Issue #23973: Update typing.py from GitHub repo.

2 files changed, 150 insertions, 348 deletions

diff --git a/Lib/test/test_typing.py b/Lib/test/test_typing.py
index c37e113..b34007d 100644
--- a/Lib/test/test_typing.py
+++ b/Lib/test/test_typing.py
@@ -41,11 +41,9 @@ class ManagingFounder(Manager, Founder):
 
 class AnyTests(TestCase):
 
-    def test_any_instance(self):
-        self.assertIsInstance(Employee(), Any)
-        self.assertIsInstance(42, Any)
-        self.assertIsInstance(None, Any)
-        self.assertIsInstance(object(), Any)
+    def test_any_instance_type_error(self):
+        with self.assertRaises(TypeError):
+            isinstance(42, Any)
 
     def test_any_subclass(self):
         self.assertTrue(issubclass(Employee, Any))
@@ -109,9 +107,6 @@ class TypeVarTests(TestCase):
 
     def test_basic_plain(self):
         T = TypeVar('T')
-        # Nothing is an instance if T.
-        with self.assertRaises(TypeError):
-            isinstance('', T)
         # Every class is a subclass of T.
         assert issubclass(int, T)
         assert issubclass(str, T)
@@ -119,12 +114,16 @@ class TypeVarTests(TestCase):
         assert T == T
         # T is a subclass of itself.
         assert issubclass(T, T)
+        # T is an instance of TypeVar
+        assert isinstance(T, TypeVar)
+
+    def test_typevar_instance_type_error(self):
+        T = TypeVar('T')
+        with self.assertRaises(TypeError):
+            isinstance(42, T)
 
     def test_basic_constrained(self):
         A = TypeVar('A', str, bytes)
-        # Nothing is an instance of A.
-        with self.assertRaises(TypeError):
-            isinstance('', A)
         # Only str and bytes are subclasses of A.
         assert issubclass(str, A)
         assert issubclass(bytes, A)
@@ -213,8 +212,6 @@ class UnionTests(TestCase):
     def test_basics(self):
         u = Union[int, float]
         self.assertNotEqual(u, Union)
-        self.assertIsInstance(42, u)
-        self.assertIsInstance(3.14, u)
         self.assertTrue(issubclass(int, u))
         self.assertTrue(issubclass(float, u))
 
@@ -247,7 +244,6 @@ class UnionTests(TestCase):
 
     def test_subclass(self):
         u = Union[int, Employee]
-        self.assertIsInstance(Manager(), u)
         self.assertTrue(issubclass(Manager, u))
 
     def test_self_subclass(self):
@@ -256,7 +252,6 @@ class UnionTests(TestCase):
 
     def test_multiple_inheritance(self):
         u = Union[int, Employee]
-        self.assertIsInstance(ManagingFounder(), u)
         self.assertTrue(issubclass(ManagingFounder, u))
 
     def test_single_class_disappears(self):
@@ -309,9 +304,6 @@ class UnionTests(TestCase):
         o = Optional[int]
         u = Union[int, None]
         self.assertEqual(o, u)
-        self.assertIsInstance(42, o)
-        self.assertIsInstance(None, o)
-        self.assertNotIsInstance(3.14, o)
 
     def test_empty(self):
         with self.assertRaises(TypeError):
@@ -321,11 +313,9 @@ class UnionTests(TestCase):
         assert issubclass(Union[int, str], Union)
         assert not issubclass(int, Union)
 
-    def test_isinstance_union(self):
-        # Nothing is an instance of bare Union.
-        assert not isinstance(42, Union)
-        assert not isinstance(int, Union)
-        assert not isinstance(Union[int, str], Union)
+    def test_union_instance_type_error(self):
+        with self.assertRaises(TypeError):
+            isinstance(42, Union[int, str])
 
 
 class TypeVarUnionTests(TestCase):
@@ -352,22 +342,11 @@ class TypeVarUnionTests(TestCase):
         TU = TypeVar('TU', Union[int, float], None)
         assert issubclass(int, TU)
         assert issubclass(float, TU)
-        with self.assertRaises(TypeError):
-            isinstance(42, TU)
-        with self.assertRaises(TypeError):
-            isinstance('', TU)
 
 
 class TupleTests(TestCase):
 
     def test_basics(self):
-        self.assertIsInstance((42, 3.14, ''), Tuple)
-        self.assertIsInstance((42, 3.14, ''), Tuple[int, float, str])
-        self.assertIsInstance((42,), Tuple[int])
-        self.assertNotIsInstance((3.14,), Tuple[int])
-        self.assertNotIsInstance((42, 3.14), Tuple[int, float, str])
-        self.assertNotIsInstance((42, 3.14, 100), Tuple[int, float, str])
-        self.assertNotIsInstance((42, 3.14, 100), Tuple[int, float])
         self.assertTrue(issubclass(Tuple[int, str], Tuple))
         self.assertTrue(issubclass(Tuple[int, str], Tuple[int, str]))
         self.assertFalse(issubclass(int, Tuple))
@@ -382,14 +361,11 @@ class TupleTests(TestCase):
             pass
         self.assertTrue(issubclass(MyTuple, Tuple))
 
-    def test_tuple_ellipsis(self):
-        t = Tuple[int, ...]
-        assert isinstance((), t)
-        assert isinstance((1,), t)
-        assert isinstance((1, 2), t)
-        assert isinstance((1, 2, 3), t)
-        assert not isinstance((3.14,), t)
-        assert not isinstance((1, 2, 3.14,), t)
+    def test_tuple_instance_type_error(self):
+        with self.assertRaises(TypeError):
+            isinstance((0, 0), Tuple[int, int])
+        with self.assertRaises(TypeError):
+            isinstance((0, 0), Tuple)
 
     def test_tuple_ellipsis_subclass(self):
 
@@ -419,18 +395,6 @@ class TupleTests(TestCase):
 
 class CallableTests(TestCase):
 
-    def test_basics(self):
-        c = Callable[[int, float], str]
-
-        def flub(a: int, b: float) -> str:
-            return str(a * b)
-
-        def flob(a: int, b: int) -> str:
-            return str(a * b)
-
-        self.assertIsInstance(flub, c)
-        self.assertNotIsInstance(flob, c)
-
     def test_self_subclass(self):
         self.assertTrue(issubclass(Callable[[int], int], Callable))
         self.assertFalse(issubclass(Callable, Callable[[int], int]))
@@ -453,91 +417,6 @@ class CallableTests(TestCase):
         self.assertNotEqual(Callable[[int], int], Callable[[], int])
         self.assertNotEqual(Callable[[int], int], Callable)
 
-    def test_with_none(self):
-        c = Callable[[None], None]
-
-        def flub(self: None) -> None:
-            pass
-
-        def flab(self: Any) -> None:
-            pass
-
-        def flob(self: None) -> Any:
-            pass
-
-        self.assertIsInstance(flub, c)
-        self.assertIsInstance(flab, c)
-        self.assertNotIsInstance(flob, c)  # Test contravariance.
-
-    def test_with_subclasses(self):
-        c = Callable[[Employee, Manager], Employee]
-
-        def flub(a: Employee, b: Employee) -> Manager:
-            return Manager()
-
-        def flob(a: Manager, b: Manager) -> Employee:
-            return Employee()
-
-        self.assertIsInstance(flub, c)
-        self.assertNotIsInstance(flob, c)
-
-    def test_with_default_args(self):
-        c = Callable[[int], int]
-
-        def flub(a: int, b: float = 3.14) -> int:
-            return a
-
-        def flab(a: int, *, b: float = 3.14) -> int:
-            return a
-
-        def flob(a: int = 42) -> int:
-            return a
-
-        self.assertIsInstance(flub, c)
-        self.assertIsInstance(flab, c)
-        self.assertIsInstance(flob, c)
-
-    def test_with_varargs(self):
-        c = Callable[[int], int]
-
-        def flub(*args) -> int:
-            return 42
-
-        def flab(*args: int) -> int:
-            return 42
-
-        def flob(*args: float) -> int:
-            return 42
-
-        self.assertIsInstance(flub, c)
-        self.assertIsInstance(flab, c)
-        self.assertNotIsInstance(flob, c)
-
-    def test_with_method(self):
-
-        class C:
-
-            def imethod(self, arg: int) -> int:
-                self.last_arg = arg
-                return arg + 1
-
-            @classmethod
-            def cmethod(cls, arg: int) -> int:
-                cls.last_cls_arg = arg
-                return arg + 1
-
-            @staticmethod
-            def smethod(arg: int) -> int:
-                return arg + 1
-
-        ct = Callable[[int], int]
-        self.assertIsInstance(C().imethod, ct)
-        self.assertIsInstance(C().cmethod, ct)
-        self.assertIsInstance(C.cmethod, ct)
-        self.assertIsInstance(C().smethod, ct)
-        self.assertIsInstance(C.smethod, ct)
-        self.assertIsInstance(C.imethod, Callable[[Any, int], int])
-
     def test_cannot_subclass(self):
         with self.assertRaises(TypeError):
 
@@ -556,21 +435,21 @@ class CallableTests(TestCase):
         with self.assertRaises(TypeError):
             c()
 
-    def test_varargs(self):
-        ct = Callable[..., int]
-
-        def foo(a, b) -> int:
-            return 42
-
-        def bar(a=42) -> int:
-            return a
-
-        def baz(*, x, y, z) -> int:
-            return 100
+    def test_callable_instance_works(self):
+        f = lambda: None
+        assert isinstance(f, Callable)
+        assert not isinstance(None, Callable)
 
-        self.assertIsInstance(foo, ct)
-        self.assertIsInstance(bar, ct)
-        self.assertIsInstance(baz, ct)
+    def test_callable_instance_type_error(self):
+        f = lambda: None
+        with self.assertRaises(TypeError):
+            assert isinstance(f, Callable[[], None])
+        with self.assertRaises(TypeError):
+            assert isinstance(f, Callable[[], Any])
+        with self.assertRaises(TypeError):
+            assert not isinstance(None, Callable[[], None])
+        with self.assertRaises(TypeError):
+            assert not isinstance(None, Callable[[], Any])
 
     def test_repr(self):
         ct0 = Callable[[], bool]
@@ -580,6 +459,14 @@ class CallableTests(TestCase):
         ctv = Callable[..., str]
         self.assertEqual(repr(ctv), 'typing.Callable[..., str]')
 
+    def test_callable_with_ellipsis(self):
+
+        def foo(a: Callable[..., T]):
+            pass
+
+        self.assertEqual(get_type_hints(foo, globals(), locals()),
+                         {'a': Callable[..., T]})
+
 
 XK = TypeVar('XK', str, bytes)
 XV = TypeVar('XV')
@@ -659,6 +546,10 @@ class ProtocolTests(TestCase):
         assert issubclass(list, typing.Reversible)
         assert not issubclass(int, typing.Reversible)
 
+    def test_protocol_instance_type_error(self):
+        with self.assertRaises(TypeError):
+            isinstance([], typing.Reversible)
+
 
 class GenericTests(TestCase):
 
@@ -672,6 +563,14 @@ class GenericTests(TestCase):
         with self.assertRaises(TypeError):
             Y[str, bytes]
 
+    def test_init(self):
+        T = TypeVar('T')
+        S = TypeVar('S')
+        with self.assertRaises(TypeError):
+            Generic[T, T]
+        with self.assertRaises(TypeError):
+            Generic[T, S, T]
+
     def test_repr(self):
         self.assertEqual(repr(SimpleMapping),
                          __name__ + '.' + 'SimpleMapping[~XK, ~XV]')
@@ -824,11 +723,11 @@ class VarianceTests(TestCase):
                               typing.Sequence[Manager])
 
     def test_covariance_mapping(self):
-        # Ditto for Mapping (a generic class with two parameters).
+        # Ditto for Mapping (covariant in the value, invariant in the key).
         assert issubclass(typing.Mapping[Employee, Manager],
                           typing.Mapping[Employee, Employee])
-        assert issubclass(typing.Mapping[Manager, Employee],
-                          typing.Mapping[Employee, Employee])
+        assert not issubclass(typing.Mapping[Manager, Employee],
+                              typing.Mapping[Employee, Employee])
         assert not issubclass(typing.Mapping[Employee, Manager],
                               typing.Mapping[Manager, Manager])
         assert not issubclass(typing.Mapping[Manager, Employee],
@@ -889,6 +788,11 @@ class ForwardRefTests(TestCase):
         right_hints = get_type_hints(t.add_right, globals(), locals())
         assert right_hints['node'] == Optional[Node[T]]
 
+    def test_forwardref_instance_type_error(self):
+        fr = typing._ForwardRef('int')
+        with self.assertRaises(TypeError):
+            isinstance(42, fr)
+
     def test_union_forward(self):
 
         def foo(a: Union['T']):
@@ -913,6 +817,14 @@ class ForwardRefTests(TestCase):
         self.assertEqual(get_type_hints(foo, globals(), locals()),
                          {'a': Callable[[T], T]})
 
+    def test_callable_with_ellipsis_forward(self):
+
+        def foo(a: 'Callable[..., T]'):
+            pass
+
+        self.assertEqual(get_type_hints(foo, globals(), locals()),
+                         {'a': Callable[..., T]})
+
     def test_syntax_error(self):
 
         with self.assertRaises(SyntaxError):
@@ -1069,50 +981,17 @@ class CollectionsAbcTests(TestCase):
 
     def test_list(self):
         assert issubclass(list, typing.List)
-        assert isinstance([], typing.List)
-        assert not isinstance((), typing.List)
-        t = typing.List[int]
-        assert isinstance([], t)
-        assert isinstance([42], t)
-        assert not isinstance([''], t)
 
     def test_set(self):
         assert issubclass(set, typing.Set)
         assert not issubclass(frozenset, typing.Set)
-        assert isinstance(set(), typing.Set)
-        assert not isinstance({}, typing.Set)
-        t = typing.Set[int]
-        assert isinstance(set(), t)
-        assert isinstance({42}, t)
-        assert not isinstance({''}, t)
 
     def test_frozenset(self):
         assert issubclass(frozenset, typing.FrozenSet)
         assert not issubclass(set, typing.FrozenSet)
-        assert isinstance(frozenset(), typing.FrozenSet)
-        assert not isinstance({}, typing.FrozenSet)
-        t = typing.FrozenSet[int]
-        assert isinstance(frozenset(), t)
-        assert isinstance(frozenset({42}), t)
-        assert not isinstance(frozenset({''}), t)
-        assert not isinstance({42}, t)
-
-    def test_mapping_views(self):
-        # TODO: These tests are kind of lame.
-        assert isinstance({}.keys(), typing.KeysView)
-        assert isinstance({}.items(), typing.ItemsView)
-        assert isinstance({}.values(), typing.ValuesView)
 
     def test_dict(self):
         assert issubclass(dict, typing.Dict)
-        assert isinstance({}, typing.Dict)
-        assert not isinstance([], typing.Dict)
-        t = typing.Dict[int, str]
-        assert isinstance({}, t)
-        assert isinstance({42: ''}, t)
-        assert not isinstance({42: 42}, t)
-        assert not isinstance({'': 42}, t)
-        assert not isinstance({'': ''}, t)
 
     def test_no_list_instantiation(self):
         with self.assertRaises(TypeError):
@@ -1191,8 +1070,6 @@ class CollectionsAbcTests(TestCase):
             yield 42
         g = foo()
         assert issubclass(type(g), typing.Generator)
-        assert isinstance(g, typing.Generator)
-        assert not isinstance(foo, typing.Generator)
         assert issubclass(typing.Generator[Manager, Employee, Manager],
                           typing.Generator[Employee, Manager, Employee])
         assert not issubclass(typing.Generator[Manager, Manager, Manager],
@@ -1228,12 +1105,6 @@ class CollectionsAbcTests(TestCase):
         assert len(MMB[str, str]()) == 0
         assert len(MMB[KT, VT]()) == 0
 
-    def test_recursive_dict(self):
-        D = typing.Dict[int, 'D']  # Uses a _ForwardRef
-        assert isinstance({}, D)  # Easy
-        assert isinstance({0: {}}, D)  # Touches _ForwardRef
-        assert isinstance({0: {0: {}}}, D)  # Etc...
-
 
 class NamedTupleTests(TestCase):
 
@@ -1294,8 +1165,6 @@ class RETests(TestCase):
     def test_basics(self):
         pat = re.compile('[a-z]+', re.I)
         assert issubclass(pat.__class__, Pattern)
-        assert isinstance(pat, Pattern[str])
-        assert not isinstance(pat, Pattern[bytes])
         assert issubclass(type(pat), Pattern)
         assert issubclass(type(pat), Pattern[str])
 
@@ -1307,12 +1176,10 @@ class RETests(TestCase):
         assert issubclass(type(mat), Match[str])
 
         p = Pattern[Union[str, bytes]]
-        assert isinstance(pat, p)
         assert issubclass(Pattern[str], Pattern)
         assert issubclass(Pattern[str], p)
 
         m = Match[Union[bytes, str]]
-        assert isinstance(mat, m)
         assert issubclass(Match[bytes], Match)
         assert issubclass(Match[bytes], m)
 
@@ -1327,6 +1194,12 @@ class RETests(TestCase):
         with self.assertRaises(TypeError):
             # Too complicated?
             m[str]
+        with self.assertRaises(TypeError):
+            # We don't support isinstance().
+            isinstance(42, Pattern)
+        with self.assertRaises(TypeError):
+            # We don't support isinstance().
+            isinstance(42, Pattern[str])
 
     def test_repr(self):
         assert repr(Pattern) == 'Pattern[~AnyStr]'
diff --git a/Lib/typing.py b/Lib/typing.py
index 38e07ad..ddaec3e 100644
--- a/Lib/typing.py
+++ b/Lib/typing.py
@@ -128,6 +128,8 @@ class TypingMeta(type):
 class Final:
     """Mix-in class to prevent instantiation."""
 
+    __slots__ = ()
+
     def __new__(self, *args, **kwds):
         raise TypeError("Cannot instantiate %r" % self.__class__)
 
@@ -176,6 +178,9 @@ class _ForwardRef(TypingMeta):
             self.__forward_evaluated__ = True
         return self.__forward_value__
 
+    def __instancecheck__(self, obj):
+        raise TypeError("Forward references cannot be used with isinstance().")
+
     def __subclasscheck__(self, cls):
         if not self.__forward_evaluated__:
             globalns = self.__forward_frame__.f_globals
@@ -186,16 +191,6 @@ class _ForwardRef(TypingMeta):
                 return False  # Too early.
         return issubclass(cls, self.__forward_value__)
 
-    def __instancecheck__(self, obj):
-        if not self.__forward_evaluated__:
-            globalns = self.__forward_frame__.f_globals
-            localns = self.__forward_frame__.f_locals
-            try:
-                self._eval_type(globalns, localns)
-            except NameError:
-                return False  # Too early.
-        return isinstance(obj, self.__forward_value__)
-
     def __repr__(self):
         return '_ForwardRef(%r)' % (self.__forward_arg__,)
 
@@ -211,6 +206,8 @@ class _TypeAlias:
     False.
     """
 
+    __slots__ = ('name', 'type_var', 'impl_type', 'type_checker')
+
     def __new__(cls, *args, **kwds):
         """Constructor.
 
@@ -259,8 +256,7 @@ class _TypeAlias:
                               self.impl_type, self.type_checker)
 
     def __instancecheck__(self, obj):
-        return (isinstance(obj, self.impl_type) and
-                isinstance(self.type_checker(obj), self.type_var))
+        raise TypeError("Type aliases cannot be used with isinstance().")
 
     def __subclasscheck__(self, cls):
         if cls is Any:
@@ -332,8 +328,8 @@ class AnyMeta(TypingMeta):
         self = super().__new__(cls, name, bases, namespace, _root=_root)
         return self
 
-    def __instancecheck__(self, instance):
-        return True
+    def __instancecheck__(self, obj):
+        raise TypeError("Any cannot be used with isinstance().")
 
     def __subclasscheck__(self, cls):
         if not isinstance(cls, type):
@@ -349,6 +345,8 @@ class Any(Final, metaclass=AnyMeta, _root=True):
     - As a special case, Any and object are subclasses of each other.
     """
 
+    __slots__ = ()
+
 
 class TypeVar(TypingMeta, metaclass=TypingMeta, _root=True):
     """Type variable.
@@ -447,7 +445,6 @@ KT = TypeVar('KT')  # Key type.
 VT = TypeVar('VT')  # Value type.
 T_co = TypeVar('T_co', covariant=True)  # Any type covariant containers.
 V_co = TypeVar('V_co', covariant=True)  # Any type covariant containers.
-KT_co = TypeVar('KT_co', covariant=True)  # Key type covariant containers.
 VT_co = TypeVar('VT_co', covariant=True)  # Value type covariant containers.
 T_contra = TypeVar('T_contra', contravariant=True)  # Ditto contravariant.
 
@@ -548,9 +545,8 @@ class UnionMeta(TypingMeta):
     def __hash__(self):
         return hash(self.__union_set_params__)
 
-    def __instancecheck__(self, instance):
-        return (self.__union_set_params__ is not None and
-                any(isinstance(instance, t) for t in self.__union_params__))
+    def __instancecheck__(self, obj):
+        raise TypeError("Unions cannot be used with isinstance().")
 
     def __subclasscheck__(self, cls):
         if cls is Any:
@@ -645,6 +641,8 @@ class Optional(Final, metaclass=OptionalMeta, _root=True):
     Optional[X] is equivalent to Union[X, type(None)].
     """
 
+    __slots__ = ()
+
 
 class TupleMeta(TypingMeta):
     """Metaclass for Tuple."""
@@ -709,18 +707,8 @@ class TupleMeta(TypingMeta):
     def __hash__(self):
         return hash(self.__tuple_params__)
 
-    def __instancecheck__(self, t):
-        if not isinstance(t, tuple):
-            return False
-        if self.__tuple_params__ is None:
-            return True
-        if self.__tuple_use_ellipsis__:
-            p = self.__tuple_params__[0]
-            return all(isinstance(x, p) for x in t)
-        else:
-            return (len(t) == len(self.__tuple_params__) and
-                    all(isinstance(x, p)
-                        for x, p in zip(t, self.__tuple_params__)))
+    def __instancecheck__(self, obj):
+        raise TypeError("Tuples cannot be used with isinstance().")
 
     def __subclasscheck__(self, cls):
         if cls is Any:
@@ -754,6 +742,8 @@ class Tuple(Final, metaclass=TupleMeta, _root=True):
     To specify a variable-length tuple of homogeneous type, use Sequence[T].
     """
 
+    __slots__ = ()
+
 
 class CallableMeta(TypingMeta):
     """Metaclass for Callable."""
@@ -787,7 +777,10 @@ class CallableMeta(TypingMeta):
     def _eval_type(self, globalns, localns):
         if self.__args__ is None and self.__result__ is None:
             return self
-        args = [_eval_type(t, globalns, localns) for t in self.__args__]
+        if self.__args__ is Ellipsis:
+            args = self.__args__
+        else:
+            args = [_eval_type(t, globalns, localns) for t in self.__args__]
         result = _eval_type(self.__result__, globalns, localns)
         if args == self.__args__ and result == self.__result__:
             return self
@@ -826,57 +819,14 @@ class CallableMeta(TypingMeta):
     def __hash__(self):
         return hash(self.__args__) ^ hash(self.__result__)
 
-    def __instancecheck__(self, instance):
-        if not callable(instance):
-            return False
+    def __instancecheck__(self, obj):
+        # For unparametrized Callable we allow this, because
+        # typing.Callable should be equivalent to
+        # collections.abc.Callable.
         if self.__args__ is None and self.__result__ is None:
-            return True
-        assert self.__args__ is not None
-        assert self.__result__ is not None
-        my_args, my_result = self.__args__, self.__result__
-        import inspect  # TODO: Avoid this import.
-        # Would it be better to use Signature objects?
-        try:
-            (args, varargs, varkw, defaults, kwonlyargs, kwonlydefaults,
-             annotations) = inspect.getfullargspec(instance)
-        except TypeError:
-            return False  # We can't find the signature.  Give up.
-        msg = ("When testing isinstance(<callable>, Callable[...], "
-               "<calleble>'s annotations must be types.")
-        if my_args is not Ellipsis:
-            if kwonlyargs and (not kwonlydefaults or
-                               len(kwonlydefaults) < len(kwonlyargs)):
-                return False
-            if isinstance(instance, types.MethodType):
-                # For methods, getfullargspec() includes self/cls,
-                # but it's not part of the call signature, so drop it.
-                del args[0]
-            min_call_args = len(args)
-            if defaults:
-                min_call_args -= len(defaults)
-            if varargs:
-                max_call_args = 999999999
-                if len(args) < len(my_args):
-                    args += [varargs] * (len(my_args) - len(args))
-            else:
-                max_call_args = len(args)
-            if not min_call_args <= len(my_args) <= max_call_args:
-                return False
-            for my_arg_type, name in zip(my_args, args):
-                if name in annotations:
-                    annot_type = _type_check(annotations[name], msg)
-                else:
-                    annot_type = Any
-                if not issubclass(my_arg_type, annot_type):
-                    return False
-                # TODO: If mutable type, check invariance?
-        if 'return' in annotations:
-            annot_return_type = _type_check(annotations['return'], msg)
-            # Note contravariance here!
-            if not issubclass(annot_return_type, my_result):
-                return False
-        # Can't find anything wrong...
-        return True
+            return isinstance(obj, collections_abc.Callable)
+        else:
+            raise TypeError("Callable[] cannot be used with isinstance().")
 
     def __subclasscheck__(self, cls):
         if cls is Any:
@@ -900,6 +850,8 @@ class Callable(Final, metaclass=CallableMeta, _root=True):
     such function types are rarely used as callback types.
     """
 
+    __slots__ = ()
+
 
 def _gorg(a):
     """Return the farthest origin of a generic class."""
@@ -1010,6 +962,8 @@ class GenericMeta(TypingMeta, abc.ABCMeta):
                 if not isinstance(p, TypeVar):
                     raise TypeError("Initial parameters must be "
                                     "type variables; got %s" % p)
+            if len(set(params)) != len(params):
+                raise TypeError("All type variables in Generic[...] must be distinct.")
         else:
             if len(params) != len(self.__parameters__):
                 raise TypeError("Cannot change parameter count from %d to %d" %
@@ -1073,13 +1027,6 @@ class GenericMeta(TypingMeta, abc.ABCMeta):
             return False
         return issubclass(cls, self.__extra__)
 
-    def __instancecheck__(self, obj):
-        if super().__instancecheck__(obj):
-            return True
-        if self.__extra__ is None:
-            return False
-        return isinstance(obj, self.__extra__)
-
 
 class Generic(metaclass=GenericMeta):
     """Abstract base class for generic types.
@@ -1109,6 +1056,8 @@ class Generic(metaclass=GenericMeta):
           # Same body as above.
     """
 
+    __slots__ = ()
+
     def __new__(cls, *args, **kwds):
         next_in_mro = object
         # Look for the last occurrence of Generic or Generic[...].
@@ -1234,6 +1183,9 @@ class _ProtocolMeta(GenericMeta):
     from Generic.
     """
 
+    def __instancecheck__(self, obj):
+        raise TypeError("Protocols cannot be used with isinstance().")
+
     def __subclasscheck__(self, cls):
         if not self._is_protocol:
             # No structural checks since this isn't a protocol.
@@ -1272,6 +1224,7 @@ class _ProtocolMeta(GenericMeta):
                         attr != '__abstractmethods__' and
                         attr != '_is_protocol' and
                         attr != '__dict__' and
+                        attr != '__slots__' and
                         attr != '_get_protocol_attrs' and
                         attr != '__parameters__' and
                         attr != '__origin__' and
@@ -1289,6 +1242,8 @@ class _Protocol(metaclass=_ProtocolMeta):
     such as Hashable).
     """
 
+    __slots__ = ()
+
     _is_protocol = True
 
 
@@ -1299,14 +1254,15 @@ Hashable = collections_abc.Hashable  # Not generic.
 
 
 class Iterable(Generic[T_co], extra=collections_abc.Iterable):
-    pass
+    __slots__ = ()
 
 
 class Iterator(Iterable[T_co], extra=collections_abc.Iterator):
-    pass
+    __slots__ = ()
 
 
 class SupportsInt(_Protocol):
+    __slots__ = ()
 
     @abstractmethod
     def __int__(self) -> int:
@@ -1314,6 +1270,7 @@ class SupportsInt(_Protocol):
 
 
 class SupportsFloat(_Protocol):
+    __slots__ = ()
 
     @abstractmethod
     def __float__(self) -> float:
@@ -1321,6 +1278,7 @@ class SupportsFloat(_Protocol):
 
 
 class SupportsComplex(_Protocol):
+    __slots__ = ()
 
     @abstractmethod
     def __complex__(self) -> complex:
@@ -1328,30 +1286,34 @@ class SupportsComplex(_Protocol):
 
 
 class SupportsBytes(_Protocol):
+    __slots__ = ()
 
     @abstractmethod
     def __bytes__(self) -> bytes:
         pass
 
 
-class SupportsAbs(_Protocol[T]):
+class SupportsAbs(_Protocol[T_co]):
+    __slots__ = ()
 
     @abstractmethod
-    def __abs__(self) -> T:
+    def __abs__(self) -> T_co:
         pass
 
 
-class SupportsRound(_Protocol[T]):
+class SupportsRound(_Protocol[T_co]):
+    __slots__ = ()
 
     @abstractmethod
-    def __round__(self, ndigits: int = 0) -> T:
+    def __round__(self, ndigits: int = 0) -> T_co:
         pass
 
 
-class Reversible(_Protocol[T]):
+class Reversible(_Protocol[T_co]):
+    __slots__ = ()
 
     @abstractmethod
-    def __reversed__(self) -> 'Iterator[T]':
+    def __reversed__(self) -> 'Iterator[T_co]':
         pass
 
 
@@ -1359,7 +1321,7 @@ Sized = collections_abc.Sized  # Not generic.
 
 
 class Container(Generic[T_co], extra=collections_abc.Container):
-    pass
+    __slots__ = ()
 
 
 # Callable was defined earlier.
@@ -1374,7 +1336,8 @@ class MutableSet(AbstractSet[T], extra=collections_abc.MutableSet):
     pass
 
 
-class Mapping(Sized, Iterable[KT_co], Container[KT_co], Generic[KT_co, VT_co],
+# NOTE: Only the value type is covariant.
+class Mapping(Sized, Iterable[KT], Container[KT], Generic[VT_co],
               extra=collections_abc.Mapping):
     pass
 
@@ -1399,19 +1362,7 @@ class ByteString(Sequence[int], extra=collections_abc.ByteString):
 ByteString.register(type(memoryview(b'')))
 
 
-class _ListMeta(GenericMeta):
-
-    def __instancecheck__(self, obj):
-        if not super().__instancecheck__(obj):
-            return False
-        itemtype = self.__parameters__[0]
-        for x in obj:
-            if not isinstance(x, itemtype):
-                return False
-        return True
-
-
-class List(list, MutableSequence[T], metaclass=_ListMeta):
+class List(list, MutableSequence[T]):
 
     def __new__(cls, *args, **kwds):
         if _geqv(cls, List):
@@ -1420,19 +1371,7 @@ class List(list, MutableSequence[T], metaclass=_ListMeta):
         return list.__new__(cls, *args, **kwds)
 
 
-class _SetMeta(GenericMeta):
-
-    def __instancecheck__(self, obj):
-        if not super().__instancecheck__(obj):
-            return False
-        itemtype = self.__parameters__[0]
-        for x in obj:
-            if not isinstance(x, itemtype):
-                return False
-        return True
-
-
-class Set(set, MutableSet[T], metaclass=_SetMeta):
+class Set(set, MutableSet[T]):
 
     def __new__(cls, *args, **kwds):
         if _geqv(cls, Set):
@@ -1441,7 +1380,7 @@ class Set(set, MutableSet[T], metaclass=_SetMeta):
         return set.__new__(cls, *args, **kwds)
 
 
-class _FrozenSetMeta(_SetMeta):
+class _FrozenSetMeta(GenericMeta):
     """This metaclass ensures set is not a subclass of FrozenSet.
 
     Without this metaclass, set would be considered a subclass of
@@ -1454,13 +1393,9 @@ class _FrozenSetMeta(_SetMeta):
             return False
         return super().__subclasscheck__(cls)
 
-    def __instancecheck__(self, obj):
-        if issubclass(obj.__class__, Set):
-            return False
-        return super().__instancecheck__(obj)
-
 
 class FrozenSet(frozenset, AbstractSet[T_co], metaclass=_FrozenSetMeta):
+    __slots__ = ()
 
     def __new__(cls, *args, **kwds):
         if _geqv(cls, FrozenSet):
@@ -1473,13 +1408,13 @@ class MappingView(Sized, Iterable[T_co], extra=collections_abc.MappingView):
     pass
 
 
-class KeysView(MappingView[KT_co], AbstractSet[KT_co],
+class KeysView(MappingView[KT], AbstractSet[KT],
                extra=collections_abc.KeysView):
     pass
 
 
-# TODO: Enable Set[Tuple[KT_co, VT_co]] instead of Generic[KT_co, VT_co].
-class ItemsView(MappingView, Generic[KT_co, VT_co],
+# TODO: Enable Set[Tuple[KT, VT_co]] instead of Generic[KT, VT_co].
+class ItemsView(MappingView, Generic[KT, VT_co],
                 extra=collections_abc.ItemsView):
     pass
 
@@ -1488,20 +1423,7 @@ class ValuesView(MappingView[VT_co], extra=collections_abc.ValuesView):
     pass
 
 
-class _DictMeta(GenericMeta):
-
-    def __instancecheck__(self, obj):
-        if not super().__instancecheck__(obj):
-            return False
-        keytype, valuetype = self.__parameters__
-        for key, value in obj.items():
-            if not (isinstance(key, keytype) and
-                    isinstance(value, valuetype)):
-                return False
-        return True
-
-
-class Dict(dict, MutableMapping[KT, VT], metaclass=_DictMeta):
+class Dict(dict, MutableMapping[KT, VT]):
 
     def __new__(cls, *args, **kwds):
         if _geqv(cls, Dict):
@@ -1521,6 +1443,7 @@ else:
 
 class Generator(Iterator[T_co], Generic[T_co, T_contra, V_co],
                 extra=_G_base):
+    __slots__ = ()
 
     def __new__(cls, *args, **kwds):
         if _geqv(cls, Generator):
@@ -1564,6 +1487,8 @@ class IO(Generic[AnyStr]):
     way to track the other distinctions in the type system.
     """
 
+    __slots__ = ()
+
     @abstractproperty
     def mode(self) -> str:
         pass
@@ -1648,6 +1573,8 @@ class IO(Generic[AnyStr]):
 class BinaryIO(IO[bytes]):
     """Typed version of the return of open() in binary mode."""
 
+    __slots__ = ()
+
     @abstractmethod
     def write(self, s: Union[bytes, bytearray]) -> int:
         pass
@@ -1660,6 +1587,8 @@ class BinaryIO(IO[bytes]):
 class TextIO(IO[str]):
     """Typed version of the return of open() in text mode."""
 
+    __slots__ = ()
+
     @abstractproperty
     def buffer(self) -> BinaryIO:
         pass