Make Boolean test for shared handles explicit

Author: chschulte

changelog.in

@@ -72,6 +72,7 @@ New stuff!
 Module: int
 What:   removed
 Rank:   minor
+Thanks: Jens Krueger
 [DESCRIPTION]
 The bool test operator has been removed from IntSet (it created
 problems and is actually useless for IntSet as they are always

gecode/kernel/shared-object.hpp

@@ -80,7 +80,7 @@ namespace Gecode {
     /// Destructor that maintains reference count
     ~SharedHandle(void);
     /// Whether handle points to an object
-    operator bool(void) const;
+    explicit operator bool(void) const;
   protected:
     /// Access to the shared object
     SharedHandle::Object* object(void) const;