PEP 747: Round of updates based on feedback (#4269)

Updated draft PEP 747 to fix a few bugs found by reviewers and to reflect the majority consensus on assignability of `type[T]` to `TypeForm[T]`.

Author: erictraut

peps/pep-0747.rst

@@ -200,12 +200,17 @@ that represent a type assignable to ``str | None``::
   err1: TypeForm[str | None] = str | int  # Error
   err2: TypeForm[str | None] = list[str | None]  # Error
 
-By this same definition, ``TypeForm[Any]`` describes a type form object
-that represents the type ``Any`` or any type that is assignable to ``Any``.
-Since all types in the Python type system are assignable to ``Any``,
-``TypeForm[Any]`` describes the set of all type form objects
+By this same definition, ``TypeForm[object]`` describes a type form object
+that represents the type ``object`` or any type that is assignable to ``object``.
+Since all types in the Python type system are assignable to ``object``,
+``TypeForm[object]`` describes the set of all type form objects
 evaluated from all valid type expressions.
 
+``TypeForm[Any]`` describes a ``TypeForm`` type whose type argument is not
+statically known but is a valid type form object. It is thus assignable both
+to and from any other ``TypeForm`` type (because ``Any`` is assignable both
+to and from any type).
+
 The type expression ``TypeForm``, with no type argument provided, is
 equivalent to ``TypeForm[Any]``.
 
@@ -289,9 +294,9 @@ This explicit syntax serves two purposes. First, it documents the developer's
 intent to use the value as a type form object. Second, static type checkers
 validate that all rules for type expressions are followed::
 
-  x4 = type(int)  # No error, evaluates to "type[int]"
+  x4 = type(1)  # No error, evaluates to "type[int]"
 
-  x5 = TypeForm(type(int))  # Error: call not allowed in type expression
+  x5 = TypeForm(type(1))  # Error: call not allowed in type expression
 
 
 Assignability
@@ -306,6 +311,14 @@ Assignability
   t1: TypeForm[int | str] = get_type_form()  # OK
   t2: TypeForm[str] = get_type_form()  # Error
 
+``type[T]`` is a subtype of ``TypeForm[T]``, which means that ``type[B]`` is
+assignable to ``TypeForm[A]`` if ``B`` is assignable to ``A``::
+
+  def get_type() -> type[int]: ...
+
+  t3: TypeForm[int | str] = get_type()  # OK
+  t4: TypeForm[str] = get_type()  # Error
+
 ``TypeForm`` is a subtype of ``object`` and is assumed to have all of the
 attributes and methods of ``object``.
 
@@ -530,34 +543,6 @@ of ``type``.
 .. _designed: https://mail.python.org/archives/list/[email protected]/message/D5FHORQVPHX3BHUDGF3A3TBZURBXLPHD/
 
 
-Treat ``type[T]`` as a subtype of ``TypeForm[T]``
--------------------------------------------------
-
-It was suggested that type ``type[T]`` should be considered a subtype
-of ``TypeForm[T]``. This was ultimately rejected because there are ways to
-create an object of type ``type[T]`` that does not encode a valid type expression.
-For example, the expression ``type[1]`` is not a valid type expression. Likewise,
-the  expression ``type[S]`` is not a valid type expression if ``S`` is an
-out-of-scope type variable. This same argument applies to other special forms
-like objects of type ``UnionType``, which may or may not encode a valid type
-expression. Rather than carving out a special case for ``type[T]`` that would
-allow for potential unsoundness, it was decided to treat all type forms
-consistently. Therefore, ``type[T]`` is not considered a subtype of ``TypeForm[T]``.
-
-It was also pointed out that the expression ``C | C`` (where ``C`` is a class
-object) is a valid type expression - and therefore a valid ``TypeForm``,
-but its runtime type form encoding is an instance of ``UnionType`` and
-therefore is not compatible with ``type[C]``.
-
-If a function wishes to indicate that it accepts values of type ``TypeForm[T]``
-_and_ ``type[T]``, the parameter can simply be annotated with a union of these
-two types.
-
-::
-
-   def func[T](t: TypeForm[T] | type[T]) -> None: ...
-
-
 
 Accept arbitrary annotation expressions
 ---------------------------------------