Minor tweaks to PEP 544 (#1046)

This PR contains mostly minor wording tweaks plus a paragraph explicitly allowing class objects as implementations of protocols, previously there were questions whether it is actually allowed, see python/mypy#4536.

pep-0544.txt

@@ -703,8 +703,8 @@ intersection type construct could be added in future as specified by PEP 483,
 see `rejected`_ ideas for more details.
 
 
-``Type[]`` with protocols
--------------------------
+``Type[]`` and class objects vs protocols
+-----------------------------------------
 
 Variables and parameters annotated with ``Type[Proto]`` accept only concrete
 (non-protocol) subtypes of ``Proto``. The main reason for this is to allow
@@ -735,6 +735,23 @@ not explicitly typed, and such assignment creates a type alias.
 For normal (non-abstract) classes, the behavior of ``Type[]`` is
 not changed.
 
+A class object is considered an implementation of a protocol if accessing
+all members on it results in types compatible with the protocol members.
+For example::
+
+  from typing import Any, Protocol
+
+  class ProtoA(Protocol):
+      def meth(self, x: int) -> int: ...
+  class ProtoB(Protocol):
+      def meth(self, obj: Any, x: int) -> int: ...
+
+  class C:
+      def meth(self, x: int) -> int: ...
+
+  a: ProtoA = C  # Type check error, signatures don't match!
+  b: ProtoB = C  # OK
+
 
 ``NewType()`` and type aliases
 ------------------------------
@@ -762,8 +779,8 @@ aliases::
   CompatReversible = Union[Reversible[T], SizedIterable[T]]
 
 
-Modules as subtypes of protocols
---------------------------------
+Modules as implementations of protocols
+---------------------------------------
 
 A module object is accepted where a protocol is expected if the public
 interface of the given module is compatible with the expected protocol.
@@ -1177,7 +1194,8 @@ For example::
 
   def do_stuff(o: Union[P, X]) -> int:
       if isinstance(o, P):
-          return o.common_method_name(1)  # oops, what if it's an X instance?
+          return o.common_method_name(1)  # Results in TypeError not caught
+                                          # statically if o is an X instance.
 
 Another potentially problematic case is assignment of attributes
 *after* instantiation::
@@ -1196,13 +1214,13 @@ Another potentially problematic case is assignment of attributes
 
   def f(x: Union[P, int]) -> None:
       if isinstance(x, P):
-          # static type of x is P here
+          # Static type of x is P here.
           ...
       else:
-          # type of x is "int" here?
+          # Static type of x is int, but can be other type at runtime...
           print(x + 1)
 
-  f(C())   # oops
+  f(C())  # ...causing a TypeError.
 
 We argue that requiring an explicit class decorator would be better, since
 one can then attach warnings about problems like this in the documentation.
@@ -1273,7 +1291,7 @@ Consider this example::
 
   c = C()
   f(c)  # Would typecheck if covariant subtyping
-        # of mutable attributes were allowed
+        # of mutable attributes were allowed.
   c.x >> 1  # But this fails at runtime
 
 It was initially proposed to allow this for practical reasons, but it was
@@ -1292,7 +1310,7 @@ However, it was decided not to do this because of several downsides:
 
     T = TypeVar('T')
 
-    class P(Protocol[T]):  # Declared as invariant
+    class P(Protocol[T]):  # Protocol is declared as invariant.
         def meth(self) -> T:
             ...
     class C: