Backport missing ipv4 mapping functionality

Author: ambv

Lib/ipaddress.py

@@ -310,7 +310,7 @@ def collapse_addresses(addresses):
                            [IPv4Network('192.0.2.0/24')]
 
     Args:
-        addresses: An iterator of IPv4Network or IPv6Network objects.
+        addresses: An iterable of IPv4Network or IPv6Network objects.
 
     Returns:
         An iterator of the collapsed IPv(4|6)Network objects.
@@ -1840,9 +1840,6 @@ def _string_from_ip_int(cls, ip_int=None):
     def _explode_shorthand_ip_string(self):
         """Expand a shortened IPv6 address.
 
-        Args:
-            ip_str: A string, the IPv6 address.
-
         Returns:
             A string, the expanded IPv6 address.
 
@@ -2100,6 +2097,9 @@ def is_loopback(self):
             RFC 2373 2.5.3.
 
         """
+        ipv4_mapped = self.ipv4_mapped
+        if ipv4_mapped is not None:
+            return ipv4_mapped.is_loopback
         return self._ip == 1
 
     @property
@@ -2216,7 +2216,7 @@ def is_unspecified(self):
 
     @property
     def is_loopback(self):
-        return self._ip == 1 and self.network.is_loopback
+        return super().is_loopback and self.network.is_loopback
 
 
 class IPv6Network(_BaseV6, _BaseNetwork):
@@ -2329,6 +2329,8 @@ class _IPv6Constants:
         IPv6Network('2001:db8::/32'),
         # IANA says N/A, let's consider it not globally reachable to be safe
         IPv6Network('2002::/16'),
+        # RFC 9637: https://www.rfc-editor.org/rfc/rfc9637.html#section-6-2.2
+        IPv6Network('3fff::/20'),
         IPv6Network('fc00::/7'),
         IPv6Network('fe80::/10'),
         ]