CheckResolvedIP: an IPv4-mapped prefix is not one of NAT64, a test

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

locator/resolved_test.go

@@ -64,3 +64,12 @@ func TestResolvedIPWellKnownPrefix(t *testing.T) {
 		}
 	}
 }
+
+// An IPv4-mapped prefix is not one of NAT64: the texts of TypeScript and Dart
+// follow this one.
+func TestResolvedIPMappedPrefix(t *testing.T) {
+	err := locator.CheckResolvedIP(netip.MustParseAddr("8.8.8.8"), netip.MustParsePrefix("::ffff:0.0.0.0/96"))
+	if err == nil || err.Error() != "locator: the NAT64 prefix ::ffff:0.0.0.0/96 is not an IPv6 prefix" {
+		t.Fatal(err)
+	}
+}