Explain subtype treatment in constrained TypeVars. Explain bare List is equivalent to List[Any].
diff --git a/pep-0484.txt b/pep-0484.txt
index 03cb3f0..7b1416f 100644
--- a/pep-0484.txt
+++ b/pep-0484.txt
@@ -280,17 +280,26 @@
or two ``bytes`` arguments, but not with a mix of ``str`` and ``bytes``
arguments.
-Note that subtypes are not valid values for a constrained type variable.
-Consider this example::
+Note that subtypes of types constrained by a type variable are treated
+as their respective explicitly listed base types in the context of the
+type variable. Consider this example::
class MyStr(str): ...
x = concat(MyStr('apple'), MyStr('pie'))
-The type variable ``AnyStr`` will get type ``str`` instead of ``MyStr`` (but the
-call is valid), and thus the inferred type of ``x`` will also be ``str``.
+The call is valid but the type variable ``AnyStr`` will be set to
+``str`` and not ``MyStr``. In effect, the inferred type of the return
+value assigned to ``x`` will also be ``str``.
Additionally, ``Any`` is a valid value for every type variable.
+Consider the following::
+
+ def count_truthy(elements: List[Any]) -> int:
+ return sum(1 for elem in elements if element)
+
+This is equivalent to omitting the generic notation and just saying
+``elements: List``.
User-defined generic types
