[3.15] gh-161100: Fix IPv6Interface.is_unspecified for IPv4-mapped addresses (GH-161101)

Defer to IPv6Address.is_unspecified, as the is_loopback override does,
so ::ffff:0.0.0.0/128 interfaces are unspecified like the address.

ANT-2026-DIFE27WF

(cherry picked from commit 6e6e83e)

Author: gpshead

Lib/ipaddress.py

@@ -2272,7 +2272,7 @@ def with_hostmask(self):
 
     @property
     def is_unspecified(self):
-        return self._ip == 0 and self.network.is_unspecified
+        return super().is_unspecified and self.network.is_unspecified
 
     @property
     def is_loopback(self):

Lib/test/test_ipaddress.py

@@ -2447,6 +2447,14 @@ def testReservedIpv6(self):
         self.assertEqual(False, ipaddress.ip_network('::1').is_unspecified)
         self.assertEqual(False, ipaddress.ip_network('::/127').is_unspecified)
 
+        self.assertTrue(ipaddress.ip_interface('::/128').is_unspecified)
+        self.assertTrue(
+            ipaddress.ip_interface('::ffff:0.0.0.0/128').is_unspecified)
+        self.assertFalse(ipaddress.ip_interface('::/127').is_unspecified)
+        self.assertFalse(
+            ipaddress.ip_interface('::ffff:0.0.0.0/127').is_unspecified)
+        self.assertFalse(ipaddress.ip_interface('::1/128').is_unspecified)
+
         self.assertEqual(True,
                          ipaddress.ip_network('2001::1/128').is_private)
         self.assertEqual(True,
@@ -2536,6 +2544,17 @@ def testIpv4MappedProperties(self):
                 self.assertEqual(ipv4.is_link_local, ipv6.is_link_local)
                 self.assertEqual(ipv4.is_loopback, ipv6.is_loopback)
 
+                ipv4 = ipaddress.IPv4Interface(addr4)
+                ipv6 = ipaddress.IPv6Interface(f"::ffff:{addr4}")
+
+                self.assertEqual(ipv4.is_global, ipv6.is_global)
+                self.assertEqual(ipv4.is_private, ipv6.is_private)
+                self.assertEqual(ipv4.is_reserved, ipv6.is_reserved)
+                self.assertEqual(ipv4.is_multicast, ipv6.is_multicast)
+                self.assertEqual(ipv4.is_unspecified, ipv6.is_unspecified)
+                self.assertEqual(ipv4.is_link_local, ipv6.is_link_local)
+                self.assertEqual(ipv4.is_loopback, ipv6.is_loopback)
+
     def testIpv4MappedPrivateCheck(self):
         self.assertEqual(
                 True, ipaddress.ip_address('::ffff:192.168.1.1').is_private)

Misc/NEWS.d/next/Library/2026-10-22-05-20-06.gh-issue-161100.pLgnlA.rst

@@ -0,0 +1,3 @@
+Fix :attr:`!ipaddress.IPv6Interface.is_unspecified` returning ``False`` for
+the IPv4-mapped address ``::ffff:0.0.0.0``.  It now gives the same result as
+:attr:`ipaddress.IPv6Address.is_unspecified`.