Mypy crashes on @classmethod #4575

from dateutil.parser import parse
from utils.general_utils import verify_meta
from io import StringIO
...

class Details(NamedTuple):
    token: Optional[str] = None
    version: Optional[str] = None
    last1: Optional[datetime.datetime] = None
    loc1: Optional[Decimal] = None
    loc2: Optional[Decimal] = None
    last2: Optional[datetime] = None

    @classmethod <-- Crashes here --------------------------
    def from_stream(cls, stream: StringIO) -> 'Details':
        details_json = json.load(stream)
        verify_meta(details_json)

        token = details_json.get('token', None)
        version = details_json.get('version', None)
        last1 = details_json.get('last1', None)
        loc1 = details_json.get('loc1', None)
        loc2 = details_json.get('loc2', None)
        last2 = details_json.get('last2', None)

        return DeviceDetailsMessage(token, version, last1,
                                    Decimal(loc1) if loc1 else None,
                                    Decimal(loc2) if loc2 else None,
                                    parse(last2) if last2 else None)

13:01 $ mypy --show-traceback --ignore-missing-imports --python-version 3.6 .
./profiles/file_process.py:29: error: INTERNAL ERROR -- please report a bug at https://github.com/python/mypy/issues version: 0.560
Traceback (most recent call last):
  File "/Users/code/source/env_env/bin/mypy", line 11, in <module>
    sys.exit(console_entry())
  File "/Users/code/source/env_env/lib/python3.6/site-packages/mypy/__main__.py", line 7, in console_entry
    main(None)
  File "/Users/code/source/env_env/lib/python3.6/site-packages/mypy/main.py", line 66, in main
    res = type_check_only(sources, bin_dir, options)
  File "/Users/code/source/env_env/lib/python3.6/site-packages/mypy/main.py", line 119, in type_check_only
    options=options)
  File "/Users/code/source/env_env/lib/python3.6/site-packages/mypy/build.py", line 218, in build
    graph = dispatch(sources, manager)
  File "/Users/code/source/env_env/lib/python3.6/site-packages/mypy/build.py", line 1997, in dispatch
    process_graph(graph, manager)
  File "/Users/code/source/env_env/lib/python3.6/site-packages/mypy/build.py", line 2299, in process_graph
    process_stale_scc(graph, scc, manager)
  File "/Users/code/source/env_env/lib/python3.6/site-packages/mypy/build.py", line 2467, in process_stale_scc
    graph[id].semantic_analysis()
  File "/Users/code/source/env_env/lib/python3.6/site-packages/mypy/build.py", line 1855, in semantic_analysis
    self.manager.semantic_analyzer.visit_file(self.tree, self.xpath, self.options, patches)
  File "/Users/code/source/env_env/lib/python3.6/site-packages/mypy/semanal.py", line 287, in visit_file
    self.accept(d)
  File "/Users/code/source/env_env/lib/python3.6/site-packages/mypy/semanal.py", line 3861, in accept
    node.accept(self)
  File "/Users/code/source/env_env/lib/python3.6/site-packages/mypy/nodes.py", line 696, in accept
    return visitor.visit_class_def(self)
  File "/Users/code/source/env_env/lib/python3.6/site-packages/mypy/semanal.py", line 666, in visit_class_def
    defn.defs.accept(self)
  File "/Users/code/source/env_env/lib/python3.6/site-packages/mypy/nodes.py", line 756, in accept
    return visitor.visit_block(self)
  File "/Users/code/source/env_env/lib/python3.6/site-packages/mypy/semanal.py", line 1610, in visit_block
    self.accept(s)
  File "/Users/code/source/env_env/lib/python3.6/site-packages/mypy/semanal.py", line 3861, in accept
    node.accept(self)
  File "/Users/code/source/env_env/lib/python3.6/site-packages/mypy/nodes.py", line 573, in accept
    return visitor.visit_decorator(self)
  File "/Users/code/source/env_env/lib/python3.6/site-packages/mypy/semanal.py", line 2933, in visit_decorator
    dec.func.accept(self)
  File "/Users/code/source/env_env/lib/python3.6/site-packages/mypy/nodes.py", line 497, in accept
    return visitor.visit_func_def(self)
  File "/Users/code/source/env_env/lib/python3.6/site-packages/mypy/semanal.py", line 386, in visit_func_def
    self.prepare_method_signature(defn, self.type)
  File "/Users/code/source/env_env/lib/python3.6/site-packages/mypy/semanal.py", line 438, in prepare_method_signature
    leading_type = self.class_type(info)
  File "/Users/code/source/env_env/lib/python3.6/site-packages/mypy/semanal.py", line 1234, in class_type
    leading_type = checkmember.type_object_type(info, self.builtin_type)
  File "/Users/code/source/env_env/lib/python3.6/site-packages/mypy/checkmember.py", line 543, in type_object_type
    return type_object_type_from_function(init_method, info, fallback)
  File "/Users/code/source/env_env/lib/python3.6/site-packages/mypy/checkmember.py", line 548, in type_object_type_from_function
    signature = bind_self(function_type(init_or_new, fallback))
  File "/Users/code/source/env_env/lib/python3.6/site-packages/mypy/checkmember.py", line 682, in bind_self
    arg_types = [expand(x) for x in func.arg_types[1:]]
  File "/Users/code/source/env_env/lib/python3.6/site-packages/mypy/checkmember.py", line 682, in <listcomp>
    arg_types = [expand(x) for x in func.arg_types[1:]]
  File "/Users/code/source/env_env/lib/python3.6/site-packages/mypy/checkmember.py", line 680, in expand
    return expand_type(target, {func.variables[0].id: typearg})
  File "/Users/code/source/env_env/lib/python3.6/site-packages/mypy/expandtype.py", line 16, in expand_type
    return typ.accept(ExpandTypeVisitor(env))
  File "/Users/code/source/env_env/lib/python3.6/site-packages/mypy/types.py", line 1230, in accept
    return visitor.visit_union_type(self)
  File "/Users/code/source/env_env/lib/python3.6/site-packages/mypy/expandtype.py", line 117, in visit_union_type
    return UnionType.make_simplified_union(self.expand_types(t.items), t.line, t.column)
  File "/Users/code/source/env_env/lib/python3.6/site-packages/mypy/expandtype.py", line 132, in expand_types
    a.append(t.accept(self))
  File "/Users/code/source/env_env/lib/python3.6/site-packages/mypy/types.py", line 1415, in accept
    return visitor.visit_forwardref_type(self)
  File "/Users/code/source/env_env/lib/python3.6/site-packages/mypy/types.py", line 1500, in visit_forwardref_type
    raise RuntimeError('Internal error: unresolved forward reference')
RuntimeError: Internal error: unresolved forward reference
./profiles/file_process.py:29: : note: use --pdb to drop into pdb