013, in process_graph
    process_stale_scc(graph, scc, manager)
  File "/home/dev/.local/lib/python3.7/site-packages/mypy/build.py", line 3114, in process_stale_scc
    graph[id].finish_passes()
  File "/home/dev/.local/lib/python3.7/site-packages/mypy/build.py", line 2209, in finish_passes
    free_tree(self.tree)
  File "/usr/lib/python3.7/contextlib.py", line 130, in __exit__
    self.gen.throw(type, value, traceback)
  File "/home/dev/.local/lib/python3.7/site-packages/mypy/build.py", line 1954, in wrap_context
    yield
  File "/home/dev/.local/lib/python3.7/site-packages/mypy/build.py", line 2196, in finish_passes
    self._patch_indirect_dependencies(self.type_checker().module_refs, self.type_map())
  File "/home/dev/.local/lib/python3.7/site-packages/mypy/build.py", line 2223, in _patch_indirect_dependencies
    encountered = self.manager.indirection_detector.find_modules(types) | module_refs
  File "/home/dev/.local/lib/python3.7/site-packages/mypy/indirection.py", line 27, in find_modules
    return self._visit(typs)
  File "/home/dev/.local/lib/python3.7/site-packages/mypy/indirection.py", line 41, in _visit
    modules = typ.accept(self)
  File "/home/dev/.local/lib/python3.7/site-packages/mypy/types.py", line 1737, in accept
    assert isinstance(visitor, SyntheticTypeVisitor)
AssertionError: 
crashy.py: : note: use --pdb to drop into pdb

This happens on the current master