Always postpone 'unable to satisfy constraints' failure in collect_build_prerequisites() and fix unexpected 'unable to satisfy dependency' failure

7 files changed, 784 insertions, 261 deletions

diff --git a/bpkg/pkg-build-collect.cxx b/bpkg/pkg-build-collect.cxx
index b16c518..a20aa65 100644
--- a/bpkg/pkg-build-collect.cxx
+++ b/bpkg/pkg-build-collect.cxx
@@ -432,13 +432,14 @@ namespace bpkg
   //
   void unsatisfied_dependents::
   add (const package_key& dpt,
-       const build_package* dep,
-       const version_constraint& c)
+       const package_key& dep,
+       const version_constraint& c,
+       vector<unsatisfied_constraint>&& ucs,
+       vector<package_key>&& dc)
   {
     if (unsatisfied_dependent* ud = find_dependent (dpt))
     {
-      vector<pair<const build_package*, version_constraint>>& deps (
-        ud->dependencies);
+      vector<ignored_constraint>& ics (ud->ignored_constraints);
 
       // Skip the dependency if it is already in the list.
       //
@@ -446,15 +447,17 @@ namespace bpkg
       // constraint (think of multiple depends clauses with the same
       // dependency), in which case we leave it unchanged.
       //
-      if (find_if (deps.begin (), deps.end (),
-                   [dep] (const auto& v) {return v.first == dep;}) ==
-          deps.end ())
+      if (find_if (ics.begin (), ics.end (),
+                   [dep] (const auto& v) {return v.dependency == dep;}) ==
+          ics.end ())
       {
-        deps.push_back (make_pair (dep, c));
+        ics.emplace_back (dep, c, move (ucs), move (dc));
       }
     }
     else
-      push_back (unsatisfied_dependent {dpt, {make_pair (dep, c)}});
+      push_back (
+        unsatisfied_dependent {
+          dpt, {ignored_constraint (dep, c, move (ucs), move (dc))}});
   }
 
   unsatisfied_dependent* unsatisfied_dependents::
@@ -474,73 +477,118 @@ namespace bpkg
     assert (!empty ());
 
     const unsatisfied_dependent& dpt (front ());
+    const package_key& dk (dpt.dependent);
 
-    assert (!dpt.dependencies.empty ());
+    assert (!dpt.ignored_constraints.empty ());
 
-    const package_key& dk (dpt.dependent);
-    const build_package& p (*dpt.dependencies.front ().first);
-    const version_constraint& c (dpt.dependencies.front ().second);
+    const ignored_constraint& ic (dpt.ignored_constraints.front ());
 
-    database& pdb (p.db);
-    const shared_ptr<selected_package>& sp (p.selected);
+    const build_package* p (pkgs.entered_build (ic.dependency));
+    assert (p != nullptr); // The dependency must be collected.
 
-    const package_name& n (sp->name);
+    const version_constraint& c (ic.constraint);
+    const vector<unsatisfied_constraint>& ucs (ic.unsatisfied_constraints);
 
-    // Otherwise, this would be a dependency adjustment (not an
-    // up/down-grade), and thus the dependent must be satisfied with the
-    // already configured dependency.
-    //
-    assert (p.available != nullptr);
+    const package_name& n (p->name ());
 
-    const version& av (p.available_version ());
+    //             "  info: ..."
+    string indent ("          ");
 
-    // See if we are upgrading or downgrading this package.
-    //
-    int ud (sp->version.compare (av));
+    if (ucs.empty ()) // 'unable to up/downgrade package' failure.
+    {
+      database& pdb (p->db);
+      const shared_ptr<selected_package>& sp (p->selected);
 
-    // Otherwise, the dependent must be satisfied with the already configured
-    // dependency.
-    //
-    assert (ud != 0);
+      // Otherwise, this would be a dependency adjustment (not an
+      // up/down-grade), and thus the dependent must be satisfied with the
+      // already configured dependency.
+      //
+      assert (p->available != nullptr);
 
-    diag_record dr (fail);
-    dr << "unable to " << (ud < 0 ? "up" : "down") << "grade "
-       << "package " << *sp << pdb << " to ";
+      const version& av (p->available_version ());
 
-    // Print both (old and new) package names in full if the system
-    // attribution changes.
-    //
-    if (p.system != sp->system ())
-      dr << p.available_name_version ();
-    else
-      dr << av; // Can't be the wildcard otherwise would satisfy.
+      // See if we are upgrading or downgrading this package.
+      //
+      int ud (sp->version.compare (av));
 
-    dr << info << "because package " << dk << " depends on (" << n << ' '
-               << c << ')';
+      // Otherwise, the dependent must be satisfied with the already configured
+      // dependency.
+      //
+      assert (ud != 0);
 
-    {
-      //             "  info: ..."
-      string indent ("          ");
-      set<package_key> printed;
-      pkgs.print_constraints (dr, dk, indent, printed);
-    }
+      diag_record dr (fail);
+      dr << "unable to " << (ud < 0 ? "up" : "down") << "grade "
+         << "package " << *sp << pdb << " to ";
 
-    string rb;
-    if (!p.user_selection ())
-    {
-      for (const package_key& pk: p.required_by)
-        rb += (rb.empty () ? " " : ", ") + pk.string ();
+      // Print both (old and new) package names in full if the system
+      // attribution changes.
+      //
+      if (p->system != sp->system ())
+        dr << p->available_name_version ();
+      else
+        dr << av; // Can't be the wildcard otherwise would satisfy.
+
+      dr << info << "because package " << dk << " depends on (" << n << ' '
+                 << c << ')';
+
+      {
+        set<package_key> printed;
+        pkgs.print_constraints (dr, dk, indent, printed);
+      }
+
+      string rb;
+      if (!p->user_selection ())
+      {
+        for (const package_key& pk: p->required_by)
+          rb += (rb.empty () ? " " : ", ") + pk.string ();
+      }
+
+      if (!rb.empty ())
+        dr << info << "package " << p->available_name_version ()
+                   << " required by" << rb;
+
+      dr << info << "consider re-trying with --upgrade|-u potentially combined "
+                 << "with --recursive|-r" <<
+        info << "or explicitly request up/downgrade of package " << dk.name <<
+        info << "or explicitly specify package " << n << " version to "
+             << "manually satisfy these constraints" << endf;
     }
+    else              // 'unable to satisfy constraints' failure.
+    {
+      diag_record dr (fail);
+      dr << "unable to satisfy constraints on package " << n;
+
+      for (const unsatisfied_constraint& uc: ucs)
+      {
+        const build_package::constraint_type& c (uc.constraint);
+
+        dr << info << c.dependent << c.db << " depends on (" << n
+                   << ' ' << c.value << ")";
+
+        if (const build_package* d = pkgs.dependent_build (c))
+        {
+          set<package_key> printed;
+          pkgs.print_constraints (dr, *d, indent, printed);
+        }
+      }
 
-    if (!rb.empty ())
-      dr << info << "package " << p.available_name_version ()
-                 << " required by" << rb;
+      for (const unsatisfied_constraint& uc: ucs)
+      {
+        dr << info << "available "
+           << package_string (n, uc.available_version, uc.available_system);
+      }
+
+      for (const package_key& d: reverse_iterate (ic.dependency_chain))
+      {
+        const build_package* p (pkgs.entered_build (d));
+        assert (p != nullptr);
+
+        dr << info << "while satisfying " << p->available_name_version_db ();
+      }
 
-    dr << info << "consider re-trying with --upgrade|-u potentially combined "
-               << "with --recursive|-r" <<
-          info << "or explicitly request up/downgrade of package " << dk.name <<
-          info << "or explicitly specify package " << n << " version to "
-               << "manually satisfy these constraints" << endf;
+      dr << info << "explicitly specify " << n << " version to "
+                 << "manually satisfy both constraints" << endf;
+    }
   }
 
   // postponed_configuration
@@ -1263,8 +1311,9 @@ namespace bpkg
 
     // See the above notes.
     //
-    bool recursive (dep_chain != nullptr);
-    assert ((fdb               != nullptr) == recursive &&
+    bool recursive (fdb != nullptr);
+
+    assert ((!recursive || dep_chain != nullptr)        &&
             (rpt_depts         != nullptr) == recursive &&
             (postponed_repo    != nullptr) == recursive &&
             (postponed_alts    != nullptr) == recursive &&
@@ -1315,10 +1364,51 @@ namespace bpkg
     }
 
     // Add the version replacement entry, call the verification function if
-    // specified, and throw replace_version.
+    // specified, and throw replace_version, unless the package is in the
+    // unsatisfied dependents list on the dependency side and the version
+    // replacement doesn't satisfy the ignored constraint. In the later case,
+    // report the first encountered ignored (unsatisfied) dependency
+    // constraint and fail.
+    //
+    // The thinking for the above failure is as follows: if we add the
+    // replacement entry and throw, then on the next iteration there won't be
+    // any information preserved that this version is actually unsatisfactory
+    // for some dependent and we need to ignore the respective constraint
+    // during simulating the configuration of this dependent and to fail if
+    // the problem doesn't resolve naturally (see unsatisfied_depts for
+    // details). This approach may potentially end up with some failures which
+    // we could potentially avoid if postpone them for some longer. Let's,
+    // however, keep it simple for now and reconsider if it turn out to be an
+    // issue.
     //
-    auto replace_ver = [&pk, &vpb, &vi, &replaced_vers] (const build_package& p)
+    auto replace_ver = [&pk,
+                        &vpb,
+                        &vi,
+                        &replaced_vers,
+                        &unsatisfied_depts,
+                        &trace,
+                        this] (const build_package& p)
     {
+
+      const version& v (p.available_version ());
+
+      for (const unsatisfied_dependent& ud: unsatisfied_depts)
+      {
+        for (const auto& c: ud.ignored_constraints)
+        {
+          if (c.dependency == pk && !satisfies (v, c.constraint))
+          {
+            l5 ([&]{trace << "dependency " << p.available_name_version_db ()
+                          << " is unsatisfactory for dependent "
+                          << ud.dependent << " (" << p.name () << ' '
+                          << c.constraint << "), so the failure can't be "
+                          << "postponed anymore";});
+
+            unsatisfied_depts.diag (*this);
+          }
+        }
+      }
+
       replaced_version rv (p.available, p.repository_fragment, p.system);
 
       if (vi != replaced_vers.end ())
@@ -1388,11 +1478,9 @@ namespace bpkg
         // satisfactory, then keep the preferred.
         //
         // If neither of the versions is satisfactory, then ignore those
-        // unsatisfied constraints which are imposed by the existing
-        // dependents which are not being up/downgraded and record them in
-        // unsatisfied_depts in the hope that the problem will resolve
-        // naturally (see unsatisfied_dependents for details). Fail if neither
-        // of the versions is still satisfactory after that.
+        // unsatisfied constraints which prevent us from picking the package
+        // version which is currently in the map (this way we try to avoid the
+        // postponed failure; see replace_ver() lambda for details).
         //
         if (p1->available_version () != p2->available_version ())
         {
@@ -1417,9 +1505,8 @@ namespace bpkg
             return nullptr;
           };
 
-          // Iterate until some of the version becomes satisfactory due to
-          // ignoring some of the constraints or until there is nothing more
-          // to ignore, in which case we fail.
+          // Iterate until one of the versions becomes satisfactory due to
+          // ignoring some of the constraints.
           //
           for (;;)
           {
@@ -1431,79 +1518,63 @@ namespace bpkg
               //
               if (auto c1 = test (p2, p1))
               {
-                // Return true if a constraint is imposed by a being
-                // reconfigured (not being up/downgraded) dependent.
-                //
-                auto existing_dependent = [this] (const constraint_type& c)
-                {
-                  const build_package* p (dependent_build (c));
-                  return p != nullptr           &&
-                         p->selected != nullptr &&
-                         (p->available == nullptr ||
-                          p->selected->version.compare (
-                            p->available_version ()) == 0);
-                };
-
                 // Add a constraint to the igrore-list and the dependent to
                 // the unsatisfied-list.
                 //
-                auto ignore_constraint = [&ics,
-                                          &bp,
-                                          &unsatisfied_depts,
-                                          this] (const constraint_type& c)
+                const constraint_type* c (p1 == &bp ? c2 : c1);
+                const build_package* p (dependent_build (*c));
+
+                // Note that if one of the constraints is imposed on the
+                // package by the command line, then another constraint must
+                // be imposed by a dependent. Also, in this case it feels that
+                // the map must contain the dependency constrained by the
+                // command line and so p may not be NULL. If that (suddenly)
+                // is not the case, then we will have to ignore the constraint
+                // imposed by the dependent which is not in the map, replace
+                // the version, and as a result fail immediately (see
+                // replace_ver() lambda for details).
+                //
+                if (p == nullptr)
                 {
-                  ics.push_back (&c);
+                  c = (c == c1 ? c2 : c1);
+                  p = dependent_build (*c);
 
-                  const build_package* p (dependent_build (c));
+                  // One of the dependents must be a real package.
+                  //
                   assert (p != nullptr);
-
-                  unsatisfied_depts.add (package_key (p->db, p->name ()),
-                                         &bp,
-                                         c.value);
-                };
-
-                if (existing_dependent (*c2))
-                {
-                  ignore_constraint (*c2);
-                  continue;
-                }
-
-                if (existing_dependent (*c1))
-                {
-                  ignore_constraint (*c1);
-                  swap (p1, p2);
-                  continue;
                 }
 
-                const package_name& n (i->first.name);
+                ics.push_back (c);
 
-                //             "  info: ..."
-                string indent ("          ");
+                package_key d (p->db, p->name ());
 
-                diag_record dr (fail);
-                dr << "unable to satisfy constraints on package " << n <<
-                  info << c1->dependent << c1->db << " depends on (" << n
-                       << ' ' << c1->value << ")";
+                l5 ([&]{trace << "postpone failure for dependent " << d
+                              << " unsatisfied with dependency "
+                              << bp.available_name_version_db () << " ("
+                              << c->value << ')';});
 
-                if (const build_package* d = dependent_build (*c1))
-                {
-                  set<package_key> printed;
-                  print_constraints (dr, *d, indent, printed);
-                }
+                // Note that in contrast to collect_order_dependents(), here
+                // we also save both unsatisfied constraints and the
+                // dependency chain, for the sake of the diagnostics.
+                //
+                vector<unsatisfied_constraint> ucs {
+                  unsatisfied_constraint {
+                    *c1, p1->available_version (), p1->system},
+                  unsatisfied_constraint {
+                    *c2, p2->available_version (), p2->system}};
 
-                dr << info << c2->dependent << c2->db << " depends on (" << n
-                           << ' ' << c2->value << ")";
+                vector<package_key> dc;
 
-                if (const build_package* d = dependent_build (*c2))
+                if (dep_chain != nullptr)
                 {
-                  set<package_key> printed;
-                  print_constraints (dr, *d, indent, printed);
+                  dc.reserve (dep_chain->size ());
+
+                  for (const build_package& p: *dep_chain)
+                    dc.emplace_back (p.db, p.name ());
                 }
 
-                dr << info << "available " << p1->available_name_version () <<
-                      info << "available " << p2->available_name_version () <<
-                      info << "explicitly specify " << n << " version to "
-                           << "manually satisfy both constraints";
+                unsatisfied_depts.add (d, pk, c->value, move (ucs), move (dc));
+                continue;
               }
               else
                 swap (p1, p2);
@@ -1520,7 +1591,7 @@ namespace bpkg
         // collect its prerequisites since that should have already been done.
         // Remember, p1 points to the object we want to keep.
         //
-        bool replace (p1 != &i->second.package);
+        bool replace (p1 != &bp);
 
         if (replace)
         {
@@ -3371,7 +3442,7 @@ namespace bpkg
                              replaced_vers,
                              postponed_cfgs,
                              unsatisfied_depts,
-                             nullptr /* dep_chain */,
+                             &dep_chain,
                              nullptr /* fdb */,
                              nullptr /* apc */,
                              nullptr /* rpt_depts */,
@@ -5304,7 +5375,8 @@ namespace bpkg
             replaced_vers_ (replaced_vers),
             postponed_edeps_ (postponed_edeps),
             postponed_deps_ (postponed_deps),
-            postponed_cfgs_ (postponed_cfgs)
+            postponed_cfgs_ (postponed_cfgs),
+            unsatisfied_depts_ (unsatisfied_depts)
       {
         auto save = [] (vector<package_key>& d, const postponed_packages& s)
         {
@@ -5317,21 +5389,6 @@ namespace bpkg
         save (postponed_repo_, postponed_repo);
         save (postponed_alts_, postponed_alts);
         save (postponed_recs_, postponed_recs);
-
-        unsatisfied_depts_.reserve (unsatisfied_depts.size ());
-
-        for (const auto& ud: unsatisfied_depts)
-        {
-          vector<pair<package_key, version_constraint>> deps;
-          deps.reserve (ud.dependencies.size());
-
-          for (const auto& d: ud.dependencies)
-            deps.emplace_back (package_key (d.first->db, d.first->name ()),
-                               d.second);
-
-          unsatisfied_depts_.push_back (
-            unsatisfied_dependent {ud.dependent, move (deps)});
-        }
       }
 
       void
@@ -5368,24 +5425,7 @@ namespace bpkg
         restore (postponed_alts, postponed_alts_);
         restore (postponed_recs, postponed_recs_);
 
-        unsatisfied_depts.clear ();
-        unsatisfied_depts.reserve (unsatisfied_depts_.size ());
-
-        for (auto& ud: unsatisfied_depts_)
-        {
-          vector<pair<const build_package*, version_constraint>> deps;
-          deps.reserve (ud.dependencies.size());
-
-          for (const auto& d: ud.dependencies)
-          {
-            build_package* b (pkgs.entered_build (d.first));
-            assert (b != nullptr);
-            deps.emplace_back (b, move (d.second));
-          }
-
-          unsatisfied_depts.push_back (
-            bpkg::unsatisfied_dependent {move (ud.dependent), move (deps)});
-        }
+        unsatisfied_depts = move (unsatisfied_depts_);
       }
 
     private:
@@ -5401,13 +5441,7 @@ namespace bpkg
       postponed_existing_dependencies postponed_edeps_;
       postponed_dependencies          postponed_deps_;
       postponed_configurations        postponed_cfgs_;
-
-      struct unsatisfied_dependent
-      {
-        package_key dependent;
-        vector<pair<package_key, version_constraint>> dependencies;
-      };
-      vector<unsatisfied_dependent> unsatisfied_depts_;
+      unsatisfied_dependents          unsatisfied_depts_;
     };
 
     size_t depth (pcfg != nullptr ? pcfg->depth : 0);
@@ -7004,7 +7038,16 @@ namespace bpkg
           // up/downgraded, etc; see unsatisfied_dependents for details).
           //
           if (!satisfies (av, c))
-            unsatisfied_depts.add (package_key (ddb, dn), &p, c);
+          {
+            package_key d (ddb, dn);
+
+            l5 ([&]{trace << "postpone failure for existing dependent " << d
+                          << " unsatisfied with dependency "
+                          << p.available_name_version_db () << " ("
+                          << c << ')';});
+
+            unsatisfied_depts.add (d, package_key (p.db, p.name ()), c);
+          }
         }
 
         auto adjustment = [&dn, &ddb, &n, &pdb] () -> build_package
diff --git a/bpkg/pkg-build-collect.hxx b/bpkg/pkg-build-collect.hxx
index 5362c65..5a5a397 100644
--- a/bpkg/pkg-build-collect.hxx
+++ b/bpkg/pkg-build-collect.hxx
@@ -702,40 +702,81 @@ namespace bpkg
     cancel_bogus (tracer&, bool scratch);
   };
 
-  // Existing dependents with their up/downgraded dependencies which don't
-  // satisfy the version constraints.
+
+  // Dependents with their unsatisfactory dependencies and the respective
+  // ignored constraints.
+  //
+  // Note that during the collecting of all the explicitly specified packages
+  // and their dependencies for the build, we may discover that a being
+  // up/downgraded dependency doesn't satisfy all the being reconfigured,
+  // up/downgraded, or newly built dependents. Rather than fail immediately in
+  // such a case, we postpone the failure, add the unsatisfied dependents and
+  // their respective constraints to the unsatisfied dependents list, and
+  // continue the collection/ordering in the hope that these problems will be
+  // resolved naturally as a result of the requested recollection from scratch
+  // or execution plan refinement (dependents will also be up/downgraded or
+  // dropped, dependencies will be up/downgraded to a different versions,
+  // etc).
   //
-  // Note that after collecting/ordering of all the explicitly specified
+  // Also note that after collecting/ordering of all the explicitly specified
   // packages and their dependencies for the build we also collect/order their
   // existing dependents for reconfiguration, recursively. It may happen that
   // some of the up/downgraded dependencies don't satisfy the version
   // constraints which some of the existing dependents impose on them. Rather
-  // than fail immediately in such a case, we postpone the failure in the hope
-  // that these problems will be resolved naturally as a result of the
-  // execution plan refinement (dependents will also be up/downgraded or
-  // dropped, dependencies will be up/downgraded to a different versions,
-  // etc).
+  // than fail immediately in such a case, we postpone the failure, add this
+  // dependent and the unsatisfactory dependency to the unsatisfied dependents
+  // list, and continue the collection/ordering in the hope that these
+  // problems will be resolved naturally as a result of the execution plan
+  // refinement.
   //
-  // Also note that we may discover that a being up/downgraded dependency
-  // doesn't satisfy an existing dependent which we re-collect recursively for
-  // some reason (configuration variables are specified, etc). In this case we
-  // may also postpone the failure in the hope that the problem will resolve
-  // naturally as it is described above (see collect_build() implementation
-  // for details).
-  //
-  // Specifically, we cache such unsatisfied constraints, pretend that the
-  // dependents don't impose them and proceed with the remaining
+  // Specifically, we cache such unsatisfied dependents/constraints, pretend
+  // that the dependents don't impose them and proceed with the remaining
   // collecting/ordering, simulating the plan execution, and evaluating the
-  // dependency versions. After that we check if the execution plan is
-  // finalized or a further refinement is required. In the former case we
-  // report the first encountered unsatisfied dependency constraint and
+  // dependency versions. After that, if scratch_collection exception has not
+  // been thrown, we check if the execution plan is finalized or a further
+  // refinement is required. In the former case we report the first
+  // encountered unsatisfied (and ignored) dependency constraint and
   // fail. Otherwise, we drop the cache and proceed with the next iteration of
   // the execution plan refinement which may resolve these problems naturally.
   //
+  struct unsatisfied_constraint
+  {
+    // Note: also contains the unsatisfied dependent information.
+    //
+    build_package::constraint_type constraint;
+
+    // Available package version which satisfies the above constraint.
+    //
+    version available_version;
+    bool    available_system;
+  };
+
+  struct ignored_constraint
+  {
+    package_key dependency;
+    version_constraint constraint;
+
+    // Only specified when the failure is postponed during the collection of
+    // the explicitly specified packages and their dependencies. Only used to
+    // properly reproduce the postponed failure diagnostics.
+    //
+    vector<unsatisfied_constraint> unsatisfied_constraints;
+    vector<package_key> dependency_chain;
+
+    ignored_constraint (const package_key& d,
+                        const version_constraint& c,
+                        vector<unsatisfied_constraint>&& ucs = {},
+                        vector<package_key>&& dc = {})
+        : dependency (d),
+          constraint (c),
+          unsatisfied_constraints (move (ucs)),
+          dependency_chain (move (dc)) {}
+  };
+
   struct unsatisfied_dependent
   {
     package_key dependent;
-    vector<pair<const build_package*, version_constraint>> dependencies;
+    vector<ignored_constraint> ignored_constraints;
   };
 
   struct build_packages;
@@ -743,20 +784,23 @@ namespace bpkg
   class unsatisfied_dependents: public vector<unsatisfied_dependent>
   {
   public:
-    // Add a dependent together with the unsatisfied dependency constraint.
+    // Add a dependent together with the ignored dependency constraint and,
+    // potentially, with the unsatisfied constraints and the dependency chain.
     //
     void
     add (const package_key& dependent,
-         const build_package* dependency,
-         const version_constraint&);
+         const package_key& dependency,
+         const version_constraint&,
+         vector<unsatisfied_constraint>&& ucs = {},
+         vector<package_key>&& dc = {});
 
     // Try to find the dependent entry and return NULL if not found.
     //
     unsatisfied_dependent*
     find_dependent (const package_key&);
 
-    // Issue the diagnostics for the first unsatisfied dependency constraint
-    // and throw failed.
+    // Issue the diagnostics for the first unsatisfied (and ignored)
+    // dependency constraint and throw failed.
     //
     [[noreturn]] void
     diag (const build_packages&);
@@ -1169,7 +1213,7 @@ namespace bpkg
     // the package version needs to be replaced but in-place replacement is
     // not possible (see replaced_versions for details).
     //
-    // Also, in the recursive mode (dep_chain is not NULL):
+    // Also, in the recursive mode (find database function is not NULL):
     //
     // - Use the custom search function to find the package dependency
     //   databases.
@@ -1180,8 +1224,10 @@ namespace bpkg
     // - Call add_priv_cfg_function callback for the created private
     //   configurations.
     //
-    // Note that postponed_* and dep_chain arguments must all be either
-    // specified or not.
+    // Note that postponed_* arguments must all be either specified or not.
+    // The dep_chain argument can be specified in the non-recursive mode (for
+    // the sake of the diagnostics) and must be specified in the recursive
+    // mode.
     //
     struct replace_version: scratch_collection
     {
diff --git a/bpkg/pkg-build.cxx b/bpkg/pkg-build.cxx
index 1aaed99..588d29c 100644
--- a/bpkg/pkg-build.cxx
+++ b/bpkg/pkg-build.cxx
@@ -6413,6 +6413,7 @@ namespace bpkg
         //
         auto unconstrain_deps = [simulate,
                                  &p,
+                                 &trace,
                                  deps = vector<package_key> ()] () mutable
         {
           if (simulate)
@@ -6424,12 +6425,16 @@ namespace bpkg
             {
               assert (deps.empty ());
 
-              deps.reserve (ud->dependencies.size ());
+              deps.reserve (ud->ignored_constraints.size ());
 
-              for (const auto& d: ud->dependencies)
+              for (const auto& c: ud->ignored_constraints)
               {
-                const build_package& p (*d.first);
-                deps.emplace_back (p.db, p.name ());
+                l5 ([&]{trace << "while configuring dependent "
+                              << p.available_name_version_db ()
+                              << " in simulation mode unconstrain ("
+                              << c.dependency << ' ' << c.constraint << ')';});
+
+                deps.emplace_back (c.dependency);
               }
             }
           }
diff --git a/tests/common/dependency-alternatives/t11a/libfoo-1.0.0.tar.gz b/tests/common/dependency-alternatives/t11a/libfoo-1.0.0.tar.gz
index da545c1..e46178a 100644
--- a/tests/common/dependency-alternatives/t11a/libfoo-1.0.0.tar.gz
+++ b/tests/common/dependency-alternatives/t11a/libfoo-1.0.0.tar.gz
diff --git a/tests/common/dependency-alternatives/t11a/tvz-0.1.0.tar.gz b/tests/common/dependency-alternatives/t11a/tvz-0.1.0.tar.gz
new file mode 100644
index 0000000..55ef173
--- /dev/null
+++ b/tests/common/dependency-alternatives/t11a/tvz-0.1.0.tar.gz
diff --git a/tests/pkg-build.testscript b/tests/pkg-build.testscript
index d3f949d..0efe4ff 100644
--- a/tests/pkg-build.testscript
+++ b/tests/pkg-build.testscript
@@ -284,6 +284,9 @@
 # |   |-- tuz-1.0.0.tar.gz      -> toz {require {config.toz.extras=true}}
 # |   |-- tux-1.0.0.tar.gz      -> libbox {require {config.libbox.extras=true}},
 # |   |                            tix == 0.1.0
+# |   |-- tvz-0.1.0.tar.gz      -> toz == 0.2.0 {require {config.toz.extras=true}},
+# |   |                            bax,
+# |   |                            libfoo {require {config.libfoo.network=true}}
 # |   |-- tvz-1.0.0.tar.gz      -> toz == 0.2.0 {require {config.toz.extras=true}}
 # |   |-- dex-1.0.0.tar.gz      -> bar {require {config.bar.extras=true}},
 # |   |                            libfoo {require {config.libfoo.extras=true}}
@@ -1183,38 +1186,146 @@ test.arguments += --sys-no-query
       new libbaz/1.1.0
       EOO
 
-    : unable-satisfy
+    : libbaz-unable-satisfy
     :
-    $clone_cfg;
-    $* libfoo/1.0.0 libbaz 2>>EOE != 0
-      error: unable to satisfy constraints on package libfoo
-        info: command line depends on (libfoo == 1.0.0)
-        info: libbar depends on (libfoo == 1.1.0)
-        info: available libfoo/1.0.0
-        info: available libfoo/1.1.0
-        info: explicitly specify libfoo version to manually satisfy both constraints
-      info: while satisfying libbar/1.1.0
-      info: while satisfying libbaz/1.1.0
-      EOE
+    {
+      +$clone_cfg
 
-    : unable-satisfy-config
-    :
-    : As above but with a linked configuration.
+      : basic
+      :
+      {
+        $clone_cfg;
+
+        $* libfoo/1.0.0 libbaz 2>>EOE != 0
+        error: unable to satisfy constraints on package libfoo
+          info: command line depends on (libfoo == 1.0.0)
+          info: libbar/1.1.0 depends on (libfoo == 1.1.0)
+          info: available libfoo/1.0.0
+          info: available libfoo/1.1.0
+          info: while satisfying libbar/1.1.0
+          info: while satisfying libbaz/1.1.0
+          info: explicitly specify libfoo version to manually satisfy both constraints
+        EOE
+      }
+
+      : unable-satisfy-reorder
+      :
+      : As above but the packages are specified in a different order on the
+      : command line.
+      :
+      {
+        $clone_cfg;
+
+        $* libbaz libfoo/1.0.0 2>>EOE != 0
+          error: unable to satisfy constraints on package libfoo
+            info: command line depends on (libfoo == 1.0.0)
+            info: libbar/1.1.0 depends on (libfoo == 1.1.0)
+            info: available libfoo/1.0.0
+            info: available libfoo/1.1.0
+            info: while satisfying libbar/1.1.0
+            info: while satisfying libbaz/1.1.0
+            info: explicitly specify libfoo version to manually satisfy both constraints
+          EOE
+      }
+
+      : unable-satisfy-dependency
+      :
+      : As above but specify libfoo as a dependency.
+      :
+      {
+        $clone_cfg;
+
+        $* libbaz ?libfoo/1.0.0 2>>EOE != 0
+          error: unable to satisfy constraints on package libfoo
+            info: libbaz/1.1.0 depends on (libfoo == 1.0.0)
+            info: libbar/1.1.0 depends on (libfoo == 1.1.0)
+            info: available libfoo/1.0.0
+            info: available libfoo/1.1.0
+            info: while satisfying libbar/1.1.0
+            info: while satisfying libbaz/1.1.0
+            info: explicitly specify libfoo version to manually satisfy both constraints
+          EOE
+      }
+
+      : unable-satisfy-config
+      :
+      : As above but with a linked configuration.
+      :
+      {
+        $clone_cfg;
+        $cfg_create -d cfg2 &cfg2/***;
+        $cfg_link -d cfg cfg2;
+        $rep_add -d cfg2 $rep/t4c && $rep_fetch -d cfg2;
+        $* libbaz ?libbar +{ --config-id 1 } libfoo/1.0.0 +{ --config-id 1 } 2>>~%EOE% != 0
+          error: unable to satisfy constraints on package libfoo
+            info: command line depends on (libfoo == 1.0.0)
+          %  info: libbar/1.1.0 \[cfg2.\] depends on \(libfoo == 1.1.0\)%
+            info: available libfoo/1.0.0
+            info: available libfoo/1.1.0
+          %  info: while satisfying libbar/1.1.0 \[cfg2.\]%
+            info: while satisfying libbaz/1.1.0
+            info: explicitly specify libfoo version to manually satisfy both constraints
+          EOE
+      }
+    }
+
+    : libbar-unable-satisfy
     :
-    $clone_cfg;
-    $cfg_create -d cfg2 &cfg2/***;
-    $cfg_link -d cfg cfg2;
-    $rep_add -d cfg2 $rep/t4c && $rep_fetch -d cfg2;
-    $* libbaz ?libbar +{ --config-id 1 } libfoo/1.0.0 +{ --config-id 1 } 2>>~%EOE% != 0
-      error: unable to satisfy constraints on package libfoo
-        info: command line depends on (libfoo == 1.0.0)
-      %  info: libbar \[cfg2.\] depends on \(libfoo == 1.1.0\)%
-        info: available libfoo/1.0.0
-        info: available libfoo/1.1.0
-        info: explicitly specify libfoo version to manually satisfy both constraints
-      %info: while satisfying libbar/1.1.0 \[cfg2.\]%
-      info: while satisfying libbaz/1.1.0
-      EOE
+    {
+      +$clone_cfg
+      +$rep_add $rep/t4b && $rep_fetch
+
+      : basic
+      :
+      {
+        $clone_cfg;
+
+        $* libfoo/1.0.0 libbar 2>>EOE != 0
+        error: unable to satisfy constraints on package libfoo
+          info: command line depends on (libfoo == 1.0.0)
+          info: libbar/1.1.0 depends on (libfoo == 1.1.0)
+          info: available libfoo/1.0.0
+          info: available libfoo/1.1.0
+          info: while satisfying libbar/1.1.0
+          info: explicitly specify libfoo version to manually satisfy both constraints
+        EOE
+      }
+
+      : unable-satisfy-reorder
+      :
+      : As above but the packages are specified in a different order on the
+      : command line.
+      :
+      {
+        $clone_cfg;
+
+        $* libbar libfoo/1.0.0 2>>EOE != 0
+          error: unable to satisfy constraints on package libfoo
+            info: command line depends on (libfoo == 1.0.0)
+            info: libbar/1.1.0 depends on (libfoo == 1.1.0)
+            info: available libfoo/1.0.0
+            info: available libfoo/1.1.0
+            info: while satisfying libbar/1.1.0
+            info: explicitly specify libfoo version to manually satisfy both constraints
+          EOE
+      }
+
+      : unable-satisfy-dependency
+      :
+      : As above but specify libfoo as a dependency.
+      :
+      {
+        $clone_cfg;
+
+        $* libbar ?libfoo/1.0.0 2>>EOE != 0
+          error: unable to satisfy constraints on package libfoo
+            info: libbar depends on (libfoo == 1.1.0)
+            info: command line depends on (libfoo == 1.0.0)
+            info: specify libfoo version to satisfy libbar constraint
+          info: while satisfying libbar/1.1.0
+          EOE
+      }
+    }
 
     : not-available
     :
@@ -2057,9 +2168,9 @@ test.arguments += --sys-no-query
             info: libbox depends on (libbar ^1.0.0)
             info: available libbar/2.0.0
             info: available libbar/1.0.0
+            info: while satisfying libbox/1.0.0
+            info: while satisfying foo/1.0.0
             info: explicitly specify libbar version to manually satisfy both constraints
-          info: while satisfying libbox/1.0.0
-          info: while satisfying foo/1.0.0
           EOE
 
         $* foo ?libbaz/1.0.0 2>!;
@@ -2152,9 +2263,9 @@ test.arguments += --sys-no-query
             info: libbox depends on (libbar ^1.0.0)
             info: available libbar/2.0.0
             info: available libbar/1.0.0
+            info: while satisfying libbox/1.1.0
+            info: while satisfying foo/1.0.0
             info: explicitly specify libbar version to manually satisfy both constraints
-          info: while satisfying libbox/1.1.0
-          info: while satisfying foo/1.0.0
           EOE
 
         $* foo ?libbaz/1.0.0 2>!;
@@ -2234,8 +2345,8 @@ test.arguments += --sys-no-query
             info: libbox depends on (libbar ^1.0.0)
             info: available libbar/2.0.0
             info: available libbar/1.0.0
+            info: while satisfying libbox/1.1.0
             info: explicitly specify libbar version to manually satisfy both constraints
-          info: while satisfying libbox/1.1.0
           EOE
 
         $* ?libbox 2>!;
@@ -2726,8 +2837,8 @@ test.arguments += --sys-no-query
             info: libbiz depends on (libbaz <= 0.0.3)
             info: available libbaz/0.1.0
             info: available libbaz/0.0.2
+            info: while satisfying libbiz/0.0.2
             info: explicitly specify libbaz version to manually satisfy both constraints
-          info: while satisfying libbiz/0.0.2
           EOE
 
         $* libfix libbiz ?libbaz/0.0.3 2>>EOE;
@@ -2800,9 +2911,9 @@ test.arguments += --sys-no-query
             info: libfox depends on (libfoo == 0.0.1)
             info: available libfoo/1.0.0
             info: available libfoo/0.0.1
+            info: while satisfying libbox/0.0.2
+            info: while satisfying libbiz/0.0.1
             info: explicitly specify libfoo version to manually satisfy both constraints
-          info: while satisfying libbox/0.0.2
-          info: while satisfying libbiz/0.0.1
           EOE
 
         $* libbiz ?libfox/0.0.1 2>>EOE;
@@ -4332,15 +4443,16 @@ test.arguments += --sys-no-query
 
             # While at it, test the reused-only alternative selection mode.
             # Also test the postponement of the 'unable to satisfy constraints
-            # on package' failure which ends up with the 'unable to downgrade
-            # package' failure.
+            # on package' failure.
             #
             $* box +{ config.box.extras=true } libbox/0.1.0 2>>EOE != 0;
-              error: unable to downgrade package libbox/1.0.0 to 0.1.0
-                info: because package box depends on (libbox >= 0.1.1)
-                info: consider re-trying with --upgrade|-u potentially combined with --recursive|-r
-                info: or explicitly request up/downgrade of package box
-                info: or explicitly specify package libbox version to manually satisfy these constraints
+              error: unable to satisfy constraints on package libbox
+                info: command line depends on (libbox == 0.1.0)
+                info: box depends on (libbox >= 0.1.1)
+                info: available libbox/0.1.0
+                info: available libbox/1.0.0
+                info: while satisfying box/1.0.0
+                info: explicitly specify libbox version to manually satisfy both constraints
               EOE
 
             $* box +{ config.box.extras=true } libbox/0.1.0 ?libbiz 2>>~%EOE%;
@@ -9425,6 +9537,322 @@ test.arguments += --sys-no-query
 
         $pkg_drop tax toz tez
       }
+
+      : unconstrain-deps
+      :
+      {
+        $clone_cfg;
+
+        #  Dependencies:
+        #
+        #  tvz: depends: toz == 0.2.0 (c)
+        #
+        #  toz/0.2.0: depends: libfoo(c)
+        #             depends: libbar(c)
+        #
+        #  tez: depends: libbox(c)
+        #       depends: toz == 0.1.0 (c)
+        #       depends: libbar(c)
+        #
+        $* tvz 2>!;
+
+        $pkg_status -r >>EOO;
+          !tvz configured 1.0.0
+            toz configured 0.2.0 available 1.0.0
+              libbar configured 1.0.0
+              libfoo configured 1.0.0
+          EOO
+
+        $* tvz +{ config.tvz.extras=true } tez 2>&1 != 0 | $filter 2>>~%EOE%;
+          %.*
+          trace: pkg_build: refine package collection/plan execution from scratch
+          trace: collect_build: add tvz/1.0.0
+          trace: collect_build: add tez/1.0.0
+          trace: collect_build_prerequisites: begin tvz/1.0.0
+          trace: collect_build: add toz/0.2.0
+          trace: collect_build_prerequisites: cfg-postpone dependency toz/0.2.0 of dependent tvz/1.0.0
+          trace: postponed_configurations::add: create {tvz | toz->{tvz/1,1}}
+          trace: collect_build_prerequisites: postpone tvz/1.0.0
+          trace: collect_build_prerequisites: begin tez/1.0.0
+          trace: collect_build: add libbox/1.0.0
+          trace: collect_build_prerequisites: cfg-postpone dependency libbox/1.0.0 of dependent tez/1.0.0
+          trace: postponed_configurations::add: create {tez | libbox->{tez/1,1}}
+          trace: collect_build_prerequisites: postpone tez/1.0.0
+          trace: collect_build_postponed (0): begin
+          trace: collect_build_postponed (1): begin {tvz | toz->{tvz/1,1}}
+          trace: collect_build_postponed (1): skip being built existing dependent tvz of dependency toz
+          trace: collect_build_postponed (1): cfg-negotiate begin {tvz | toz->{tvz/1,1}}
+          trace: collect_build_postponed (1): recursively collect cfg-negotiated dependencies
+          trace: collect_build_prerequisites: begin toz/0.2.0
+          trace: collect_build: add libfoo/1.0.0
+          trace: collect_build_prerequisites: cfg-postpone dependency libfoo/1.0.0 of dependent toz/0.2.0
+          trace: postponed_configurations::add: create {toz | libfoo->{toz/1,1}}
+          trace: collect_build_prerequisites: postpone toz/0.2.0
+          trace: collect_build_postponed (1): recursively collect cfg-negotiated dependents
+          trace: collect_build_postponed (1): select cfg-negotiated dependency alternative for dependent tvz/1.0.0
+          trace: collect_build_prerequisites: resume tvz/1.0.0
+          trace: collect_build_prerequisites: end tvz/1.0.0
+          trace: collect_build_postponed (1): cfg-negotiate end {tvz | toz->{tvz/1,1}}!
+          trace: collect_build_postponed (2): begin {tez | libbox->{tez/1,1}}
+          trace: collect_build_postponed (2): cfg-negotiate begin {tez | libbox->{tez/1,1}}
+          trace: collect_build_postponed (2): recursively collect cfg-negotiated dependencies
+          trace: collect_build_prerequisites: begin libbox/1.0.0
+          trace: collect_build_prerequisites: end libbox/1.0.0
+          trace: collect_build_postponed (2): recursively collect cfg-negotiated dependents
+          trace: collect_build_postponed (2): select cfg-negotiated dependency alternative for dependent tez/1.0.0
+          trace: collect_build_prerequisites: resume tez/1.0.0
+          trace: collect_build: postpone failure for dependent tez unsatisfied with dependency toz/0.2.0 (== 0.1.0)
+          trace: collect_build: pick toz/0.2.0 over toz/0.1.0
+          trace: collect_build_prerequisites: cfg-postpone dependency toz/0.2.0 of dependent tez/1.0.0
+          trace: postponed_configurations::add: add {tez 2,1: toz} to {tvz | toz->{tvz/1,1}}!
+          trace: collect_build_prerequisites: configuration for cfg-postponed dependencies of dependent tez/1.0.0 is negotiated
+          trace: collect_build_prerequisites: dependency toz/0.2.0 of dependent tez/1.0.0 is already (being) recursively collected, skipping
+          trace: collect_build: add libbar/1.0.0
+          trace: collect_build_prerequisites: cfg-postpone dependency libbar/1.0.0 of dependent tez/1.0.0
+          trace: postponed_configurations::add: create {tez | libbar->{tez/3,1}}
+          trace: collect_build_prerequisites: postpone tez/1.0.0
+          trace: collect_build_postponed (2): cfg-negotiate end {tez | libbox->{tez/1,1}}!
+          trace: collect_build_postponed (3): begin {toz | libfoo->{toz/1,1}}
+          trace: collect_build_postponed (3): skip being built existing dependent toz of dependency libfoo
+          trace: collect_build_postponed (3): cfg-negotiate begin {toz | libfoo->{toz/1,1}}
+          trace: collect_build_postponed (3): recursively collect cfg-negotiated dependencies
+          trace: collect_build_prerequisites: skip configured libfoo/1.0.0
+          trace: collect_build_postponed (3): recursively collect cfg-negotiated dependents
+          trace: collect_build_postponed (3): select cfg-negotiated dependency alternative for dependent toz/0.2.0
+          trace: collect_build_prerequisites: resume toz/0.2.0
+          trace: collect_build_prerequisites: cfg-postpone dependency libbar/1.0.0 of dependent toz/0.2.0
+          trace: postponed_configurations::add: add {toz 2,1: libbar} to {tez | libbar->{tez/3,1}}
+          trace: collect_build_prerequisites: postpone toz/0.2.0
+          trace: collect_build_postponed (3): cfg-negotiate end {toz | libfoo->{toz/1,1}}!
+          trace: collect_build_postponed (4): begin {tez toz | libbar->{tez/3,1 toz/2,1}}
+          trace: collect_build_postponed (4): skip being built existing dependent toz of dependency libbar
+          trace: collect_build_postponed (4): cfg-negotiate begin {tez toz | libbar->{tez/3,1 toz/2,1}}
+          trace: collect_build_postponed (4): recursively collect cfg-negotiated dependencies
+          trace: collect_build_prerequisites: skip configured libbar/1.0.0
+          trace: collect_build_postponed (4): recursively collect cfg-negotiated dependents
+          trace: collect_build_postponed (4): select cfg-negotiated dependency alternative for dependent tez/1.0.0
+          trace: collect_build_prerequisites: resume tez/1.0.0
+          trace: collect_build_prerequisites: end tez/1.0.0
+          trace: collect_build_postponed (4): select cfg-negotiated dependency alternative for dependent toz/0.2.0
+          trace: collect_build_prerequisites: resume toz/0.2.0
+          trace: collect_build_prerequisites: end toz/0.2.0
+          trace: collect_build_postponed (4): cfg-negotiate end {tez toz | libbar->{tez/3,1 toz/2,1}}!
+          trace: collect_build_postponed (4): end {tez toz | libbar->{tez/3,1 toz/2,1}}
+          trace: collect_build_postponed (3): end {toz | libfoo->{toz/1,1}}
+          trace: collect_build_postponed (2): end {tez | libbox->{tez/1,1}}
+          trace: collect_build_postponed (1): end {tvz | toz->{tvz/1,1}}
+          trace: collect_build_postponed (0): end
+          trace: execute_plan: simulate: yes
+          %.*
+          trace: execute_plan: while configuring dependent tez/1.0.0 in simulation mode unconstrain (toz == 0.1.0)
+          %.*
+          error: unable to satisfy constraints on package toz
+            info: tvz depends on (toz == 0.2.0)
+            info: tez depends on (toz == 0.1.0)
+            info: available toz/0.2.0
+            info: available toz/0.1.0
+            info: while satisfying tez/1.0.0
+            info: explicitly specify toz version to manually satisfy both constraints
+          %.*
+          EOE
+
+        $pkg_drop tvz
+      }
+
+      : restore-unsatisfied-depts
+      :
+      {
+        $clone_cfg;
+
+        #  Dependencies:
+        #
+        #  tez: depends: libbox(c)
+        #       depends: toz == 0.1.0 (c)
+        #       depends: libbar(c)
+        #
+        #  toz/0.1.0:
+        #
+        #  toz/0.2.0: depends: libfoo(c)
+        #             depends: libbar(c)
+        #
+        #  tvz: depends: toz == 0.2.0 (c)
+        #       depends: bax
+        #       depends: libfoo(c)
+        #
+        #  bax: depends: libfoo(c)
+        #       depends: {libbox libbar} (c)
+        #
+        $* tez 2>!;
+
+        $pkg_status -r >>EOO;
+          !tez configured 1.0.0
+            libbar configured 1.0.0
+            libbox configured 1.0.0
+            toz configured 0.1.0 available 1.0.0 0.2.0
+          EOO
+
+        $* tvz/0.1.0 tez +{ config.tvz.extras=true } 2>&1 != 0 | $filter 2>>~%EOE%;
+          %.*
+          trace: pkg_build: refine package collection/plan execution from scratch
+          trace: collect_build: add tvz/0.1.0
+          trace: collect_build: add tez/1.0.0
+          trace: collect_build_prerequisites: begin tvz/0.1.0
+          trace: collect_build: add toz/0.2.0
+          info: package tvz dependency on (toz == 0.2.0) is forcing upgrade of toz/0.1.0 to 0.2.0
+          trace: collect_build_prerequisites: cfg-postpone dependency toz/0.2.0 of dependent tvz/0.1.0
+          trace: postponed_configurations::add: create {tvz | toz->{tvz/1,1}}
+          trace: collect_build_prerequisites: postpone tvz/0.1.0
+          trace: collect_build_prerequisites: begin tez/1.0.0
+          trace: collect_build: add libbox/1.0.0
+          trace: collect_build_prerequisites: cfg-postpone dependency libbox/1.0.0 of dependent tez/1.0.0
+          trace: postponed_configurations::add: create {tez | libbox->{tez/1,1}}
+          trace: collect_build_prerequisites: postpone tez/1.0.0
+          trace: collect_build_postponed (0): begin
+          trace: collect_build_postponed (1): begin {tvz | toz->{tvz/1,1}}
+          trace: collect_build_postponed (1): skip being built existing dependent tez of dependency toz
+          trace: collect_build_postponed (1): cfg-negotiate begin {tvz | toz->{tvz/1,1}}
+          trace: collect_build_postponed (1): recursively collect cfg-negotiated dependencies
+          trace: collect_build_prerequisites: begin toz/0.2.0
+          trace: collect_build: add libfoo/1.0.0
+          trace: collect_build_prerequisites: cfg-postpone dependency libfoo/1.0.0 of dependent toz/0.2.0
+          trace: postponed_configurations::add: create {toz | libfoo->{toz/1,1}}
+          trace: collect_build_prerequisites: postpone toz/0.2.0
+          trace: collect_build_postponed (1): recursively collect cfg-negotiated dependents
+          trace: collect_build_postponed (1): select cfg-negotiated dependency alternative for dependent tvz/0.1.0
+          trace: collect_build_prerequisites: resume tvz/0.1.0
+          trace: collect_build: add bax/1.0.0
+          trace: collect_build_prerequisites: no cfg-clause for dependency bax/1.0.0 of dependent tvz/0.1.0
+          trace: collect_build_prerequisites: begin bax/1.0.0
+          trace: collect_build_prerequisites: cfg-postpone dependency libfoo/1.0.0 of dependent bax/1.0.0
+          trace: postponed_configurations::add: add {bax 1,1: libfoo} to {toz | libfoo->{toz/1,1}}
+          trace: collect_build_prerequisites: postpone bax/1.0.0
+          trace: collect_build_prerequisites: cfg-postpone dependency libfoo/1.0.0 of dependent tvz/0.1.0
+          trace: postponed_configurations::add: add {tvz 3,1: libfoo} to {bax toz | libfoo->{bax/1,1 toz/1,1}}
+          trace: collect_build_prerequisites: postpone tvz/0.1.0
+          trace: collect_build_postponed (1): cfg-negotiate end {tvz | toz->{tvz/1,1}}!
+          trace: collect_build_postponed (2): begin {tez | libbox->{tez/1,1}}
+          trace: collect_build_postponed (2): skip being built existing dependent tez of dependency libbox
+          trace: collect_build_postponed (2): cfg-negotiate begin {tez | libbox->{tez/1,1}}
+          trace: collect_build_postponed (2): recursively collect cfg-negotiated dependencies
+          trace: collect_build_prerequisites: skip configured libbox/1.0.0
+          trace: collect_build_postponed (2): recursively collect cfg-negotiated dependents
+          trace: collect_build_postponed (2): select cfg-negotiated dependency alternative for dependent tez/1.0.0
+          trace: collect_build_prerequisites: resume tez/1.0.0
+          trace: collect_build: postpone failure for dependent tez unsatisfied with dependency toz/0.2.0 (== 0.1.0)
+          trace: collect_build: pick toz/0.2.0 over toz/0.1.0
+          trace: collect_build_prerequisites: cfg-postpone dependency toz/0.2.0 of dependent tez/1.0.0
+          trace: postponed_configurations::add: add {tez 2,1: toz} to {tvz | toz->{tvz/1,1}}!
+          trace: collect_build_prerequisites: configuration for cfg-postponed dependencies of dependent tez/1.0.0 is negotiated
+          trace: collect_build_prerequisites: dependency toz/0.2.0 of dependent tez/1.0.0 is already (being) recursively collected, skipping
+          trace: collect_build: add libbar/1.0.0
+          trace: collect_build_prerequisites: cfg-postpone dependency libbar/1.0.0 of dependent tez/1.0.0
+          trace: postponed_configurations::add: create {tez | libbar->{tez/3,1}}
+          trace: collect_build_prerequisites: postpone tez/1.0.0
+          trace: collect_build_postponed (2): cfg-negotiate end {tez | libbox->{tez/1,1}}!
+          trace: collect_build_postponed (3): begin {bax toz tvz | libfoo->{bax/1,1 toz/1,1 tvz/3,1}}
+          trace: collect_build_postponed (3): cfg-negotiate begin {bax toz tvz | libfoo->{bax/1,1 toz/1,1 tvz/3,1}}
+          trace: collect_build_postponed (3): recursively collect cfg-negotiated dependencies
+          trace: collect_build_prerequisites: begin libfoo/1.0.0
+          trace: collect_build_prerequisites: end libfoo/1.0.0
+          trace: collect_build_postponed (3): recursively collect cfg-negotiated dependents
+          trace: collect_build_postponed (3): select cfg-negotiated dependency alternative for dependent bax/1.0.0
+          trace: collect_build_prerequisites: resume bax/1.0.0
+          trace: collect_build_prerequisites: cfg-postpone dependency libbox/1.0.0 of dependent bax/1.0.0
+          trace: collect_build_prerequisites: cfg-postpone dependency libbar/1.0.0 of dependent bax/1.0.0
+          trace: postponed_configurations::add: add {bax 2,1: libbox libbar} to {tez | libbox->{tez/1,1}}!
+          trace: postponed_configurations::add: merge {tez | libbar->{tez/3,1}} into {bax tez | libbox->{bax/2,1 tez/1,1} libbar->{bax/2,1}}!
+          trace: collect_build_prerequisites: cfg-postponing dependent bax/1.0.0 merges non-negotiated and/or being negotiated configurations in and results in {bax tez | libbox->{bax/2,1 tez/1,1} libbar->{bax/2,1 tez/3,1}}!, throwing merge_configuration
+          trace: collect_build_postponed (1): cfg-negotiation of {tez | libbox->{tez/1,1}} failed due to non-negotiated clusters, force-merging based on shadow cluster {bax tez | libbox->{bax/2,1 tez/1,1} libbar->{bax/2,1 tez/3,1}}!
+          trace: collect_build_postponed (2): begin {tez | libbox->{tez/1,1}}
+          trace: collect_build_postponed (2): skip being built existing dependent tez of dependency libbox
+          trace: collect_build_postponed (2): cfg-negotiate begin {tez | libbox->{tez/1,1}}
+          trace: collect_build_postponed (2): recursively collect cfg-negotiated dependencies
+          trace: collect_build_prerequisites: skip configured libbox/1.0.0
+          trace: collect_build_postponed (2): recursively collect cfg-negotiated dependents
+          trace: collect_build_postponed (2): select cfg-negotiated dependency alternative for dependent tez/1.0.0
+          trace: collect_build_prerequisites: resume tez/1.0.0
+          trace: collect_build: postpone failure for dependent tez unsatisfied with dependency toz/0.2.0 (== 0.1.0)
+          trace: collect_build: pick toz/0.2.0 over toz/0.1.0
+          trace: collect_build_prerequisites: cfg-postpone dependency toz/0.2.0 of dependent tez/1.0.0
+          trace: postponed_configurations::add: add {tez 2,1: toz} to {tvz | toz->{tvz/1,1}}!
+          trace: collect_build_prerequisites: configuration for cfg-postponed dependencies of dependent tez/1.0.0 is negotiated
+          trace: collect_build_prerequisites: dependency toz/0.2.0 of dependent tez/1.0.0 is already (being) recursively collected, skipping
+          trace: collect_build: add libbar/1.0.0
+          trace: collect_build_prerequisites: cfg-postpone dependency libbar/1.0.0 of dependent tez/1.0.0
+          trace: postponed_configurations::add: add {tez 3,1: libbar} to {tez | libbox->{tez/1,1}}? (shadow cluster-based)
+          trace: collect_build_prerequisites: skip being built existing dependent tez of dependency libbar
+          trace: collect_build_prerequisites: cfg-postponing dependent tez/1.0.0 involves (being) negotiated configurations and results in {tez | libbox->{tez/1,1} libbar->{tez/3,1}}?, throwing retry_configuration
+          trace: collect_build_postponed (1): cfg-negotiation of {tez | libbox->{tez/1,1}} failed due to dependent tez, refining configuration
+          trace: collect_build_postponed (2): begin {tez | libbox->{tez/1,1}}
+          trace: collect_build_postponed (2): skip being built existing dependent tez of dependency libbox
+          trace: collect_build_postponed (2): cfg-negotiate begin {tez | libbox->{tez/1,1}}
+          trace: collect_build_postponed (2): recursively collect cfg-negotiated dependencies
+          trace: collect_build_prerequisites: skip configured libbox/1.0.0
+          trace: collect_build_postponed (2): recursively collect cfg-negotiated dependents
+          trace: collect_build_postponed (2): select cfg-negotiated dependency alternative for dependent tez/1.0.0
+          trace: collect_build_prerequisites: resume tez/1.0.0
+          trace: collect_build: postpone failure for dependent tez unsatisfied with dependency toz/0.2.0 (== 0.1.0)
+          trace: collect_build: pick toz/0.2.0 over toz/0.1.0
+          trace: collect_build_prerequisites: cfg-postpone dependency toz/0.2.0 of dependent tez/1.0.0
+          trace: postponed_configurations::add: add {tez 2,1: toz} to {tvz | toz->{tvz/1,1}}!
+          trace: collect_build_prerequisites: configuration for cfg-postponed dependencies of dependent tez/1.0.0 is negotiated
+          trace: collect_build_prerequisites: dependency toz/0.2.0 of dependent tez/1.0.0 is already (being) recursively collected, skipping
+          trace: collect_build: add libbar/1.0.0
+          trace: collect_build_prerequisites: cfg-postpone dependency libbar/1.0.0 of dependent tez/1.0.0
+          trace: postponed_configurations::add: add {tez 3,1: libbar} to {tez | libbox->{tez/1,1}}? (shadow cluster-based)
+          trace: collect_build_prerequisites: skip being built existing dependent tez of dependency libbar
+          trace: collect_build_prerequisites: configuration for cfg-postponed dependencies of dependent tez/1.0.0 is shadow-negotiated
+          trace: collect_build_prerequisites: collecting cfg-postponed dependency libbar/1.0.0 of dependent tez/1.0.0
+          trace: collect_build_prerequisites: skip configured libbar/1.0.0
+          trace: collect_build_prerequisites: end tez/1.0.0
+          trace: collect_build_postponed (2): cfg-negotiate end {tez | libbox->{tez/1,1} libbar->{tez/3,1}}!
+          trace: collect_build_postponed (3): begin {bax toz tvz | libfoo->{bax/1,1 toz/1,1 tvz/3,1}}
+          trace: collect_build_postponed (3): cfg-negotiate begin {bax toz tvz | libfoo->{bax/1,1 toz/1,1 tvz/3,1}}
+          trace: collect_build_postponed (3): recursively collect cfg-negotiated dependencies
+          trace: collect_build_prerequisites: begin libfoo/1.0.0
+          trace: collect_build_prerequisites: end libfoo/1.0.0
+          trace: collect_build_postponed (3): recursively collect cfg-negotiated dependents
+          trace: collect_build_postponed (3): select cfg-negotiated dependency alternative for dependent bax/1.0.0
+          trace: collect_build_prerequisites: resume bax/1.0.0
+          trace: collect_build_prerequisites: cfg-postpone dependency libbox/1.0.0 of dependent bax/1.0.0
+          trace: collect_build_prerequisites: cfg-postpone dependency libbar/1.0.0 of dependent bax/1.0.0
+          trace: postponed_configurations::add: add {bax 2,1: libbox libbar} to {tez | libbox->{tez/1,1} libbar->{tez/3,1}}! (shadow cluster-based)
+          trace: collect_build_prerequisites: configuration for cfg-postponed dependencies of dependent bax/1.0.0 is shadow-negotiated
+          trace: collect_build_prerequisites: dependency libbox/1.0.0 of dependent bax/1.0.0 is already (being) recursively collected, skipping
+          trace: collect_build_prerequisites: dependency libbar/1.0.0 of dependent bax/1.0.0 is already (being) recursively collected, skipping
+          trace: collect_build_prerequisites: end bax/1.0.0
+          trace: collect_build_postponed (3): select cfg-negotiated dependency alternative for dependent toz/0.2.0
+          trace: collect_build_prerequisites: resume toz/0.2.0
+          trace: collect_build_prerequisites: cfg-postpone dependency libbar/1.0.0 of dependent toz/0.2.0
+          trace: postponed_configurations::add: add {toz 2,1: libbar} to {bax tez | libbox->{bax/2,1 tez/1,1} libbar->{bax/2,1 tez/3,1}}!
+          trace: collect_build_prerequisites: configuration for cfg-postponed dependencies of dependent toz/0.2.0 is negotiated
+          trace: collect_build_prerequisites: dependency libbar/1.0.0 of dependent toz/0.2.0 is already (being) recursively collected, skipping
+          trace: collect_build_prerequisites: end toz/0.2.0
+          trace: collect_build_postponed (3): select cfg-negotiated dependency alternative for dependent tvz/0.1.0
+          trace: collect_build_prerequisites: resume tvz/0.1.0
+          trace: collect_build_prerequisites: end tvz/0.1.0
+          trace: collect_build_postponed (3): cfg-negotiate end {bax toz tvz | libfoo->{bax/1,1 toz/1,1 tvz/3,1}}!
+          trace: collect_build_postponed (3): end {bax toz tvz | libfoo->{bax/1,1 toz/1,1 tvz/3,1}}
+          trace: collect_build_postponed (2): end {tez | libbox->{tez/1,1}}
+          trace: collect_build_postponed (1): end {tvz | toz->{tvz/1,1}}
+          trace: collect_build_postponed (0): end
+          trace: execute_plan: simulate: yes
+          %.*
+          trace: execute_plan: while configuring dependent tez/1.0.0 in simulation mode unconstrain (toz == 0.1.0)
+          %.*
+          error: unable to satisfy constraints on package toz
+            info: tvz depends on (toz == 0.2.0)
+                    command line requires (tvz == 0.1.0)
+            info: tez depends on (toz == 0.1.0)
+            info: available toz/0.2.0
+            info: available toz/0.1.0
+            info: while satisfying tez/1.0.0
+            info: explicitly specify toz version to manually satisfy both constraints
+          %.*
+          EOE
+
+        $pkg_drop tez
+      }
     }
 
     : cycle
@@ -16528,12 +16956,13 @@ test.arguments += --sys-no-query
      # requires toz of the 0.2.0 version, which can't be satisfied.
      #
      $* tvz --verbose 1 2>>EOE != 0;
-       error: unable to upgrade package toz/0.1.0 to 0.2.0
-         info: because package tez depends on (toz == 0.1.0)
-         info: package toz/0.2.0 required by tez, tvz
-         info: consider re-trying with --upgrade|-u potentially combined with --recursive|-r
-         info: or explicitly request up/downgrade of package tez
-         info: or explicitly specify package toz version to manually satisfy these constraints
+       error: unable to satisfy constraints on package toz
+         info: tvz depends on (toz == 0.2.0)
+         info: tez depends on (toz == 0.1.0)
+         info: available toz/0.2.0
+         info: available toz/0.1.0
+         info: while satisfying tez/1.0.0
+         info: explicitly specify toz version to manually satisfy both constraints
        EOE
 
       $pkg_drop bax tiz tez
@@ -27759,8 +28188,8 @@ else
             info: libbar depends on (libfoo == 1.1.0)
             info: available libfoo/1.0.0
             info: available libfoo/1.1.0
+            info: while satisfying libbar/1.1.0
             info: explicitly specify libfoo version to manually satisfy both constraints
-          info: while satisfying libbar/1.1.0
           EOE
       }
 
@@ -27779,8 +28208,8 @@ else
                     command line requires (libbar == 1.1.0)
             info: available libfoo/1.0.0
             info: available libfoo/1.1.0
+            info: while satisfying libbar/1.1.0
             info: explicitly specify libfoo version to manually satisfy both constraints
-          info: while satisfying libbar/1.1.0
           EOE
       }
 
@@ -28189,8 +28618,8 @@ else
             info: libbar depends on (libfoo == 1.1.0)
             info: available libfoo/1.0.0
             info: available libfoo/1.1.0
+            info: while satisfying libbar/1.1.0
             info: explicitly specify libfoo version to manually satisfy both constraints
-          info: while satisfying libbar/1.1.0
           EOE
       }
 
@@ -28209,8 +28638,8 @@ else
                     command line requires (libbar == 1.1.0)
             info: available libfoo/1.0.0
             info: available libfoo/1.1.0
+            info: while satisfying libbar/1.1.0
             info: explicitly specify libfoo version to manually satisfy both constraints
-          info: while satisfying libbar/1.1.0
           EOE
       }
 
diff --git a/tests/pkg-system.testscript b/tests/pkg-system.testscript
index 0da2bc9..8aaabd9 100644
--- a/tests/pkg-system.testscript
+++ b/tests/pkg-system.testscript
@@ -714,8 +714,8 @@ rep_remove += -d cfg 2>!
         info: foo depends on (libbar >= 2)
         info: available sys:libbar/1
         info: available sys:libbar/2
+        info: while satisfying foo/2
         info: explicitly specify libbar version to manually satisfy both constraints
-      info: while satisfying foo/2
       EOE
 
     $pkg_drop libbar