Crash in union with a forward reference to a bad type.

[ilevkivskyi]

This code causes a crash:

Alias = Union[Bad, C, D]

Bad = object()

class C:
    pass
class D:
    pass

option: Alias
if isinstance(option, C):
    pass

Here is a traceback:

tmp.py:3: error: Invalid type "tmp.Bad"
tmp.py:13: error: INTERNAL ERROR -- please report a bug at https://github.com/python/mypy/issues version: 0.570-dev-d2c04190b8d407476c535f30d07a9f6712e7b12a-dirty
Traceback (most recent call last):
  File "/Users/ilevkivskyi/mypy/bin/mypy", line 11, in <module>
    load_entry_point('mypy===0.570-dev-d2c04190b8d407476c535f30d07a9f6712e7b12a-dirty', 'console_scripts', 'mypy')()
  File "/Users/ilevkivskyi/mypy/lib/python3.6/site-packages/mypy/__main__.py", line 7, in console_entry
    main(None)
  File "/Users/ilevkivskyi/mypy/lib/python3.6/site-packages/mypy/main.py", line 80, in main
    type_check_only(sources, bin_dir, options, flush_errors)
  File "/Users/ilevkivskyi/mypy/lib/python3.6/site-packages/mypy/main.py", line 129, in type_check_only
    flush_errors=flush_errors)
  File "/Users/ilevkivskyi/mypy/lib/python3.6/site-packages/mypy/build.py", line 172, in build
    result = _build(sources, options, alt_lib_path, bin_dir, saved_cache, flush_errors)
  File "/Users/ilevkivskyi/mypy/lib/python3.6/site-packages/mypy/build.py", line 258, in _build
    graph = dispatch(sources, manager)
  File "/Users/ilevkivskyi/mypy/lib/python3.6/site-packages/mypy/build.py", line 2048, in dispatch
    process_graph(graph, manager)
  File "/Users/ilevkivskyi/mypy/lib/python3.6/site-packages/mypy/build.py", line 2349, in process_graph
    process_stale_scc(graph, scc, manager)
  File "/Users/ilevkivskyi/mypy/lib/python3.6/site-packages/mypy/build.py", line 2525, in process_stale_scc
    graph[id].type_check_first_pass()
  File "/Users/ilevkivskyi/mypy/lib/python3.6/site-packages/mypy/build.py", line 1923, in type_check_first_pass
    self.type_checker().check_first_pass()
  File "/Users/ilevkivskyi/mypy/lib/python3.6/site-packages/mypy/checker.py", line 228, in check_first_pass
    self.accept(d)
  File "/Users/ilevkivskyi/mypy/lib/python3.6/site-packages/mypy/checker.py", line 323, in accept
    stmt.accept(self)
  File "/Users/ilevkivskyi/mypy/lib/python3.6/site-packages/mypy/nodes.py", line 928, in accept
    return visitor.visit_if_stmt(self)
  File "/Users/ilevkivskyi/mypy/lib/python3.6/site-packages/mypy/checker.py", line 2232, in visit_if_stmt
    if_map, else_map = self.find_isinstance_check(e)
  File "/Users/ilevkivskyi/mypy/lib/python3.6/site-packages/mypy/checker.py", line 2835, in find_isinstance_check
    return conditional_type_map(expr, vartype, type)
  File "/Users/ilevkivskyi/mypy/lib/python3.6/site-packages/mypy/checker.py", line 3191, in conditional_type_map
    remaining_type = restrict_subtype_away(current_type, proposed_precise_type)
  File "/Users/ilevkivskyi/mypy/lib/python3.6/site-packages/mypy/subtypes.py", line 817, in restrict_subtype_away
    new_items = [item for item in t.relevant_items()
  File "/Users/ilevkivskyi/mypy/lib/python3.6/site-packages/mypy/subtypes.py", line 818, in <listcomp>
    if (not (is_proper_subtype(erase_type(item), erased_s) or
  File "/Users/ilevkivskyi/mypy/lib/python3.6/site-packages/mypy/erasetype.py", line 25, in erase_type
    return typ.accept(EraseTypeVisitor())
  File "/Users/ilevkivskyi/mypy/lib/python3.6/site-packages/mypy/types.py", line 194, in accept
    return visitor.visit_unbound_type(self)
  File "/Users/ilevkivskyi/mypy/lib/python3.6/site-packages/mypy/erasetype.py", line 31, in visit_unbound_type
    assert False, 'Not supported'
AssertionError: Not supported

It looks like the problem is that clearly erroneous types are not wrapped in ForwardRef and are not post-processed in third pass. There are two options that I see now for UnboundTypes (maybe there are other):

• Expect them in type checker and amend the corresponding visitors
• Replace them with Any after giving an Invalid type error (maybe with better error message, but we already have an issue for this.)

I would prefer the second option, since this seems simpler and would probably minimize the amount of not useful errors (because of the initially invalid type).

[ilevkivskyi]

Note: It looks like the crash happens only if there are at least three elements in the union.

[ilevkivskyi]

Another possible scenario for this crash is #4653