# Buggy code
def process_newtype_declaration(self, s: AssignmentStmt) -> None:
    """Check if s declares a NewType; if yes, store it in symbol table."""
    # Extract and check all information from newtype declaration
    name, call = self.analyze_newtype_declaration(s)
    if name is None or call is None:
        return

    old_type = self.check_newtype_args(name, call, s)
    call.analyzed = NewTypeExpr(name, old_type, line=call.line)
    if old_type is None:
        return

    # Create the corresponding class definition if the aliased type is subtypeable
    if isinstance(old_type, TupleType):
        newtype_class_info = self.build_newtype_typeinfo(
            name, old_type, old_type.fallback
        )
        newtype_class_info.tuple_type = old_type
    elif isinstance(old_type, Instance):
        if old_type.type.is_protocol:
            self.fail("NewType cannot be used with protocol classes", s)
        newtype_class_info = self.build_newtype_typeinfo(name, old_type, old_type)
    else:
        message = "Argument 2 to NewType(...) must be subclassable (got {})"
        self.fail(message.format(old_type), s)
        return

    check_for_explicit_any(
        old_type, self.options, self.is_typeshed_stub_file, self.msg, context=s
    )

    if "unimported" in self.options.disallow_any and has_any_from_unimported_type(
        old_type
    ):
        self.msg.unimported_type_becomes_any("Argument 2 to NewType(...)", old_type, s)

    # If so, add it to the symbol table.
    node = self.lookup(name, s)
    if node is None:
        self.fail("Could not find {} in current namespace".format(name), s)
        return
    # TODO: why does NewType work in local scopes despite always being of kind GDEF?
    node.kind = GDEF
    call.analyzed.info = node.node = newtype_class_info


# Traceback (TypeError)
mypy --show-traceback Deduction.py
Deduction.py:102: error: The return type of "__init__" must be None
Deduction.py:103: error: INTERNAL ERROR -- please report a bug at https://github.com/python/mypy/issues version: 0.521-858f7512cf3b4e39c0f4e8de5a13eee0e1e138fb-dirty
Traceback (most recent call last):
File "/usr/local/Cellar/mypy/0.521/libexec/bin/mypy", line 11, in <module>
load_entry_point('mypy===0.521-858f7512cf3b4e39c0f4e8de5a13eee0e1e138fb-dirty', 'console_scripts', 'mypy')()
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/__main__.py", line 7, in console_entry
main(None)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/main.py", line 50, in main
res = type_check_only(sources, bin_dir, options)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/main.py", line 97, in type_check_only
options=options)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/build.py", line 196, in build
graph = dispatch(sources, manager)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/build.py", line 1801, in dispatch
process_graph(graph, manager)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/build.py", line 2044, in process_graph
process_stale_scc(graph, scc, manager)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/build.py", line 2147, in process_stale_scc
graph[id].type_check_first_pass()
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/build.py", line 1716, in type_check_first_pass
self.type_checker.check_first_pass()
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/checker.py", line 185, in check_first_pass
self.accept(d)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/checker.py", line 273, in accept
stmt.accept(self)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/nodes.py", line 754, in accept
return visitor.visit_class_def(self)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/checker.py", line 1109, in visit_class_def
self.accept(defn.defs)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/checker.py", line 273, in accept
stmt.accept(self)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/nodes.py", line 815, in accept
return visitor.visit_block(self)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/checker.py", line 1202, in visit_block
self.accept(s)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/checker.py", line 273, in accept
stmt.accept(self)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/nodes.py", line 565, in accept
return visitor.visit_func_def(self)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/checker.py", line 519, in visit_func_def
self.check_func_item(defn, name=defn.name())
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/checker.py", line 578, in check_func_item
self.check_func_def(defn, typ, name)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/checker.py", line 738, in check_func_def
self.accept(item.body)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/checker.py", line 273, in accept
stmt.accept(self)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/nodes.py", line 815, in accept
return visitor.visit_block(self)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/checker.py", line 1202, in visit_block
self.accept(s)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/checker.py", line 273, in accept
stmt.accept(self)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/nodes.py", line 974, in accept
return visitor.visit_if_stmt(self)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/checker.py", line 1977, in visit_if_stmt
if_map, else_map = self.find_isinstance_check(e)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/checker.py", line 2559, in find_isinstance_check
return find_isinstance_check(n, self.type_map)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/checker.py", line 2810, in find_isinstance_check
return conditional_type_map(expr, vartype, type)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/checker.py", line 2613, in conditional_type_map
and is_proper_subtype(current_type, proposed_type)):
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/subtypes.py", line 569, in is_proper_subtype
return left.accept(ProperSubtypeVisitor(right))
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/types.py", line 1114, in accept
return visitor.visit_union_type(self)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/subtypes.py", line 693, in visit_union_type
return all([is_proper_subtype(item, self.right) for item in left.items])
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/subtypes.py", line 693, in <listcomp>
return all([is_proper_subtype(item, self.right) for item in left.items])
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/subtypes.py", line 569, in is_proper_subtype
return left.accept(ProperSubtypeVisitor(right))
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/types.py", line 900, in accept
return visitor.visit_tuple_type(self)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/subtypes.py", line 671, in visit_tuple_type
if not is_proper_subtype(l, r):
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/subtypes.py", line 569, in is_proper_subtype
return left.accept(ProperSubtypeVisitor(right))
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/types.py", line 435, in accept
return visitor.visit_instance(self)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/subtypes.py", line 605, in visit_instance
for base in left.type.mro:
TypeError: 'NoneType' object is not iterable
Deduction.py:103: note: use --pdb to drop into pdb

# Fixed code
def process_newtype_declaration(self, s: AssignmentStmt) -> None:
    """Check if s declares a NewType; if yes, store it in symbol table."""
    # Extract and check all information from newtype declaration
    name, call = self.analyze_newtype_declaration(s)
    if name is None or call is None:
        return

    old_type = self.check_newtype_args(name, call, s)
    call.analyzed = NewTypeExpr(name, old_type, line=call.line)
    if old_type is None:
        return

    # Create the corresponding class definition if the aliased type is subtypeable
    if isinstance(old_type, TupleType):
        newtype_class_info = self.build_newtype_typeinfo(
            name, old_type, old_type.fallback
        )
        newtype_class_info.tuple_type = old_type
    elif isinstance(old_type, Instance):
        if old_type.type.is_protocol:
            self.fail("NewType cannot be used with protocol classes", s)
        newtype_class_info = self.build_newtype_typeinfo(name, old_type, old_type)
    else:
        message = "Argument 2 to NewType(...) must be subclassable (got {})"
        self.fail(message.format(self.msg.format(old_type)), s)
        return

    check_for_explicit_any(
        old_type, self.options, self.is_typeshed_stub_file, self.msg, context=s
    )

    if "unimported" in self.options.disallow_any and has_any_from_unimported_type(
        old_type
    ):
        self.msg.unimported_type_becomes_any("Argument 2 to NewType(...)", old_type, s)

    # If so, add it to the symbol table.
    node = self.lookup(name, s)
    if node is None:
        self.fail("Could not find {} in current namespace".format(name), s)
        return
    # TODO: why does NewType work in local scopes despite always being of kind GDEF?
    node.kind = GDEF
    call.analyzed.info = node.node = newtype_class_info