Improve pkg-build's 'unable to upgrade package' diagnostics

9 files changed, 417 insertions, 155 deletions

diff --git a/bpkg/database.hxx b/bpkg/database.hxx
index 684dc9d..a9e1da5 100644
--- a/bpkg/database.hxx
+++ b/bpkg/database.hxx
@@ -504,8 +504,9 @@ namespace bpkg
     linked_databases implicit_links_;
   };
 
-  // NOTE: remember to update config_package comparison operators and
-  // compare_lazy_ptr if changing the database comparison operators.
+  // NOTE: remember to update package_key and package_version_key comparison
+  // operators and compare_lazy_ptr if changing the database comparison
+  // operators.
   //
   // Note that here we use the database address as the database identity since
   // we don't suppose two database instances for the same configuration to
diff --git a/bpkg/package.cxx b/bpkg/package.cxx
index aca3550..4beba8e 100644
--- a/bpkg/package.cxx
+++ b/bpkg/package.cxx
@@ -70,6 +70,39 @@ namespace bpkg
     return r != 0 ? (r < 0) : (db < v.db);
   }
 
+  // package_version_key
+  //
+  string package_version_key::
+  string (bool ignore_version) const
+  {
+    std::string r (name.string ());
+
+    if (version && !version->empty () && !ignore_version)
+    {
+      r += '/';
+      r += version->string ();
+    }
+
+    const std::string& d (db.get ().string);
+
+    if (!d.empty ())
+    {
+      r += ' ';
+      r += d;
+    }
+
+    return r;
+  }
+
+  bool package_version_key::
+  operator< (const package_version_key& v) const
+  {
+    if (int r = name.compare (v.name))
+      return r < 0;
+
+    return version != v.version ? (version < v.version) : (db < v.db);
+  }
+
   // available_package
   //
   const version* available_package::
diff --git a/bpkg/package.hxx b/bpkg/package.hxx
index 06278cd..8f4b46c 100644
--- a/bpkg/package.hxx
+++ b/bpkg/package.hxx
@@ -1614,12 +1614,6 @@ namespace bpkg
 
     package_key (database& d, package_name n): db (d), name (move (n)) {}
 
-    // Create a pseudo-package (command line as a dependent, etc).
-    //
-    package_key (database& d, string n)
-        : db (d),
-          name (n.empty () ? package_name () : package_name (move (n))) {}
-
     bool
     operator== (const package_key& v) const
     {
@@ -1651,6 +1645,66 @@ namespace bpkg
     return os << p.string ();
   }
 
+  // Database, package name, and package version.
+  //
+  // It is normally used as a key for maps containing data for package
+  // versions across multiple linked configurations. Assumes that the
+  // respective databases are not detached during such map lifetimes.
+  // Considers all package name, package version, and database for objects
+  // comparison.
+  //
+  // The package name can be a pseudo-package (command line as a dependent,
+  // etc), in which case the version is absent. The version can also be empty,
+  // denoting a package of an unknown version.
+  //
+  struct package_version_key
+  {
+    reference_wrapper<database> db;
+    package_name                name;
+    optional<bpkg::version>     version;
+
+    package_version_key (database& d, package_name n, bpkg::version v)
+        : db (d), name (move (n)), version (move (v)) {}
+
+    // Create a pseudo-package (command line as a dependent, etc).
+    //
+    package_version_key (database& d, string n)
+        : db (d),
+          name (move (n), package_name::raw_string) {}
+
+    bool
+    operator== (const package_version_key& v) const
+    {
+      // See operator==(database, database).
+      //
+      return name == v.name       &&
+             version == v.version &&
+             &db.get () == &v.db.get ();
+    }
+
+    bool
+    operator!= (const package_version_key& v) const
+    {
+      return !(*this == v);
+    }
+
+    bool
+    operator< (const package_version_key&) const;
+
+    // Return the package string representation in the form:
+    //
+    // <name>[/<version>] [ <config-dir>]
+    //
+    std::string
+    string (bool ignore_version = false) const;
+  };
+
+  inline ostream&
+  operator<< (ostream& os, const package_version_key& p)
+  {
+    return os << p.string ();
+  }
+
   // Return a count of repositories that contain this repository fragment.
   //
   #pragma db view table("main.repository_fragments")
diff --git a/bpkg/pkg-build-collect.cxx b/bpkg/pkg-build-collect.cxx
index 918242c..b1306b5 100644
--- a/bpkg/pkg-build-collect.cxx
+++ b/bpkg/pkg-build-collect.cxx
@@ -62,7 +62,8 @@ namespace bpkg
   bool build_package::
   user_selection () const
   {
-    return required_by.find (package_key {db.get ().main_database (), ""}) !=
+    return required_by.find (package_version_key (db.get ().main_database (),
+                                                  "command line")) !=
            required_by.end ();
   }
 
@@ -303,7 +304,7 @@ namespace bpkg
 
       // Propagate the user-selection tag.
       //
-      required_by.emplace (db.get ().main_database (), "");
+      required_by.emplace (db.get ().main_database (), "command line");
     }
 
     // Copy the required-by package names only if semantics matches.
@@ -320,10 +321,7 @@ namespace bpkg
         if (find_if (constraints.begin (), constraints.end (),
                      [&c] (const constraint_type& v)
                      {
-                       return v.db.get () == c.db.get () &&
-                              v.dependent == c.dependent &&
-                              v.value     == c.value;
-
+                       return v.dependent == c.dependent && v.value == c.value;
                      }) == constraints.end ())
         {
           constraints.push_back (move (c));
@@ -525,8 +523,8 @@ namespace bpkg
       //
       int ud (sp->version.compare (av));
 
-      // Otherwise, the dependent must be satisfied with the already configured
-      // dependency.
+      // Otherwise, the dependent must be satisfied with the already
+      // configured dependency.
       //
       assert (ud != 0);
 
@@ -542,24 +540,81 @@ namespace bpkg
       else
         dr << av; // Can't be the wildcard otherwise would satisfy.
 
-      dr << info << "because package " << dk << " depends on (" << n << ' '
-                 << c << ')';
+      shared_ptr<selected_package> dsp (
+        dk.db.get ().load<selected_package> (dk.name));
+
+      assert (dsp != nullptr); // By definition.
 
+      dr << info << "because configured package " << *dsp << dk.db
+                 << " depends on (" << n << ' ' << c << ')';
+
+      // Print the dependency constraints tree for this unsatisfied dependent,
+      // which only contains constraints which come from its selected
+      // dependents, recursively.
+      //
       {
         set<package_key> printed;
-        pkgs.print_constraints (dr, dk, indent, printed);
+        pkgs.print_constraints (dr,
+                                dk,
+                                indent,
+                                printed,
+                                true /* selected_dependent */);
       }
 
-      string rb;
+      // If the dependency we failed to up/downgrade is not explicitly
+      // specified on the command line, then print its dependency constraints
+      // tree which only contains constraints which come from its being built
+      // dependents, recursively.
+      //
       if (!p->user_selection ())
       {
-        for (const package_key& pk: p->required_by)
-          rb += (rb.empty () ? " " : ", ") + pk.string ();
-      }
+        // The dependency upgrade is always required by someone, the command
+        // line or a package.
+        //
+        assert (!p->required_by.empty ());
+
+        dr << info << "package " << p->available_name_version_db ();
+
+        // Note that if the required_by member contains the dependencies,
+        // rather than the dependents, we will subsequently print the
+        // dependency constraints trees for these dependencies rather than a
+        // single constraints tree (rooted in the dependency we failed to
+        // up/downgrade). Also note that in this case we will still reuse the
+        // same printed packages cache for all print_constraints() calls,
+        // since it will likely be considered as a single dependency graph by
+        // the user.
+        //
+        bool rbd (p->required_by_dependents);
+        dr << (rbd ? " required by" : " dependent of");
 
-      if (!rb.empty ())
-        dr << info << "package " << p->available_name_version ()
-                   << " required by" << rb;
+        set<package_key> printed;
+        for (const package_version_key& pvk: p->required_by)
+        {
+          dr << '\n' << indent << pvk;
+
+          if (rbd)
+          {
+            const vector<build_package::constraint_type>& cs (p->constraints);
+            auto i (find_if (cs.begin (), cs.end (),
+                             [&pvk] (const build_package::constraint_type& v)
+                             {
+                               return v.dependent == pvk;
+                             }));
+
+            if (i != cs.end ())
+              dr << " (" << p->name () << ' ' << i->value << ')';
+          }
+
+          indent += "  ";
+          pkgs.print_constraints (dr,
+                                  package_key (pvk.db, pvk.name),
+                                  indent,
+                                  printed,
+                                  false /* selected_dependent */);
+
+          indent.resize (indent.size () - 2);
+        }
+      }
 
       dr << info << "consider re-trying with --upgrade|-u potentially combined "
                  << "with --recursive|-r" <<
@@ -576,8 +631,8 @@ namespace bpkg
       {
         const build_package::constraint_type& c (uc.constraint);
 
-        dr << info << c.dependent << c.db << " depends on (" << n
-                   << ' ' << c.value << ")";
+        dr << info << c.dependent << " depends on (" << n << ' ' << c.value
+                   << ')';
 
         if (const build_package* d = pkgs.dependent_build (c))
         {
@@ -1268,17 +1323,16 @@ namespace bpkg
   dependent_build (const build_package::constraint_type& c) const
   {
     const build_package* r (nullptr);
-    const string& d (c.dependent);
 
-    if (d != "command line")
+    if (c.dependent.version)
     try
     {
-      r = entered_build (c.db, package_name (d));
+      r = entered_build (c.dependent.db, c.dependent.name);
       assert (r != nullptr); // Expected to be collected.
     }
     catch (const invalid_argument&)
     {
-      // Must be a package name, unless it is 'command line'.
+      // Must be a package name since the version is specified.
       //
       assert (false);
     }
@@ -1943,7 +1997,8 @@ namespace bpkg
                                           replaced_vers,
                                           postponed_recs,
                                           postponed_cfgs,
-                                          unsatisfied_depts);
+                                          unsatisfied_depts,
+                                          true /* add_required_by */);
           }
 
           // Postpone the recursive collection of a dependency if the existing
@@ -2473,15 +2528,15 @@ namespace bpkg
                     *dr << error << "unable to satisfy constraints on package "
                         << dn <<
                       info << nm << pdb << " depends on (" << dn << ' '
-                           << *dp.constraint << ")";
+                           << *dp.constraint << ')';
 
                     {
                       set<package_key> printed;
                       print_constraints (*dr, pkg, indent, printed);
                     }
 
-                    *dr << info << c.dependent << c.db << " depends on (" << dn
-                                << ' ' << c.value << ")";
+                    *dr << info << c.dependent << " depends on (" << dn << ' '
+                                << c.value << ')';
 
                     if (const build_package* d = dependent_build (c))
                     {
@@ -2725,7 +2780,7 @@ namespace bpkg
                     assert (dr == nullptr); // Should fail on the "silent" run.
 
                     fail << "multiple possible " << type << " configurations "
-                         << "for build-time dependency (" << dp << ")" <<
+                         << "for build-time dependency (" << dp << ')' <<
                       info << db->config_orig <<
                       info << ldb.config_orig <<
                       info << "use --config-* to select the configuration";
@@ -3106,7 +3161,7 @@ namespace bpkg
                       info << package_string (dn,
                                               *dap->system_version (*ddb),
                                               true /* system */)
-                        << " does not satisfy the constrains";
+                           << " does not satisfy the constrains";
 
                   return precollect_result (false /* postpone */);
                 }
@@ -3181,7 +3236,11 @@ namespace bpkg
                   {
                     using constraint_type = build_package::constraint_type;
 
-                    constraint_type c1 {pdb, nm.string (), *d.constraint};
+                    constraint_type c1 (*d.constraint,
+                                        pdb,
+                                        nm,
+                                        pkg.available_version (),
+                                        false /* selected_dependent */);
 
                     if (!satisfies (v2, c1.value))
                     {
@@ -3204,8 +3263,8 @@ namespace bpkg
 
                             *dr << error << "unable to satisfy constraints on "
                                 << "package " << n <<
-                              info << c2.dependent << c2.db << " depends on ("
-                                   << n << ' ' << c2.value << ")";
+                              info << c2.dependent << " depends on (" << n
+                                   << ' ' << c2.value << ')';
 
                             if (const build_package* d = dependent_build (c2))
                             {
@@ -3213,9 +3272,8 @@ namespace bpkg
                               print_constraints (*dr, *d, indent, printed);
                             }
 
-                            *dr << info << c1.dependent << c1.db
-                                << " depends on (" << n << ' '
-                                << c1.value << ")";
+                            *dr << info << c1.dependent << " depends on ("
+                                        << n << ' ' << c1.value << ')';
 
                             if (const build_package* d = dependent_build (c1))
                             {
@@ -3308,6 +3366,8 @@ namespace bpkg
                   postponed_deps.erase (d);
               }));
 
+          package_version_key pvk (pk.db, pk.name, pkg.available_version ());
+
           for (prebuild& b: bs)
           {
             build_package bpk {
@@ -3331,7 +3391,7 @@ namespace bpkg
               nullopt,                    // Checkout root.
               false,                      // Checkout purge.
               strings (),                 // Configuration variables.
-              {pk},                       // Required by (dependent).
+              {pvk},                      // Required by (dependent).
               true,                       // Required by dependents.
               0};                         // State flags.
 
@@ -3346,7 +3406,11 @@ namespace bpkg
             // both constraints for completeness.
             //
             if (constraint)
-              bpk.constraints.emplace_back (pdb, nm.string (), *constraint);
+              bpk.constraints.emplace_back (*constraint,
+                                            pdb,
+                                            nm,
+                                            pkg.available_version (),
+                                            false /* selected_dependent */);
 
             // Now collect this prerequisite. If it was actually collected
             // (i.e., it wasn't already there) and we are forcing a downgrade
@@ -5073,13 +5137,19 @@ namespace bpkg
 
       // Add the prerequisite replacements as the required-by packages.
       //
-      set<package_key> required_by;
+      set<package_version_key> required_by;
       for (const auto& prq: rd.second)
       {
         if (prq.second) // Prerequisite replacement?
         {
           const package_key& pk (prq.first);
-          required_by.emplace (pk.db, pk.name);
+
+          // Note that the dependency can potentially be just pre-entered, in
+          // which case its version is not known at this point.
+          //
+          assert (entered_build (pk) != nullptr);
+
+          required_by.emplace (pk.db, pk.name, version ());
         }
       }
 
@@ -5630,7 +5700,8 @@ namespace bpkg
                                               replaced_vers,
                                               postponed_recs,
                                               postponed_cfgs,
-                                              unsatisfied_depts);
+                                              unsatisfied_depts,
+                                              true /* add_required_by */);
               }
             }
           }
@@ -6515,12 +6586,18 @@ namespace bpkg
                             << "existing dependent " << *ed.selected
                             << ed.db;});
 
+              // Note that we pass false as the add_required_by argument since
+              // the package builds collection state has been restored and the
+              // originating dependency for this existing dependent may not be
+              // collected anymore.
+              //
               recollect_existing_dependent (o,
                                             ed,
                                             replaced_vers,
                                             postponed_recs,
                                             postponed_cfgs,
-                                            unsatisfied_depts);
+                                            unsatisfied_depts,
+                                            false /* add_required_by */);
             }
           }
         }
@@ -6691,12 +6768,17 @@ namespace bpkg
                           << ed.db << " due to bogus postponement of "
                           << "dependency " << pk;});
 
+            // Note that we pass false as the add_required_by argument since
+            // the postponment is bogus and thus the originating dependency
+            // for this existing dependent may not be collected.
+            //
             recollect_existing_dependent (o,
                                           ed,
                                           replaced_vers,
                                           postponed_recs,
                                           postponed_cfgs,
-                                          unsatisfied_depts);
+                                          unsatisfied_depts,
+                                          false /* add_required_by */);
             prog = true;
             break;
           }
@@ -7072,6 +7154,8 @@ namespace bpkg
           //
           assert (!dsp->system ());
 
+          package_version_key pvk (pdb, n, version ());
+
           return build_package {
             build_package::adjust,
             ddb,
@@ -7093,7 +7177,7 @@ namespace bpkg
             nullopt,                   // Checkout root.
             false,                     // Checkout purge.
             strings (),                // Configuration variables.
-            {package_key {pdb, n}},    // Required by (dependency).
+            {move (pvk)},              // Required by (dependency).
             false,                     // Required by dependents.
             build_package::adjust_reconfigure};
         };
@@ -7160,11 +7244,29 @@ namespace bpkg
           i->second.position = insert (pos, i->second.package);
         }
 
-        // Add this dependent's contraint, if present, to the dependency's
-        // constraints list for completeness.
+        // Add this dependent's constraint, if present, to the dependency's
+        // constraints list for completeness, while suppressing duplicates.
         //
         if (dc)
-          p.constraints.emplace_back (ddb, move (dn).string (), move (*dc));
+        {
+          using constraint_type = build_package::constraint_type;
+
+          constraint_type c (move (*dc),
+                             ddb,
+                             move (dn),
+                             i->second.package.selected->version,
+                             true /* selected_dependent */);
+
+          if (find_if (p.constraints.begin (), p.constraints.end (),
+                       [&c] (const constraint_type& v)
+                       {
+                         return v.dependent == c.dependent &&
+                                v.value == c.value;
+                       }) == p.constraints.end ())
+          {
+            p.constraints.emplace_back (move (c));
+          }
+        }
 
         // Recursively collect our own dependents inserting them before us.
         //
@@ -7200,7 +7302,8 @@ namespace bpkg
   print_constraints (diag_record& dr,
                      const build_package& p,
                      string& indent,
-                     set<package_key>& printed) const
+                     set<package_key>& printed,
+                     optional<bool> selected_dependent) const
   {
     using constraint_type = build_package::constraint_type;
 
@@ -7216,33 +7319,35 @@ namespace bpkg
 
         for (const constraint_type& c: cs)
         {
-          if (const build_package* d = dependent_build (c))
+          if (!selected_dependent ||
+              *selected_dependent == c.selected_dependent)
           {
-            dr << '\n' << indent << c.dependent << '/';
+            if (const build_package* d = dependent_build (c))
+            {
+              dr << '\n' << indent << c.dependent << " requires (" << pk
+                 << ' ' << c.value << ')';
 
-            // The dependent can only be collected as a build or adjustment.
-            //
-            assert (d->action                        &&
-                    d->action != build_package::drop &&
-                    (d->available || d->selected));
-
-            dr << (d->available != nullptr
-                   ? d->available_version ()
-                   : d->selected->version) << c.db << " requires (" << pk
-               << ' ' << c.value << ")";
-
-            indent += "  ";
-            print_constraints (dr, *d, indent, printed);
-            indent.resize (indent.size () - 2);
+              indent += "  ";
+              print_constraints (dr, *d, indent, printed, selected_dependent);
+              indent.resize (indent.size () - 2);
+            }
+            else
+              dr << '\n' << indent << c.dependent << " requires (" << pk << ' '
+                 << c.value << ')';
           }
-          else
-            dr << '\n' << indent << c.dependent << " requires (" << pk << ' '
-               << c.value << ")";
         }
       }
       else
       {
-        dr << '\n' << indent << "...";
+        for (const constraint_type& c: cs)
+        {
+          if (!selected_dependent ||
+              *selected_dependent == c.selected_dependent)
+          {
+            dr << '\n' << indent << "...";
+            break;
+          }
+        }
       }
     }
   }
@@ -7251,11 +7356,12 @@ namespace bpkg
   print_constraints (diag_record& dr,
                      const package_key& pk,
                      string& indent,
-                     set<package_key>& printed) const
+                     set<package_key>& printed,
+                     optional<bool> selected_dependent) const
   {
     const build_package* p (entered_build (pk));
     assert (p != nullptr); // Expected to be collected.
-    print_constraints (dr, *p, indent, printed);
+    print_constraints (dr, *p, indent, printed, selected_dependent);
   }
 
   void build_packages::
@@ -7570,8 +7676,8 @@ namespace bpkg
 
     const shared_ptr<selected_package>& dsp (ed.selected);
 
-    package_key        dpt (ed.db, dsp->name);
-    const package_key& dep (*ed.dependency);
+    package_version_key dpt (ed.db, dsp->name, dsp->version);
+    const package_key&  dep (*ed.dependency);
 
     lazy_shared_ptr<selected_package> lsp (dep.db.get (), dep.name);
     shared_ptr<selected_package>       sp (lsp.load ());
@@ -7614,9 +7720,11 @@ namespace bpkg
       assert (i != dsp->prerequisites.end ());
 
       if (i->second.constraint)
-        p.constraints.emplace_back (dpt.db,
-                                    dpt.name.string (),
-                                    *i->second.constraint);
+        p.constraints.emplace_back (*i->second.constraint,
+                                    dpt.db,
+                                    dpt.name,
+                                    *dpt.version,
+                                    true /* selected_package */);
     }
 
     // Note: not recursive.
@@ -7641,6 +7749,11 @@ namespace bpkg
          lazy_shared_ptr<repository_fragment>> rp (
            find_available_fragment (o, ed.db, ed.selected));
 
+    set<package_version_key> rb;
+
+    for (package_key& p: ds)
+      rb.emplace (p.db, move (p.name), version ());
+
     build_package p {
       build_package::build,
       ed.db,
@@ -7662,9 +7775,7 @@ namespace bpkg
       nullopt,                            // Checkout root.
       false,                              // Checkout purge.
       strings (),                         // Configuration variables.
-      set<package_key> (                  // Required by (dependency).
-        make_move_iterator (ds.begin ()),
-        make_move_iterator (ds.end ())),
+      move (rb),                          // Required by (dependency).
       false,                              // Required by dependents.
       build_package::build_reevaluate};
 
@@ -7680,7 +7791,8 @@ namespace bpkg
                                 replaced_versions& replaced_vers,
                                 postponed_packages& postponed_recs,
                                 postponed_configurations& postponed_cfgs,
-                                unsatisfied_dependents& unsatisfied_depts)
+                                unsatisfied_dependents& unsatisfied_depts,
+                                bool add_required_by)
   {
     pair<shared_ptr<available_package>,
          lazy_shared_ptr<repository_fragment>> rp (
@@ -7693,6 +7805,17 @@ namespace bpkg
     if (!ed.dependency)
       flags |= build_package::adjust_reconfigure;
 
+    set<package_version_key> rb;
+
+    if (add_required_by)
+    {
+      const package_key& pk (ed.originating_dependency);
+
+      assert (entered_build (pk) != nullptr); // Expected to be collected.
+
+      rb.emplace (pk.db, pk.name, version ());
+    }
+
     build_package p {
       build_package::build,
       ed.db,
@@ -7714,7 +7837,7 @@ namespace bpkg
       nullopt,                     // Checkout root.
       false,                       // Checkout purge.
       strings (),                  // Configuration variables.
-      {ed.originating_dependency}, // Required by (dependency).
+      move (rb),                   // Required by (dependency).
       false,                       // Required by dependents.
       flags};
 
diff --git a/bpkg/pkg-build-collect.hxx b/bpkg/pkg-build-collect.hxx
index 5a5a397..e34fc25 100644
--- a/bpkg/pkg-build-collect.hxx
+++ b/bpkg/pkg-build-collect.hxx
@@ -196,24 +196,43 @@ namespace bpkg
     optional<bool> hold_package;
     optional<bool> hold_version;
 
-    // Constraint value plus, normally, the dependent package name that placed
-    // this constraint but can also be some other name for the initial
-    // selection. This is why we use the string type, rather than
-    // package_name. Currently, the only valid non-package name is "command
-    // line", which is used when the package version is specified by the user
-    // on the command line.
+    // Constraint value plus, normally, the dependent package name/version
+    // that placed this constraint but can also be some other name (in which
+    // case the version is absent) for the initial selection. Currently, the
+    // only valid non-package name is 'command line', which is used when the
+    // package version is constrained by the user on the command line.
     //
     // Note that if the dependent is a package name, then this package is
     // expected to be collected (present in the map).
     //
     struct constraint_type
     {
-      reference_wrapper<database> db; // Main database for non-packages.
-      string dependent;
       version_constraint value;
 
-      constraint_type (database& d, string dp, version_constraint v)
-          : db (d), dependent (move (dp)), value (move (v)) {}
+      package_version_key dependent;
+
+      // False for non-packages. Otherwise, indicates whether the constraint
+      // comes from the selected dependent or not.
+      //
+      bool selected_dependent;
+
+      // Create constraint for a package dependent.
+      //
+      constraint_type (version_constraint v,
+                       database& db,
+                       package_name nm,
+                       version ver,
+                       bool s)
+          : value (move (v)),
+            dependent (db, move (nm), move (ver)),
+            selected_dependent (s) {}
+
+      // Create constraint for a non-package dependent.
+      //
+      constraint_type (version_constraint v, database& db, string nm)
+          : value (move (v)),
+            dependent (db, move (nm)),
+            selected_dependent (false) {}
     };
 
     vector<constraint_type> constraints;
@@ -273,11 +292,23 @@ namespace bpkg
     strings config_vars;
 
     // Set of packages (dependents or dependencies but not a mix) that caused
-    // this package to be built or adjusted. Empty name signifies user
-    // selection and can be present regardless of the required_by_dependents
-    // flag value.
+    // this package to be built or adjusted. The 'command line' name signifies
+    // user selection and can be present regardless of the
+    // required_by_dependents flag value.
     //
-    std::set<package_key> required_by;
+    // Note that if this is a package name, then this package is expected to
+    // be collected (present in the map), potentially just pre-entered if
+    // required_by_dependents is false.
+    //
+    // Also note that if required_by_dependents is true, then all the
+    // dependent package versions in the required_by set are expected to be
+    // known (the version members are not empty). Otherwise (the required_by
+    // set contains dependencies), since it's not always easy to deduce the
+    // dependency versions at the time of collecting the dependent build (see
+    // collect_repointed_dependents() implementation for details), the
+    // dependency package versions are expected to all be unknown.
+    //
+    std::set<package_version_key> required_by;
 
     // If this flags is true, then required_by contains dependents.
     //
@@ -1583,17 +1614,22 @@ namespace bpkg
     // constraints for the same package twice, printing "..." instead. Noop if
     // there are no constraints for this package.
     //
+    // Optionally, only print constraints from the selected or being built
+    // dependents (see build_package::constraint_type for details).
+    //
     void
     print_constraints (diag_record&,
                        const build_package&,
                        string& indent,
-                       std::set<package_key>& printed) const;
+                       std::set<package_key>& printed,
+                       optional<bool> selected_dependent = nullopt) const;
 
     void
     print_constraints (diag_record&,
                        const package_key&,
                        string& indent,
-                       std::set<package_key>& printed) const;
+                       std::set<package_key>& printed,
+                       optional<bool> selected_dependent = nullopt) const;
 
     // Verify that builds ordering is consistent across all the data
     // structures and the ordering expectations are fulfilled (real build
@@ -1698,7 +1734,8 @@ namespace bpkg
                                   replaced_versions&,
                                   postponed_packages& postponed_recs,
                                   postponed_configurations&,
-                                  unsatisfied_dependents&);
+                                  unsatisfied_dependents&,
+                                  bool add_required_by);
 
     struct package_ref
     {
diff --git a/bpkg/pkg-build.cxx b/bpkg/pkg-build.cxx
index 588d29c..5c41204 100644
--- a/bpkg/pkg-build.cxx
+++ b/bpkg/pkg-build.cxx
@@ -1660,6 +1660,10 @@ namespace bpkg
                    ? empty_string
                    : '[' + config_dirs[0].representation () + ']'));
 
+    // Command line as a dependent.
+    //
+    package_version_key cmd_line (mdb, "command line");
+
     current_configs.push_back (mdb);
 
     if (config_dirs.size () != 1)
@@ -3609,7 +3613,7 @@ namespace bpkg
            : optional<dir_path> ()),
           pa.options.checkout_purge (),
           move (pa.config_vars),
-          {package_key {mdb, ""}},    // Required by (command line).
+          {cmd_line},                 // Required by (command line).
           false,                      // Required by dependents.
           replace ? build_package::build_replace : uint16_t (0)};
 
@@ -3623,7 +3627,7 @@ namespace bpkg
         //
         if (pa.constraint)
           p.constraints.emplace_back (
-            mdb, "command line", move (*pa.constraint));
+            move (*pa.constraint), cmd_line.db, cmd_line.name.string ());
 
         pkg_confs.emplace_back (p.db, p.name ());
 
@@ -3741,7 +3745,7 @@ namespace bpkg
                 nullopt,                 // Checkout root.
                 false,                   // Checkout purge.
                 strings (),              // Configuration variables.
-                {package_key {mdb, ""}}, // Required by (command line).
+                {cmd_line},              // Required by (command line).
                 false,                   // Required by dependents.
                 deorphan ? build_package::build_replace : uint16_t (0)};
 
@@ -4082,7 +4086,8 @@ namespace bpkg
         // Also, if a dependency package already has selected package that
         // is held, then we need to unhold it.
         //
-        auto enter = [&mdb, &pkgs] (database& db, const dependency_package& p)
+        auto enter = [&pkgs, &cmd_line] (database& db,
+                                         const dependency_package& p)
         {
           build_package bp {
             nullopt,                    // Action.
@@ -4105,13 +4110,14 @@ namespace bpkg
             p.checkout_root,
             p.checkout_purge,
             p.config_vars,
-            {package_key {mdb, ""}},    // Required by (command line).
+            {cmd_line},                 // Required by (command line).
             false,                      // Required by dependents.
             0};                         // State flags.
 
           if (p.constraint)
-            bp.constraints.emplace_back (
-              mdb, "command line", *p.constraint);
+            bp.constraints.emplace_back (*p.constraint,
+                                         cmd_line.db,
+                                         cmd_line.name.string ());
 
           pkgs.enter (p.name, move (bp));
         };
@@ -4403,7 +4409,7 @@ namespace bpkg
                 nullopt,                 // Checkout root.
                 false,                   // Checkout purge.
                 strings (),              // Configuration variables.
-                {package_key {mdb, ""}}, // Required by (command line).
+                {cmd_line},              // Required by (command line).
                 false,                   // Required by dependents.
                 (d.existing || d.deorphan
                  ? build_package::build_replace
@@ -5600,12 +5606,17 @@ namespace bpkg
               // can a dependent-dependency structure change without any of
               // the package versions changing? Doesn't feel like it should.
               //
-              for (const package_key& pk: p.required_by)
+              for (const package_version_key& pvk: p.required_by)
               {
-                // Skip the command-line dependent.
+                // Skip the command-line, etc dependents and don't print the
+                // package version (which is not always available; see
+                // build_package::required_by for details).
                 //
-                if (!pk.name.empty ())
-                  rb += (rb.empty () ? " " : ", ") + pk.string ();
+                if (pvk.version) // Is it a real package?
+                {
+                  rb += (rb.empty () ? " " : ", ") +
+                        pvk.string (true /* ignore_version */);
+                }
               }
 
               // If not user-selected, then there should be another (implicit)
diff --git a/bpkg/rep-create.cxx b/bpkg/rep-create.cxx
index 67cca95..9b9bdeb 100644
--- a/bpkg/rep-create.cxx
+++ b/bpkg/rep-create.cxx
@@ -24,7 +24,7 @@ using namespace butl;
 
 namespace bpkg
 {
-  struct package_version_key
+  struct package_name_version
   {
     package_name name;
     bpkg::version version;
@@ -34,20 +34,20 @@ namespace bpkg
     // revision.
     //
     bool
-    operator< (const package_version_key& y) const
+    operator< (const package_name_version& y) const
     {
       int r (name.compare (y.name));
       return r < 0 || (r == 0 && version.compare (y.version, true) < 0);
     }
   };
 
-  struct package_version_data
+  struct package_data
   {
     path archive;
     package_manifest manifest;
   };
 
-  using package_map = map<package_version_key, package_version_data>;
+  using package_map = map<package_name_version, package_data>;
 
   static void
   collect (const rep_create_options& o,
@@ -116,8 +116,8 @@ namespace bpkg
       //
       m.location = a.leaf (root);
 
-      package_version_key k {m.name, m.version}; // Argument evaluation order.
-      auto r (map.emplace (move (k), package_version_data {a, move (m)}));
+      package_name_version k {m.name, m.version}; // Argument evaluation order.
+      auto r (map.emplace (move (k), package_data {a, move (m)}));
 
       // Diagnose duplicates.
       //
diff --git a/tests/pkg-build.testscript b/tests/pkg-build.testscript
index 0efe4ff..eec268e 100644
--- a/tests/pkg-build.testscript
+++ b/tests/pkg-build.testscript
@@ -1392,7 +1392,7 @@ test.arguments += --sys-no-query
 
       $* libfoo/1.0.0 2>>EOE != 0;
         error: unable to downgrade package libfoo/1.1.0 to 1.0.0
-          info: because package libbar depends on (libfoo == 1.1.0)
+          info: because configured package libbar/1.1.0 depends on (libfoo == 1.1.0)
           info: consider re-trying with --upgrade|-u potentially combined with --recursive|-r
           info: or explicitly request up/downgrade of package libbar
           info: or explicitly specify package libfoo version to manually satisfy these constraints
@@ -1428,7 +1428,7 @@ test.arguments += --sys-no-query
 
       $* libfoo/1.0.0 +{ --config-id 1 } 2>>~%EOE% != 0;
         %error: unable to downgrade package libfoo/1.1.0 \[cfg.\] to 1.0.0%
-        %  info: because package libbar \[cfg.\] depends on \(libfoo == 1.1.0\)%
+        %  info: because configured package libbar/1.1.0 \[cfg.\] depends on \(libfoo == 1.1.0\)%
           info: consider re-trying with --upgrade|-u potentially combined with --recursive|-r
           info: or explicitly request up/downgrade of package libbar
           info: or explicitly specify package libfoo version to manually satisfy these constraints
@@ -2164,8 +2164,8 @@ test.arguments += --sys-no-query
         #
         $* foo 2>>EOE != 0;
           error: unable to satisfy constraints on package libbar
-            info: libbaz depends on (libbar ^2.0.0)
-            info: libbox depends on (libbar ^1.0.0)
+            info: libbaz/2.0.0 depends on (libbar ^2.0.0)
+            info: libbox/1.0.0 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
@@ -2234,8 +2234,9 @@ test.arguments += --sys-no-query
         #
         $* ?libbaz 2>>EOE != 0;
           error: unable to upgrade package libbar/1.0.0 to 2.0.0
-            info: because package libbox depends on (libbar ^1.0.0)
-            info: package libbar/2.0.0 required by libbaz
+            info: because configured package libbox/1.0.0 depends on (libbar ^1.0.0)
+            info: package libbar/2.0.0 required by
+                    libbaz/2.0.0 (libbar ^2.0.0)
             info: consider re-trying with --upgrade|-u potentially combined with --recursive|-r
             info: or explicitly request up/downgrade of package libbox
             info: or explicitly specify package libbar version to manually satisfy these constraints
@@ -2259,8 +2260,8 @@ test.arguments += --sys-no-query
         #
         $* foo 2>>EOE != 0;
           error: unable to satisfy constraints on package libbar
-            info: libbaz depends on (libbar ^2.0.0)
-            info: libbox depends on (libbar ^1.0.0)
+            info: libbaz/2.0.0 depends on (libbar ^2.0.0)
+            info: libbox/1.1.0 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
@@ -2329,8 +2330,9 @@ test.arguments += --sys-no-query
         #
         $* foo ?libbaz 2>>EOE != 0;
           error: unable to upgrade package libbar/1.0.0 to 2.0.0
-            info: because package libbox depends on (libbar ^1.0.0)
-            info: package libbar/2.0.0 required by libbaz
+            info: because configured package libbox/1.0.0 depends on (libbar ^1.0.0)
+            info: package libbar/2.0.0 required by
+                    libbaz/2.0.0 (libbar ^2.0.0)
             info: consider re-trying with --upgrade|-u potentially combined with --recursive|-r
             info: or explicitly request up/downgrade of package libbox
             info: or explicitly specify package libbar version to manually satisfy these constraints
@@ -2341,8 +2343,8 @@ test.arguments += --sys-no-query
         #
         $* foo ?libbaz ?libbox 2>>EOE != 0;
           error: unable to satisfy constraints on package libbar
-            info: libbaz depends on (libbar ^2.0.0)
-            info: libbox depends on (libbar ^1.0.0)
+            info: libbaz/2.0.0 depends on (libbar ^2.0.0)
+            info: libbox/1.1.0 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
@@ -2833,8 +2835,8 @@ test.arguments += --sys-no-query
 
         $* libfix libbiz 2>>EOE != 0;
           error: unable to satisfy constraints on package libbaz
-            info: libfix depends on (libbaz >= 0.0.3)
-            info: libbiz depends on (libbaz <= 0.0.3)
+            info: libfix/0.0.3 depends on (libbaz >= 0.0.3)
+            info: libbiz/0.0.2 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
@@ -2907,8 +2909,8 @@ test.arguments += --sys-no-query
 
         $* libbiz 2>>EOE != 0;
           error: unable to satisfy constraints on package libfoo
-            info: libbox depends on (libfoo == 1.0.0)
-            info: libfox depends on (libfoo == 0.0.1)
+            info: libbox/0.0.2 depends on (libfoo == 1.0.0)
+            info: libfox/0.0.2 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
@@ -4448,7 +4450,7 @@ test.arguments += --sys-no-query
             $* box +{ config.box.extras=true } libbox/0.1.0 2>>EOE != 0;
               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: box/1.0.0 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
@@ -5815,8 +5817,9 @@ test.arguments += --sys-no-query
 
         $* tax 2>>EOE != 0;
           error: unable to downgrade package libfoo/2.0.0 to 1.0.0
-            info: because package tpx depends on (libfoo >= 2.0.0)
-            info: package libfoo/1.0.0 required by tax
+            info: because configured package tpx/1.0.0 depends on (libfoo >= 2.0.0)
+            info: package libfoo/1.0.0 required by
+                    tax/1.0.0 (libfoo == 1.0.0)
             info: consider re-trying with --upgrade|-u potentially combined with --recursive|-r
             info: or explicitly request up/downgrade of package tpx
             info: or explicitly specify package libfoo version to manually satisfy these constraints
@@ -5829,7 +5832,7 @@ test.arguments += --sys-no-query
         #
         $* tax tpx ?libfoo/1.0.0 2>>EOE != 0;
           error: unable to downgrade package libfoo/2.0.0 to 1.0.0
-            info: because package tpx depends on (libfoo >= 2.0.0)
+            info: because configured package tpx/1.0.0 depends on (libfoo >= 2.0.0)
             info: consider re-trying with --upgrade|-u potentially combined with --recursive|-r
             info: or explicitly request up/downgrade of package tpx
             info: or explicitly specify package libfoo version to manually satisfy these constraints
@@ -9647,8 +9650,8 @@ test.arguments += --sys-no-query
           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: tvz/1.0.0 depends on (toz == 0.2.0)
+            info: tez/1.0.0 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
@@ -9841,9 +9844,9 @@ test.arguments += --sys-no-query
           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: tvz/0.1.0 depends on (toz == 0.2.0)
                     command line requires (tvz == 0.1.0)
-            info: tez depends on (toz == 0.1.0)
+            info: tez/1.0.0 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
@@ -16957,8 +16960,8 @@ test.arguments += --sys-no-query
      #
      $* tvz --verbose 1 2>>EOE != 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: tvz/1.0.0 depends on (toz == 0.2.0)
+         info: tez/1.0.0 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
@@ -24765,7 +24768,7 @@ else
 
       $* ?libbaz/1.0.0 +{ --config-name h2 } 2>>~%EOE% != 0;
         %error: unable to downgrade package libbaz/1.1.0 \[h2.\] to 1.0.0%
-        %  info: because package foo \[h2.\] depends on \(libbaz \^1.1.0\)%
+        %  info: because configured package foo/1.1.0 \[h2.\] depends on \(libbaz \^1.1.0\)%
           info: consider re-trying with --upgrade|-u potentially combined with --recursive|-r
           info: or explicitly request up/downgrade of package foo
           info: or explicitly specify package libbaz version to manually satisfy these constraints
@@ -28185,7 +28188,7 @@ else
         $* libbar $src/libfoo-1.0.0.tar.gz 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: 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
@@ -28204,7 +28207,7 @@ else
         $* $src/libbar-1.1.0.tar.gz $src/libfoo-1.0.0.tar.gz 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: libbar/1.1.0 depends on (libfoo == 1.1.0)
                     command line requires (libbar == 1.1.0)
             info: available libfoo/1.0.0
             info: available libfoo/1.1.0
@@ -28615,7 +28618,7 @@ else
         $* libbar $d/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 depends on (libfoo == 1.1.0)
+            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
@@ -28634,7 +28637,7 @@ else
         $* $d/libbar-1.1.0/ $d/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 depends on (libfoo == 1.1.0)
+            info: libbar/1.1.0 depends on (libfoo == 1.1.0)
                     command line requires (libbar == 1.1.0)
             info: available libfoo/1.0.0
             info: available libfoo/1.1.0
diff --git a/tests/pkg-system.testscript b/tests/pkg-system.testscript
index 8aaabd9..11ed1cc 100644
--- a/tests/pkg-system.testscript
+++ b/tests/pkg-system.testscript
@@ -711,7 +711,7 @@ rep_remove += -d cfg 2>!
     $pkg_build foo 'sys:libbar/1' 2>>EOE != 0;
       error: unable to satisfy constraints on package libbar
         info: command line depends on (libbar == 1)
-        info: foo depends on (libbar >= 2)
+        info: foo/2 depends on (libbar >= 2)
         info: available sys:libbar/1
         info: available sys:libbar/2
         info: while satisfying foo/2