gh-105509: Simplify implementation of `typing.Annotated` (#105510)

Lib/test/test_typing.py

@@ -8189,8 +8189,7 @@ class AnnotatedTests(BaseTestCase):
 
     def test_new(self):
         with self.assertRaisesRegex(
-            TypeError,
-            'Type Annotated cannot be instantiated',
+            TypeError, 'Cannot instantiate typing.Annotated',
         ):
             Annotated()
 

Lib/typing.py

@@ -2001,7 +2001,8 @@ def __mro_entries__(self, bases):
         return (self.__origin__,)
 
 
-class Annotated:
+@_SpecialForm
+def Annotated(self, params):
     """Add context-specific metadata to a type.
 
     Example: Annotated[int, runtime_check.Unsigned] indicates to the
@@ -2048,14 +2049,6 @@ class Annotated:
       where T1, T2 etc. are TypeVars, which would be invalid, because
       only one type should be passed to Annotated.
     """
-
-    __slots__ = ()
-
-    def __new__(cls, *args, **kwargs):
-        raise TypeError("Type Annotated cannot be instantiated.")
-
-    @_tp_cache
-    def __class_getitem__(cls, params):
     if not isinstance(params, tuple) or len(params) < 2:
         raise TypeError("Annotated[...] should be used "
                         "with at least two arguments (a type and an "
@@ -2068,11 +2061,6 @@ def __class_getitem__(cls, params):
     metadata = tuple(params[1:])
     return _AnnotatedAlias(origin, metadata)
 
-    def __init_subclass__(cls, *args, **kwargs):
-        raise TypeError(
-            "Cannot subclass {}.Annotated".format(cls.__module__)
-        )
-
 
 def runtime_checkable(cls):
     """Mark a protocol class as a runtime protocol.

Misc/NEWS.d/next/Library/2023-06-08-15-56-45.gh-issue-105509.YIG57j.rst

@@ -0,0 +1,3 @@
+:data:`typing.Annotated` is now implemented as an instance of
+``typing._SpecialForm`` rather than a class. This should have no user-facing
+impact for users of the :mod:`typing` module public API.