gh-116126: Implement PEP 696 (#116129)

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>
Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com>

Doc/reference/compound_stmts.rst

@@ -1620,15 +1620,18 @@ Type parameter lists
 
 .. versionadded:: 3.12
 
+.. versionchanged:: 3.13
+   Support for default values was added (see :pep:`696`).
+
 .. index::
    single: type parameters
 
 .. productionlist:: python-grammar
    type_params: "[" `type_param` ("," `type_param`)* "]"
    type_param: `typevar` | `typevartuple` | `paramspec`
-   typevar: `identifier` (":" `expression`)?
-   typevartuple: "*" `identifier`
-   paramspec: "**" `identifier`
+   typevar: `identifier` (":" `expression`)? ("=" `expression`)?
+   typevartuple: "*" `identifier` ("=" `expression`)?
+   paramspec: "**" `identifier` ("=" `expression`)?
 
 :ref:`Functions <def>` (including :ref:`coroutines <async def>`),
 :ref:`classes <class>` and :ref:`type aliases <type>` may
@@ -1694,19 +1697,31 @@ evaluated in a separate :ref:`annotation scope <annotation-scopes>`.
 :data:`typing.TypeVarTuple`\ s and :data:`typing.ParamSpec`\ s cannot have bounds
 or constraints.
 
+All three flavors of type parameters can also have a *default value*, which is used
+when the type parameter is not explicitly provided. This is added by appending
+a single equals sign (``=``) followed by an expression. Like the bounds and
+constraints of type variables, the default value is not evaluated when the
+object is created, but only when the type parameter's ``__default__`` attribute
+is accessed. To this end, the default value is evaluated in a separate
+:ref:`annotation scope <annotation-scopes>`. If no default value is specified
+for a type parameter, the ``__default__`` attribute is set to the special
+sentinel object :data:`typing.NoDefault`.
+
 The following example indicates the full set of allowed type parameter declarations::
 
    def overly_generic[
       SimpleTypeVar,
+      TypeVarWithDefault = int,
       TypeVarWithBound: int,
       TypeVarWithConstraints: (str, bytes),
-      *SimpleTypeVarTuple,
-      **SimpleParamSpec,
+      *SimpleTypeVarTuple = (int, float),
+      **SimpleParamSpec = (str, bytearray),
    ](
       a: SimpleTypeVar,
-      b: TypeVarWithBound,
-      c: Callable[SimpleParamSpec, TypeVarWithConstraints],
-      *d: SimpleTypeVarTuple,
+      b: TypeVarWithDefault,
+      c: TypeVarWithBound,
+      d: Callable[SimpleParamSpec, TypeVarWithConstraints],
+      *e: SimpleTypeVarTuple,
    ): ...
 
 .. _generic-functions:

Doc/reference/executionmodel.rst

@@ -205,7 +205,7 @@ Annotation scopes are used in the following contexts:
 * Type parameter lists for :ref:`generic classes <generic-classes>`.
   A generic class's base classes and
   keyword arguments are executed within the annotation scope, but its decorators are not.
-* The bounds and constraints for type variables
+* The bounds, constraints, and default values for type parameters
   (:ref:`lazily evaluated <lazy-evaluation>`).
 * The value of type aliases (:ref:`lazily evaluated <lazy-evaluation>`).
 
@@ -232,13 +232,17 @@ Annotation scopes differ from function scopes in the following ways:
 .. versionadded:: 3.12
    Annotation scopes were introduced in Python 3.12 as part of :pep:`695`.
 
+.. versionchanged:: 3.13
+   Annotation scopes are also used for type parameter defaults, as
+   introduced by :pep:`696`.
+
 .. _lazy-evaluation:
 
 Lazy evaluation
 ---------------
 
 The values of type aliases created through the :keyword:`type` statement are
-*lazily evaluated*. The same applies to the bounds and constraints of type
+*lazily evaluated*. The same applies to the bounds, constraints, and default values of type
 variables created through the :ref:`type parameter syntax <type-params>`.
 This means that they are not evaluated when the type alias or type variable is
 created. Instead, they are only evaluated when doing so is necessary to resolve