# container.pyi
_T = TypeVar('_T')

class DataTypeA: ...

class DataTypeB: ...

class CoerciveContainer( Protocol, Generic[_T] ):
  @overload
  def __set__( s, obj, value: Container[_T] ) -> None: ...

  @overload
  def __set__( s, obj, value: _T ) -> None: ...

class Container( CoerciveContainer[_T], Generic[_T] ): ...

# bar.py
from container import *

class Bar:
  def __init__( s ) -> None:
    s.foo = Container[DataTypeA]()
    s.foo = DataTypeB() # crash at this line

(mypy) $ mypy bar.py --show-traceback
bar.py:6: 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: 0.720+dev.dc6da54f50d5443fe46308046719043eba03c84d
Traceback (most recent call last):
  File "///.virtualenvs/mypy/bin/mypy", line 10, in <module>
    sys.exit(console_entry())
  File "///.virtualenvs/mypy/lib/python3.7/site-packages/mypy/__main__.py", line 8, in console_entry
    main(None, sys.stdout, sys.stderr)
  File "///.virtualenvs/mypy/lib/python3.7/site-packages/mypy/main.py", line 83, in main
    res = build.build(sources, options, None, flush_errors, fscache, stdout, stderr)
  File "///.virtualenvs/mypy/lib/python3.7/site-packages/mypy/build.py", line 164, in build
    result = _build(sources, options, alt_lib_path, flush_errors, fscache, stdout, stderr)
  File "///.virtualenvs/mypy/lib/python3.7/site-packages/mypy/build.py", line 224, in _build
    graph = dispatch(sources, manager, stdout)
  File "///.virtualenvs/mypy/lib/python3.7/site-packages/mypy/build.py", line 2579, in dispatch
    process_graph(graph, manager)
  File "///.virtualenvs/mypy/lib/python3.7/site-packages/mypy/build.py", line 2892, in process_graph
    process_stale_scc(graph, scc, manager)
  File "///.virtualenvs/mypy/lib/python3.7/site-packages/mypy/build.py", line 2999, in process_stale_scc
    graph[id].type_check_first_pass()
  File "///.virtualenvs/mypy/lib/python3.7/site-packages/mypy/build.py", line 2108, in type_check_first_pass
    self.type_checker().check_first_pass()
  File "///.virtualenvs/mypy/lib/python3.7/site-packages/mypy/checker.py", line 285, in check_first_pass
    self.accept(d)
  File "///.virtualenvs/mypy/lib/python3.7/site-packages/mypy/checker.py", line 396, in accept
    stmt.accept(self)
  File "///.virtualenvs/mypy/lib/python3.7/site-packages/mypy/nodes.py", line 922, in accept
    return visitor.visit_class_def(self)
  File "///.virtualenvs/mypy/lib/python3.7/site-packages/mypy/checker.py", line 1634, in visit_class_def
    self.accept(defn.defs)
  File "///.virtualenvs/mypy/lib/python3.7/site-packages/mypy/checker.py", line 396, in accept
    stmt.accept(self)
  File "///.virtualenvs/mypy/lib/python3.7/site-packages/mypy/nodes.py", line 987, in accept
    return visitor.visit_block(self)
  File "///.virtualenvs/mypy/lib/python3.7/site-packages/mypy/checker.py", line 1830, in visit_block
    self.accept(s)
  File "///.virtualenvs/mypy/lib/python3.7/site-packages/mypy/checker.py", line 396, in accept
    stmt.accept(self)
  File "///.virtualenvs/mypy/lib/python3.7/site-packages/mypy/nodes.py", line 663, in accept
    return visitor.visit_func_def(self)
  File "///.virtualenvs/mypy/lib/python3.7/site-packages/mypy/checker.py", line 707, in visit_func_def
    self._visit_func_def(defn)
  File "///.virtualenvs/mypy/lib/python3.7/site-packages/mypy/checker.py", line 711, in _visit_func_def
    self.check_func_item(defn, name=defn.name())
  File "///.virtualenvs/mypy/lib/python3.7/site-packages/mypy/checker.py", line 773, in check_func_item
    self.check_func_def(defn, typ, name)
  File "///.virtualenvs/mypy/lib/python3.7/site-packages/mypy/checker.py", line 952, in check_func_def
    self.accept(item.body)
  File "///.virtualenvs/mypy/lib/python3.7/site-packages/mypy/checker.py", line 396, in accept
    stmt.accept(self)
  File "///.virtualenvs/mypy/lib/python3.7/site-packages/mypy/nodes.py", line 987, in accept
    return visitor.visit_block(self)
  File "///.virtualenvs/mypy/lib/python3.7/site-packages/mypy/checker.py", line 1830, in visit_block
    self.accept(s)
  File "///.virtualenvs/mypy/lib/python3.7/site-packages/mypy/checker.py", line 396, in accept
    stmt.accept(self)
  File "///.virtualenvs/mypy/lib/python3.7/site-packages/mypy/nodes.py", line 1045, in accept
    return visitor.visit_assignment_stmt(self)
  File "///.virtualenvs/mypy/lib/python3.7/site-packages/mypy/checker.py", line 1864, in visit_assignment_stmt
    self.check_assignment(s.lvalues[-1], s.rvalue, s.type is None, s.new_syntax)
  File "///.virtualenvs/mypy/lib/python3.7/site-packages/mypy/checker.py", line 1970, in check_assignment
    instance_type, lvalue_type, rvalue, lvalue)
  File "///.virtualenvs/mypy/lib/python3.7/site-packages/mypy/checker.py", line 2748, in check_member_assignment
    assert isinstance(inferred_dunder_set_type, CallableType)
AssertionError:
bar.py:6: : note: use --pdb to drop into pdb