# Buggy code
def visit_dictionary_comprehension(self, e: DictionaryComprehension) -> Type:
    """Type check a dictionary comprehension."""
    with self.chk.binder.frame_context():
        self.check_for_comp(e)

        # Infer the type of the list comprehension by using a synthetic generic
        # callable type.
        ktdef = TypeVarDef("KT", -1, [], self.chk.object_type())
        vtdef = TypeVarDef("VT", -2, [], self.chk.object_type())
        kt = TypeVarType(ktdef)
        vt = TypeVarType(vtdef)
        constructor = CallableType(
            [kt, vt],
            [nodes.ARG_POS, nodes.ARG_POS],
            [None, None],
            self.chk.named_generic_type("builtins.dict", [kt, vt]),
            self.chk.named_type("builtins.function"),
            name="<dictionary-comprehension>",
            variables=[ktdef, vtdef],
        )
        return self.check_call(
            constructor, [e.key, e.value], [nodes.ARG_POS, nodes.ARG_POS], e
        )[0]


# Traceback (AttributeError)
Traceback (most recent call last):
File "./scripts/mypy", line 6, in <module>
main(__file__)
File "mypy/mypy/main.py", line 50, in main
type_check_only(sources, bin_dir, options)
File "mypy/mypy/main.py", line 94, in type_check_only
python_path=options.python_path)
File "mypy/mypy/build.py", line 210, in build
result = manager.process(initial_states)
File "mypy/mypy/build.py", line 425, in process
next.process()
File "mypy/mypy/build.py", line 930, in process
self.type_checker().visit_file(self.tree, self.tree.path)
File "mypy/mypy/checker.py", line 409, in visit_file
self.accept(d)
File "mypy/mypy/checker.py", line 450, in accept
typ = node.accept(self)
File "mypy/mypy/nodes.py", line 707, in accept
return visitor.visit_try_stmt(self)
File "mypy/mypy/checker.py", line 1732, in visit_try_stmt
self.accept(s.else_body)
File "mypy/mypy/checker.py", line 450, in accept
typ = node.accept(self)
File "mypy/mypy/nodes.py", line 526, in accept
return visitor.visit_block(self)
File "mypy/mypy/checker.py", line 1113, in visit_block
self.accept(s)
File "mypy/mypy/checker.py", line 450, in accept
typ = node.accept(self)
File "mypy/mypy/nodes.py", line 382, in accept
return visitor.visit_func_def(self)
File "mypy/mypy/checker.py", line 594, in visit_func_def
'redefinition with type')
File "mypy/mypy/checker.py", line 2117, in check_subtype
if not is_subtype(subtype, supertype):
File "mypy/mypy/subtypes.py", line 49, in is_subtype
return left.accept(SubtypeVisitor(right, type_parameter_checker))
AttributeError: 'NoneType' object has no attribute 'accept'

# Fixed code
def visit_dictionary_comprehension(self, e: DictionaryComprehension) -> Type:
    """Type check a dictionary comprehension."""
    with self.chk.binder.frame_context(can_skip=True, fall_through=0):
        self.check_for_comp(e)

        # Infer the type of the list comprehension by using a synthetic generic
        # callable type.
        ktdef = TypeVarDef("KT", -1, [], self.chk.object_type())
        vtdef = TypeVarDef("VT", -2, [], self.chk.object_type())
        kt = TypeVarType(ktdef)
        vt = TypeVarType(vtdef)
        constructor = CallableType(
            [kt, vt],
            [nodes.ARG_POS, nodes.ARG_POS],
            [None, None],
            self.chk.named_generic_type("builtins.dict", [kt, vt]),
            self.chk.named_type("builtins.function"),
            name="<dictionary-comprehension>",
            variables=[ktdef, vtdef],
        )
        return self.check_call(
            constructor, [e.key, e.value], [nodes.ARG_POS, nodes.ARG_POS], e
        )[0]