Vendor typing._SpecialForm to fool typing._type_check

Adds a local copy of _SpecialForm in our namespace, so
typing._type_check won't raise TypeError. (#964)

typing_extensions/src_py3/test_typing_extensions.py

@@ -2199,6 +2199,12 @@ def test_no_isinstance(self):
         with self.assertRaises(TypeError):
             issubclass(int, Self)
 
+    def test_alias(self):
+        TupleSelf = Tuple[Self, Self]
+        class Alias:
+            def return_tuple(self) -> TupleSelf:
+                return (self, self)
+
 class AllTests(BaseTestCase):
 
     def test_typing_extensions_includes_standard(self):

typing_extensions/src_py3/typing_extensions.py

@@ -2048,40 +2048,61 @@ def __eq__(self, other):
 
     TypeGuard = _TypeGuard(_root=True)
 
+if not hasattr(typing, "Self") and sys.version_info[:2] >= (3, 7):
+    # Vendored from cpython typing._SpecialFrom
+    class _SpecialForm(typing._Final, _root=True):
+        __slots__ = ('_name', '__doc__', '_getitem')
 
-if hasattr(typing, "Self"):
-    Self = typing.Self
+        def __init__(self, getitem):
+            self._getitem = getitem
+            self._name = getitem.__name__
+            self.__doc__ = getitem.__doc__
+
+        def __getattr__(self, item):
+            if item in {'__name__', '__qualname__'}:
+                return self._name
+
+            raise AttributeError(item)
+
+        def __mro_entries__(self, bases):
+            raise TypeError(f"Cannot subclass {self!r}")
 
-elif sys.version_info[:2] >= (3, 9):
-    class _SelfForm(typing._SpecialForm, _root=True):
         def __repr__(self):
-            return 'typing_extensions.' + self._name
+            return 'typing.' + self._name
 
-    @_SelfForm
-    def Self(self, params):
-        """Used to spell the type of "self" in classes.
+        def __reduce__(self):
+            return self._name
 
-        Example::
+        def __call__(self, *args, **kwds):
+            raise TypeError(f"Cannot instantiate {self!r}")
 
-          from typing import Self
+        def __or__(self, other):
+            return Union[self, other]
 
-          class ReturnsSelf:
-              def parse(self, data: bytes) -> Self:
-                  ...
-                  return self
+        def __ror__(self, other):
+            return Union[other, self]
 
-        """
+        def __instancecheck__(self, obj):
+            raise TypeError(f"{self} cannot be used with isinstance()")
 
-        raise TypeError(f"{self} is not subscriptable")
+        def __subclasscheck__(self, cls):
+            raise TypeError(f"{self} cannot be used with issubclass()")
+
+        @typing._tp_cache
+        def __getitem__(self, parameters):
+            return self._getitem(self, parameters)
+
+if hasattr(typing, "Self"):
+    Self = typing.Self
 
 elif sys.version_info[:2] >= (3, 7):
-    class _SelfForm(typing._SpecialForm, _root=True):
+    class _SelfForm(_SpecialForm, _root=True):
         def __repr__(self):
             return 'typing_extensions.' + self._name
 
-    Self = _SelfForm(
-        "Self",
-        doc="""Used to spell the type of "self" in classes.
+    @_SelfForm
+    def Self(self, params):
+        """Used to spell the type of "self" in classes.
 
         Example::
 
@@ -2093,7 +2114,8 @@ def parse(self, data: bytes) -> Self:
                   return self
 
         """
-    )
+
+        raise TypeError(f"{self} is not subscriptable")
 else:
     class _Self(typing._FinalTypingBase, _root=True):
         """Used to spell the type of "self" in classes.