bpo-41923: PEP 613: Add TypeAlias to typing module (#22532)

This special marker annotation is intended to help in distinguishing
proper PEP 484-compliant type aliases from regular top-level variable
assignments.

Lib/typing.py

@@ -113,6 +113,7 @@ __all__ = [
     'runtime_checkable',
     'Text',
     'TYPE_CHECKING',
+    'TypeAlias',
 ]
 
 # The pseudo-submodules 're' and 'io' are part of the public
@@ -460,6 +461,21 @@ def Literal(self, parameters):
     return _GenericAlias(self, parameters)
 
 
+@_SpecialForm
+def TypeAlias(self, parameters):
+    """Special marker indicating that an assignment should
+    be recognized as a proper type alias definition by type
+    checkers.
+
+    For example::
+
+        Predicate: TypeAlias = Callable[..., bool]
+
+    It's invalid when used anywhere except as in the example above.
+    """
+    raise TypeError(f"{self} is not subscriptable")
+
+
 class ForwardRef(_Final, _root=True):
     """Internal wrapper to hold a forward reference."""