do not revert candidate for protected packages

In commit 21b3eac8 I promoted the check for installable dependencies to
a pre-install check, which also reverts to a known good candidate (the
installed version) if it fails. This revert was done even for user
requested candidate switches which disabled our Broken detection so that
install requests which are impossible to satisfy do not fail anymore,
but print an (incomplete) solution proposal and then exit successfully.

Closes: 745046

apt-pkg/depcache.cc

@@ -1374,7 +1374,7 @@ bool pkgDepCache::IsInstallOkDependenciesSatisfiableByCandidates(PkgIterator con
 
 	 // the dependency is critical, but can't be installed, so discard the candidate
 	 // as the problemresolver will trip over it otherwise trying to install it (#735967)
-	 if (Pkg->CurrentVer != 0)
+	 if (Pkg->CurrentVer != 0 && (PkgState[Pkg->ID].iFlags & Protected) != Protected)
 	    SetCandidateVersion(Pkg.CurrentVer());
 	 return false;
       }

test/integration/test-bug-745046-candidate-propagation-fails

@@ -0,0 +1,39 @@
+#!/bin/sh
+set -e
+
+TESTDIR=$(readlink -f $(dirname $0))
+. $TESTDIR/framework
+
+setupenvironment
+configarchitecture 'amd64'
+
+insertinstalledpackage 'gedit' 'amd64' '1'
+
+insertpackage 'unstable' 'gedit' 'amd64' '1'
+insertpackage 'experimental' 'gedit' 'amd64' '2' 'Depends: common (>= 2)'
+
+setupaptarchive
+
+testequal "Reading package lists...
+Building dependency tree...
+Selected version '2' (experimental [amd64]) for 'gedit'
+Some packages could not be installed. This may mean that you have
+requested an impossible situation or if you are using the unstable
+distribution that some required packages have not yet been created
+or been moved out of Incoming.
+The following information may help to resolve the situation:
+
+The following packages have unmet dependencies:
+ gedit : Depends: common (>= 2) but it is not installable
+E: Unable to correct problems, you have held broken packages." aptget install gedit/experimental -sq=0
+
+insertinstalledpackage 'common' 'amd64' '2'
+
+testequal "Reading package lists...
+Building dependency tree...
+Selected version '2' (experimental [amd64]) for 'gedit'
+The following packages will be upgraded:
+  gedit
+1 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
+Inst gedit [1] (2 experimental [amd64])
+Conf gedit (2 experimental [amd64])" aptget install gedit/experimental -sq=0