PEP 696: some suggestions to make the text clearer (#3641)

* Moves TypeVarTuple discussion so the text flows better. Otherwise, it
  feels to me like an awkward interlude between the paragraph
  introducing rules and the actual rules.
* In sections where the text uses `T1` and `T2` as TypeVar names, changes
  the examples to use the same names.
* Fix subtyping relationship in "Bound Rules."
* Spell out subclassing rules in the text, rather than leaning so much on the example.

Co-authored-by: Jelle Zijlstra <[email protected]>
Co-authored-by: James Hilton-Balfe <[email protected]>

peps/pep-0696.rst

@@ -216,19 +216,8 @@ default to the type of ``start`` and step default to ``int | None``.
    Foo[int](1, "")          # Invalid: Foo[int, str] cannot be assigned to self: Foo[int, int] in Foo.__init__
    Foo[int]("", 1)          # Invalid: Foo[str, int] cannot be assigned to self: Foo[int, int] in Foo.__init__
 
-When using a type parameter as the default to another type parameter.
-Where ``T1`` is the default for ``T2`` the following rules apply.
-
-``TypeVarTuple``\s are not supported because:
-
-- `Scoping Rules`_ does not allow usage of type parameters
-  from outer scopes.
-- Multiple ``TypeVarTuple``\s cannot appear in the type
-  parameter list for a single object, as specified in
-  :pep:`646#multiple-type-variable-tuples-not-allowed`.
-
-These reasons leave no current valid location where a
-``TypeVarTuple`` could be used as the default of another ``TypeVarTuple``.
+When using a type parameter as the default to another type parameter, the
+following rules apply, where ``T1`` is the default for ``T2``.
 
 Scoping Rules
 ~~~~~~~~~~~~~
@@ -237,11 +226,11 @@ Scoping Rules
 
 ::
 
-   DefaultT = TypeVar("DefaultT", default=T)
+   T2 = TypeVar("T2", default=T1)
 
-   class Foo(Generic[T, DefaultT]): ...   # Valid
-   class Foo(Generic[T]):
-       class Bar(Generic[DefaultT]): ...  # Valid
+   class Foo(Generic[T1, T2]): ...   # Valid
+   class Foo(Generic[T1]):
+       class Bar(Generic[T2]): ...   # Valid
 
    StartT = TypeVar("StartT", default="StopT")  # Swapped defaults around from previous example
    StopT = TypeVar("StopT", default=int)
@@ -253,14 +242,14 @@ Using a type parameter from an outer scope as a default is not supported.
 Bound Rules
 ~~~~~~~~~~~
 
-``T2``'s bound must be a subtype of ``T1``'s bound.
+``T1``'s bound must be a subtype of ``T2``'s bound.
 
 ::
 
-   T = TypeVar("T", bound=float)
-   TypeVar("Ok", default=T, bound=int)        # Valid
-   TypeVar("AlsoOk", default=T, bound=float)  # Valid
-   TypeVar("Invalid", default=T, bound=str)   # Invalid: str is not a subtype of float
+   T1 = TypeVar("T1", bound=int)
+   TypeVar("Ok", default=T1, bound=float)     # Valid
+   TypeVar("AlsoOk", default=T1, bound=int)   # Valid
+   TypeVar("Invalid", default=T1, bound=str)  # Invalid: int is not a subtype of str
 
 Constraint Rules
 ~~~~~~~~~~~~~~~~
@@ -329,7 +318,10 @@ Subclassing
 '''''''''''
 
 Subclasses of ``Generic``\ s with type parameters that have defaults
-behave similarly to ``Generic`` ``TypeAlias``\ es.
+behave similarly to ``Generic`` ``TypeAlias``\ es. That is, subclasses can be
+further subscripted following normal subscription rules, non-overridden
+defaults should be substituted in, and type parameters with such defaults can be
+further specialised down the line.
 
 ::
 
@@ -421,6 +413,20 @@ for the ``ParamSpec`` and one for the ``TypeVarTuple``.
    Foo[int, str]  # Ts = (int, str), P = [float, bool]
    Foo[int, str, [bytes]]  # Ts = (int, str), P = [bytes]
 
+``TypeVarTuple``\ s as Defaults
+'''''''''''''''''''''''''''''''
+
+Using a ``TypeVarTuple`` as a default is not supported because:
+
+- `Scoping Rules`_ does not allow usage of type parameters
+  from outer scopes.
+- Multiple ``TypeVarTuple``\ s cannot appear in the type
+  parameter list for a single object, as specified in
+  :pep:`646#multiple-type-variable-tuples-not-allowed`.
+
+These reasons leave no current valid location where a
+``TypeVarTuple`` could be used as the default of another ``TypeVarTuple``.
+
 Binding rules
 -------------