Index: sbuild
===================================================================
--- sbuild	(revision 1019)
+++ sbuild	(working copy)
@@ -1519,6 +1519,11 @@
 	}
 	my $status = get_dpkg_status( keys %names );
 
+	my %policy;
+	if ($conf::apt_policy) {
+		%policy = get_apt_policy( keys %names );
+	}
+
 	foreach $dep (@$dependencies) {
 		$name = $dep->{'Package'};
 		next if !$name;
@@ -1572,6 +1577,23 @@
 			if (!$stat->{'Installed'}) {
 				print "$name: pos dep, not installed\n" if $conf::debug;
 				print PLOG "$name: missing\n";
+				if ($conf::apt_policy && $rel) {
+					if (!WannaBuild::version_compare($policy{$name}->{defversion}, $rel, $vers)) {
+						print PLOG "Default version of $name not sufficient, ";
+						foreach my $cvers (@{$policy{$name}->{versions}}) {
+							if (WannaBuild::version_compare($cvers, $rel, $vers)) {
+								print PLOG "using version $cvers\n";
+								$installable = $name . "=" . $cvers if !$installable;
+								last;
+							}
+						}
+						if(!$installable) {
+							print PLOG "no suitable version found. Skipping for now, maybe there are alternatives.\n" if !$installable;
+						}
+					} else {
+						print PLOG "Using default version " . $policy{$name}->{defversion} . "\n";
+					}
+				}
 				$installable = $name if !$installable;
 				next;
 			}
@@ -1596,6 +1618,20 @@
 				print PLOG "$name: would have to downgrade!\n";
 			}
 			else {
+				if ($conf::apt_policy && !WannaBuild::version_compare($policy{$name}->{defversion}, $rel, $vers)) {
+					print PLOG "Default version of $name not sufficient, ";
+					foreach my $cvers (@{$policy{$name}->{versions}}) {
+						if(WannaBuild::version_compare($cvers, $rel, $vers)) {
+							print PLOG "using version $cvers\n";
+							$upgradeable = $name if ! $upgradeable;
+							last;
+						}
+					}
+					print PLOG "no suitable alternative found. I probably should dep-wait this one.\n" if !$upgradeable;
+					return 0;
+				} else {
+					print PLOG "Using default version " . $policy{$name}->{defversion} . "\n";
+				}
 				$upgradeable = $name if !$upgradeable;
 			}
 		}
@@ -1609,9 +1645,9 @@
 				push( @$pos_list, $installable );
 			}
 			else {
-				print PLOG "To satisfy this dependency the package(s) would ",
-						   "have\n",
-						   "to be downgraded; this is not implemented.\n";
+				print PLOG "This dependency could not be satisfied. Possible reasons:\n";
+				print PLOG "* The package has a versioned dependency that is not yet available.\n";
+				print PLOG "* The package has a versioned dependency on a package version that is\n  older than the currently-installed package. Downgrades are not implemented.\n";
 				return 0;
 			}
 		}
@@ -1696,6 +1732,28 @@
 	return $fail;
 }
 
+sub get_apt_policy {
+	my @interest = @_;
+	my $package;
+	my %packages;
+
+	$ENV{LC_ALL}='C';
+
+	my $command = get_apt_command("$conf::apt_cache", "policy @interest", $main::username, 0);
+
+	open(APTCACHE, "$command |" )
+		or die "Cannot start $conf::apt_cache $!\n";
+	while(<APTCACHE>) {
+		$package=$1 if /^([0-9a-z+.-]+):$/;
+		$packages{$package}->{curversion}=$1 if /^ {2}Installed: ([0-9a-zA-Z-.:+]*)$/;
+		$packages{$package}->{defversion}=$1 if /^ {2}Candidate: ([0-9a-zA-Z-.:+]*)$/;
+		push @{$packages{$package}->{versions}}, "$2" if /^ (\*{3}| {3}) ([0-9a-zA-Z-.:+]*) 0$/;
+	}
+	die "$conf::apt_cache exit status $?\n" if $?;
+
+	return %packages;
+}
+
 sub get_dpkg_status {
 	my @interest = @_;
 	my %result;
Index: Sbuild/Conf.pm
===================================================================
--- Sbuild/Conf.pm	(revision 1018)
+++ Sbuild/Conf.pm	(working copy)
@@ -32,7 +32,7 @@
 
     @ISA = qw(Exporter);
 
-    @EXPORT = qw($HOME $cwd $username $verbose $nolog
+    @EXPORT = qw($HOME $apt_policy $cwd $username $verbose $nolog
 		 $source_dependencies $mailprog $dpkg $sudo $su
 		 $schroot $schroot_options $fakeroot $apt_get
 		 $apt_cache $dpkg_source $md5sum $avg_time_db
@@ -81,6 +81,7 @@
 our $srcdep_lock_wait = 1; # minutes
 our $chroot_only = 1;
 our $chroot_mode = "split";
+our $apt_policy = 1;
 our @ignore_watches_no_build_deps = qw();
 our $build_dir = undef;
 our $sbuild_mode = "buildd";
Index: sbuild.conf.local
===================================================================
--- sbuild.conf.local	(revision 1018)
+++ sbuild.conf.local	(working copy)
@@ -30,5 +30,12 @@
 # Require chrooted building?
 #$chroot_only=1;
 
+# APT policy.  1 to enable additional checking of package versions
+# available in the APT cache, or 0 to disable.  0 is the traditional
+# sbuild behaviour; 1 is needed to build from additional repositories
+# such as sarge-backports or experimental, and has a small performance
+# cost.
+#$apt_policy = 1;
+
 # don't remove this, Perl needs it:
 1;