Fix unexpected 'manual configuration of dependents with prefer or require clauses is not yet supported' error (GH issue #302)

11 files changed, 458 insertions, 68 deletions

diff --git a/bpkg/pkg-build.cxx b/bpkg/pkg-build.cxx
index ed23ea5..9a73ea6 100644
--- a/bpkg/pkg-build.cxx
+++ b/bpkg/pkg-build.cxx
@@ -5699,10 +5699,9 @@ namespace bpkg
 
     // On the package reconfiguration we will try to resolve dependencies to
     // the same prerequisites (see pkg_configure() for details). For that, we
-    // will save prerequisites before disfiguring the dependents. Note,
-    // though, that this is not required for dependents with the collected
-    // prerequisites builds since the dependency alternatives are already
-    // selected for them.
+    // will save prerequisites before disfiguring a package. Note, though,
+    // that this is not required for the recursively collected packages since
+    // the dependency alternatives are already selected for them.
     //
     map<const build_package*, vector<package_name>> previous_prerequisites;
 
@@ -5727,7 +5726,7 @@ namespace bpkg
       bool external (false);
       if (!simulate)
       {
-        external = sp != nullptr && sp->external () && p.external ();
+        external = (sp != nullptr && sp->external () && p.external ());
 
         // Reset the keep_out flag if the package being unpacked is not
         // external.
@@ -5736,16 +5735,27 @@ namespace bpkg
           p.keep_out = false;
       }
 
-      if (*p.action != build_package::drop &&
-          !p.dependencies                  &&
-          !sp->prerequisites.empty ())
+      // Save prerequisites before disfiguring the package.
+      //
+      // Note that we add the prerequisites list to the map regardless if
+      // there are any prerequisites or not to, in particular, indicate the
+      // package reconfiguration mode to the subsequent
+      // pkg_configure_prerequisites() call (see the function documentation
+      // for details).
+      //
+      if (*p.action != build_package::drop && !p.dependencies && !p.system)
       {
         vector<package_name>& ps (previous_prerequisites[&p]);
 
-        ps.reserve (sp->prerequisites.size ());
+        assert (sp != nullptr); // Shouldn't be here otherwise.
+
+        if (!sp->prerequisites.empty ())
+        {
+          ps.reserve (sp->prerequisites.size ());
 
-        for (const auto& pp: sp->prerequisites)
-          ps.push_back (pp.first.object_id ());
+          for (const auto& pp: sp->prerequisites)
+            ps.push_back (pp.first.object_id ());
+        }
       }
 
       // For an external package being replaced with another external, keep
@@ -6195,12 +6205,6 @@ namespace bpkg
             info << "while configuring " << p.name () << p.db;
           }));
 
-      auto prereqs = [&p, &previous_prerequisites] ()
-      {
-        auto i (previous_prerequisites.find (&p));
-        return i != previous_prerequisites.end () ? &i->second : nullptr;
-      };
-
       configure_prerequisites_result cpr;
       if (p.system)
       {
@@ -6219,6 +6223,15 @@ namespace bpkg
       }
       else
       {
+        // Should only be called for packages whose prerequisites are saved.
+        //
+        auto prereqs = [&p, &previous_prerequisites] ()
+        {
+          auto i (previous_prerequisites.find (&p));
+          assert (i != previous_prerequisites.end ());
+          return &i->second;
+        };
+
         if (ap != nullptr)
         {
           assert (*p.action == build_package::build);
diff --git a/bpkg/pkg-configure.cxx b/bpkg/pkg-configure.cxx
index 291929b..5bb7dfa 100644
--- a/bpkg/pkg-configure.cxx
+++ b/bpkg/pkg-configure.cxx
@@ -51,11 +51,109 @@ namespace bpkg
     //
     assert (unconstrain_deps == nullptr || simulate);
 
+    // No use case for both being specified.
+    //
+    assert (alts == nullptr || prev_prereqs == nullptr);
+
     tracer_guard tg (db, trace);
 
     package_prerequisites prereqs;
     strings               vars;
 
+    // Notes on the buildfile clauses evaluation:
+    //
+    // - In the manual configuration mode (alts == NULL, prev_prereqs == NULL)
+    //   we always evaluate the enable and reflect clauses. We, however, fail
+    //   if any of the prefer or require clauses are specified in any of the
+    //   enabled dependency alternatives, assuming that this package didn't
+    //   negotiate its preferences/requirements for the dependency
+    //   configurations.
+    //
+    //   Note that evaluating the require and prefer clauses in this case is
+    //   meaningless since we don't reconfigure the dependencies nor negotiate
+    //   configurations with other dependents. What we should probably do is
+    //   load configurations of the dependencies and use them while evaluating
+    //   the dependent's enable and reflect clauses as we go along. Probably
+    //   we should still evaluate the accept clauses to make sure that the
+    //   dependency is configured acceptably for the dependent.
+    //
+    // - In the pre-selected alternatives mode (alts != NULL, prev_prereqs ==
+    //   NULL) we don't evaluate the enable, prefer, and require clauses since
+    //   they have already been evaluated as a part of the dependency
+    //   alternatives selection and the dependency configurations negotiation.
+    //   We, however always evaluate the reflect clauses.
+    //
+    // - In the reconfiguration mode (prev_prereqs != NULL, alts == NULL) we
+    //   don't evaluate the prefer and require clauses, assuming that was done
+    //   on some previous pkg-build run when this package and its dependencies
+    //   have been configured. But because of this we may not evaluate the
+    //   enable and reflect clauses which refer to dependency configuration
+    //   variables. Thus, for now we fail if such an enable or reflect clause
+    //   is encountered and evaluate all the enable and reflect clauses
+    //   otherwise. In the future it feels like such cases should somehow
+    //   be handled at the pkg-build level (e.g., detect such situations and
+    //   cause the dependent re-evaluation instead of re-configuration). Note
+    //   that if/when this is implemented, the current logic could still be
+    //   used as an optimization (reflection of dependency configuration is
+    //   not very common).
+    //
+    //   @@ It seems there is a hole in the reconfiguration mode:
+    //
+    //      foo -> libfoo {require {config.libfoo.proto=1}
+    //                     reflect {config.foo.reflect=1}} |
+    //             libfoo {require {config.libfoo.proto=2}
+    //                     reflect {config.foo.reflect=2}}
+    //          -> libbar {require {config.libfoo.proto=2}}
+    //          -> libbaz
+    //
+    //      foo is configured with config.foo.reflect=2
+    //
+    //      If afterwords libbaz is upgraded, the foo is
+    //      reconfigured with config.foo.reflect=1
+    //
+    //      It feels more and more that the proper solution here is to go
+    //      "nuclear" and to always "upgrade" re-configuration for a package
+    //      with any prefer/require into a complete re-evaluation. The most
+    //      likely drawback of this approach would be unnecessary
+    //      re-evaluations if we use the "any prefer/require in any
+    //      alternative" condition. So we may need to exclude some common
+    //      simple cases where such an upgrade is not needed. For example:
+    //
+    //      foo -> libfoo require {config.libfoo.proto=1}
+    //      foo -> libbar require {config.libbar.proto=1}
+    //      foo -> libbaz
+    //
+    //      Feels like the condition will have to be based on the presence
+    //      of clauses plus their textual analysis.
+    //
+    bool manual (alts == nullptr && prev_prereqs == nullptr);
+
+    // In the reconfiguration mode keep track of configuration variable
+    // prefixes (in the 'config.<dependency>.' form) for dependencies in the
+    // selected alternatives with the prefer or require clauses specified and
+    // fail if any enable or reflect clause refers to them.
+    //
+    // Note that the enable and reflect clauses may only refer to dependency
+    // configuration variables of already selected alternatives with the
+    // prefer or require clauses specified.
+    //
+    vector<string> banned_var_prefixes;
+
+    auto verify_banned_vars = [&ps,
+                               &banned_var_prefixes] (const string& clause,
+                                                      const char* what)
+    {
+      for (const string& p: banned_var_prefixes)
+      {
+        if (clause.find (p) != string::npos)
+        {
+          fail << "unable to reconfigure dependent " << ps.package.name
+               << " with " << what << " clause that refers to dependency "
+               << "configuration variables";
+        }
+      }
+    };
+
     // Alternatives argument must be parallel to the dependencies argument if
     // specified.
     //
@@ -75,18 +173,6 @@ namespace bpkg
                         size_t>,
                    2> edas;
 
-      // If the dependency alternatives are not pre-selected, then evaluate
-      // the enable clauses.
-      //
-      // Note that evaluating the require and prefer clauses in this case is
-      // meaningless since we don't reconfigure the dependencies nor negotiate
-      // configurations with other dependents. What we should probably do is
-      // load configurations of the dependencies and use them while evaluating
-      // the dependent's enable and reflect clauses as we go along. Probably
-      // we should still evaluate the accept clauses to make sure that the
-      // dependency is configured acceptably for the dependent. For now we
-      // fail and will support this maybe later.
-      //
       if (alts == nullptr)
       {
         if (toolchain_buildtime_dependency (o, das, &ps.package.name))
@@ -96,14 +182,23 @@ namespace bpkg
         {
           const dependency_alternative& da (das[i]);
 
-          if (!da.enable || ps.evaluate_enable (*da.enable, make_pair (di, i)))
+          // Evaluate the dependency alternative enable clause, if present,
+          // unless it refers to any banned variables in which case we fail.
+          //
+          if (da.enable)
           {
-            if (da.prefer || da.require)
-              fail << "manual configuration of dependents with prefer or "
-                   << "require clauses is not yet supported";
+            if (!banned_var_prefixes.empty ())
+              verify_banned_vars (*da.enable, "enable");
 
-            edas.push_back (make_pair (ref (da), i));
+            if (!ps.evaluate_enable (*da.enable, make_pair (di, i)))
+              continue;
           }
+
+          if (manual && (da.prefer || da.require))
+            fail << "manual configuration of dependents with prefer or "
+                 << "require clauses is not yet supported";
+
+          edas.push_back (make_pair (ref (da), i));
         }
 
         if (edas.empty ())
@@ -130,7 +225,9 @@ namespace bpkg
       //
       for (const vector<package_name>* pps (prev_prereqs);;)
       {
-        bool satisfied (false);
+        const pair<reference_wrapper<const dependency_alternative>,
+                   size_t>* selected_alt (nullptr);
+
         for (const auto& eda: edas)
         {
           const dependency_alternative& da (eda.first);
@@ -335,12 +432,7 @@ namespace bpkg
             }
           }
 
-          // Evaluate the dependency alternative reflect clause, if present.
-          //
-          if (da.reflect)
-            ps.evaluate_reflect (*da.reflect, make_pair (di, dai));
-
-          satisfied = true;
+          selected_alt = &eda;
           break;
         }
 
@@ -348,7 +440,7 @@ namespace bpkg
         // "recreate dependency decisions" mode. In the latter case fall back
         // to the "make dependency decisions" mode and retry.
         //
-        if (!satisfied)
+        if (selected_alt == nullptr)
         {
           if (pps != nullptr)
           {
@@ -359,6 +451,32 @@ namespace bpkg
           fail << "unable to satisfy dependency on " << das;
         }
 
+        const dependency_alternative& da (selected_alt->first);
+
+        // In the reconfiguration mode ban the usage of the selected
+        // alternative dependency configuration variables in the subsequent
+        // enable and reflect clauses.
+        //
+        if (alts == nullptr && !manual && (da.prefer || da.require))
+        {
+          for (const dependency& d: da)
+            banned_var_prefixes.push_back (
+              "config." + d.name.variable () + '.');
+        }
+
+        // Evaluate the selected dependency alternative reflect clause, if
+        // present, unless it refers to any banned variables in which case we
+        // fail.
+        //
+        if (da.reflect)
+        {
+          if (!banned_var_prefixes.empty ())
+            verify_banned_vars (*da.reflect, "reflect");
+
+          ps.evaluate_reflect (*da.reflect,
+                               make_pair (di, selected_alt->second));
+        }
+
         // The dependency alternative is selected and its dependencies are
         // resolved to the selected packages. So proceed to the next depends
         // value.
diff --git a/bpkg/pkg-configure.hxx b/bpkg/pkg-configure.hxx
index 24e06a3..876d11a 100644
--- a/bpkg/pkg-configure.hxx
+++ b/bpkg/pkg-configure.hxx
@@ -45,11 +45,10 @@ namespace bpkg
   // Given dependencies of a package, return its prerequisite packages,
   // configuration variables that resulted from selection of these
   // prerequisites (import, reflection, etc), and sources of the configuration
-  // variables resulted from evaluating the reflect clauses. See
-  // pkg_configure() for the semantics of the dependency list. Fail if for
-  // some of the dependency alternative lists there is no satisfactory
-  // alternative (all its dependencies are configured, satisfy the respective
-  // constraints, etc).
+  // variables resulted from evaluating the reflect clauses. Fail if for some
+  // of the dependency alternative lists there is no satisfactory alternative
+  // (all its dependencies are configured, satisfy the respective constraints,
+  // etc).
   //
   // The package dependency constraints are expected to be complete.
   //
@@ -62,12 +61,41 @@ namespace bpkg
   // be parallel to the dependencies argument and specify indexes of the
   // selected alternatives.
   //
-  // If prerequisites corresponding to the previous configured state of the
-  // package are specified, then for each depends value try to select an
-  // alternative where dependencies all belong to this list (the "recreate
+  // If the dependency alternatives are not pre-selected (alternatives ==
+  // NULL), then for each depends value select the first satisfactory
+  // alternative encountered. If, however, prerequisites corresponding to the
+  // previous configured state of the package are specified
+  // (prev_prerequisites != NULL), then for each depends value try to select
+  // an alternative where dependencies all belong to this list (the "recreate
   // dependency decisions" mode). Failed that, select an alternative as if no
   // prerequisites are specified (the "make dependency decisions" mode).
   //
+  // Note that there are actually 3 possible use cases for
+  // pkg_configure_prerequisites():
+  //
+  // - The package is being configured manually. In this case its dependency
+  //   alternatives are not pre-selected and there is no information about its
+  //   previous configured state (alternatives == NULL, prev_prerequisites ==
+  //   NULL).
+  //
+  // - The package is being built, upgraded, or re-evaluated. In this case its
+  //   dependency alternatives are pre-selected, their enable, prefer, and
+  //   require clauses are evaluated, and there is no need in the previous
+  //   configured state information (alternatives != NULL, prev_prerequisites
+  //   == NULL).
+  //
+  // - The package is being reconfigured for a reason other than any of the
+  //   mentioned above (dependency up/down-grade/reconfiguration, deorphaning,
+  //   pkg-build --disfigure is specified, etc). In this case its dependency
+  //   alternatives are not pre-selected but the previous configured state
+  //   information is provided (alternatives == NULL, prev_prerequisites !=
+  //   NULL).
+  //
+  // - There are no use cases when both dependency alternatives are
+  //   pre-selected and the previous configured state information needs to be
+  //   provided. Thus, alternatives and prev_prerequisites must never be both
+  //   NULL.
+  //
   // Optionally, remove constraints from the specified dependencies
   // (unconstrain_deps). Only allowed in the simulation mode.
   //
@@ -122,7 +150,7 @@ namespace bpkg
   //       required.
   //
   // Note: expects all the non-external packages to be configured to be
-  //       already unpackged (for subproject discovery).
+  //       already unpacked (for subproject discovery).
   //
   void
   pkg_configure (const common_options&,
diff --git a/tests/common/dependency-alternatives/t8a/libfoo-1.0.0.tar.gz b/tests/common/dependency-alternatives/t8a/libfoo-1.0.0.tar.gz
new file mode 100644
index 0000000..f2dcb15
--- /dev/null
+++ b/tests/common/dependency-alternatives/t8a/libfoo-1.0.0.tar.gz
diff --git a/tests/common/dependency-alternatives/t8a/libfoo-2.0.0.tar.gz b/tests/common/dependency-alternatives/t8a/libfoo-2.0.0.tar.gz
new file mode 100644
index 0000000..2b6f0f5
--- /dev/null
+++ b/tests/common/dependency-alternatives/t8a/libfoo-2.0.0.tar.gz
diff --git a/tests/common/dependency-alternatives/t8a/tax-1.0.0.tar.gz b/tests/common/dependency-alternatives/t8a/tax-1.0.0.tar.gz
new file mode 100644
index 0000000..7c2b99d
--- /dev/null
+++ b/tests/common/dependency-alternatives/t8a/tax-1.0.0.tar.gz
diff --git a/tests/common/dependency-alternatives/t8a/tex-1.0.0.tar.gz b/tests/common/dependency-alternatives/t8a/tex-1.0.0.tar.gz
new file mode 100644
index 0000000..663b09a
--- /dev/null
+++ b/tests/common/dependency-alternatives/t8a/tex-1.0.0.tar.gz
diff --git a/tests/common/dependency-alternatives/t8a/tix-1.0.0.tar.gz b/tests/common/dependency-alternatives/t8a/tix-1.0.0.tar.gz
new file mode 100644
index 0000000..34ea3da
--- /dev/null
+++ b/tests/common/dependency-alternatives/t8a/tix-1.0.0.tar.gz
diff --git a/tests/common/dependency-alternatives/t8a/tox-1.0.0.tar.gz b/tests/common/dependency-alternatives/t8a/tox-1.0.0.tar.gz
new file mode 100644
index 0000000..c767026
--- /dev/null
+++ b/tests/common/dependency-alternatives/t8a/tox-1.0.0.tar.gz
diff --git a/tests/common/dependency-alternatives/t8a/tux-1.0.0.tar.gz b/tests/common/dependency-alternatives/t8a/tux-1.0.0.tar.gz
new file mode 100644
index 0000000..e171e87
--- /dev/null
+++ b/tests/common/dependency-alternatives/t8a/tux-1.0.0.tar.gz
diff --git a/tests/pkg-build.testscript b/tests/pkg-build.testscript
index 945402b..80e83a2 100644
--- a/tests/pkg-build.testscript
+++ b/tests/pkg-build.testscript
@@ -146,17 +146,27 @@
 # |   |-- libbox-0.1.0.tar.gz
 # |   |-- libbox-0.1.1.tar.gz
 # |   |-- libbox-1.0.0.tar.gz
-# |   |-- foo-1.0.0.tar.gz      -> {libbar libbaz} ^1.0.0
-# |   |-- fox-1.0.0.tar.gz      -> libbar ^1.0.0 | libbaz ^1.0.0
-# |   |-- fix-1.0.0.tar.gz      -> libbaz ^1.0.0 | libbar ^1.0.0
+# |   |-- libfoo-1.0.0.tar.gz
+# |   |-- libfoo-2.0.0.tar.gz
+# |   |-- bar                   -> libbar
+# |   |-- baz                   -> libbaz
+# |   |-- box                   -> libbiz ^1.0.0 config.box.backend=libbiz |
+# |   |                            libbox >= 0.1.1 config.box.backend=libbox,
+# |   |                            libbaz
 # |   |-- fax-1.0.0.tar.gz      -> libbar ^1.0.0 ? ($cxx.target.class == 'windows') config.fax.backend=libbar |
 # |   |                            libbaz ^1.0.0 ? ($cxx.target.class != 'windows') config.fax.backend=libbaz,
 # |   |                            libbiz ? ($config.fax.libbiz) config.fax.extras='[b\i$z]',
 # |   |                            libbox ? ($config.fax.libbox && $config.fax.backend == libbaz && $config.fax.extras == '[b\i$z]')
+# |   |-- fix-1.0.0.tar.gz      -> libbaz ^1.0.0 | libbar ^1.0.0
+# |   |-- foo-1.0.0.tar.gz      -> {libbar libbaz} ^1.0.0
+# |   |-- fox-1.0.0.tar.gz      -> libbar ^1.0.0 | libbaz ^1.0.0
 # |   |-- fux                   -> libbiz ? (!$config.fux.libbiz_old) | libbiz ^0.1.0 ? ($config.fux.libbiz_old)
-# |   |-- box                   -> libbiz ^1.0.0 config.box.backend=libbiz |
-# |   |                            libbox >= 0.1.1 config.box.backend=libbox,
-# |   |                            libbaz
+# |   |-- tax                   -> libfoo == 1.0.0 | libfoo == 2.0.0
+# |   |-- tex                   -> libfoo {prefer{} accept(false) reflect {...}}
+# |   |-- tix                   -> libfoo >= 2.0.0 reflect {...} | libfoo >= 1.0.0 reflect {...}
+# |   |-- tox                   -> libfoo >= 2.0.0 {prefer{} accept(false) reflect {...}} | libfoo >= 1.0.0 reflect {...}
+# |   |-- tux                   -> libfoo {prefer{config.libfoo.protocol = "1"} accept(false)
+# |   |                         -> libbox ? (config.libfoo.protocol == "1")
 # |   `-- repositories.manifest
 # |
 # |-- t9
@@ -275,7 +285,7 @@
 # |   |-- bar-1.0.0.tar.gz      -> liba {prefer {...} accept (...)}
 # |   |-- baz-1.0.0.tar.gz      -> liba {prefer {...} accept (...)}
 # |   |-- biz-1.0.0.tar.gz      -> liba {prefer {...} accept (...)}
-# |   |-- box-1.0.0.tar.gz      -> liba {prefer {} accept (true) reflect (...)}
+# |   |-- box-1.0.0.tar.gz      -> liba {prefer {} accept (true) reflect {...}}
 # |   `-- repositories.manifest
 # |
 # |-- t13b
@@ -287,14 +297,14 @@
 # |
 # |-- t13c
 # |   |-- liba-0.1.0.tar.gz
-# |   |-- bar-1.0.0.tar.gz      -> liba {prefer {...} accept (...) reflect (...)}
+# |   |-- bar-1.0.0.tar.gz      -> liba {prefer {...} accept (...) reflect {...}}
 # |   |-- baz-1.0.0.tar.gz      -> liba {require {...}}
 # |   `-- repositories.manifest
 # |
 # |-- t13d
 # |   |-- liba-0.1.0.tar.gz
 # |   |-- libb-0.1.0.tar.gz
-# |   |-- bar-1.0.0.tar.gz      -> liba {prefer {...} accept (...) reflect (...)},
+# |   |-- bar-1.0.0.tar.gz      -> liba {prefer {...} accept (...) reflect {...}},
 # |   |                            libb ? (...)
 # |   |-- baz-1.0.0.tar.gz      -> bar, liba {require {...}}
 # |   `-- repositories.manifest
@@ -317,14 +327,14 @@
 # |-- t13g
 # |   |-- liba-0.1.0.tar.gz
 # |   |-- libb-0.1.0.tar.gz
-# |   |-- bar-1.0.0.tar.gz      -> liba {prefer {...} accept (...) reflect (...)},
-# |   |                            libb {prefer {...} accept (...) reflect (...)}
-# |   |-- baz-1.0.0.tar.gz      -> liba {prefer {...} accept (...) reflect (...)},
+# |   |-- bar-1.0.0.tar.gz      -> liba {prefer {...} accept (...) reflect {...}},
+# |   |                            libb {prefer {...} accept (...) reflect {...}}
+# |   |-- baz-1.0.0.tar.gz      -> liba {prefer {...} accept (...) reflect {...}},
 # |   |                            libb ? (...)
-# |   |-- biz-1.0.0.tar.gz      -> liba {prefer {...} accept (...) reflect (...)},
-# |   |                            libb {prefer {...} accept (...) reflect (...)}
-# |   |-- box-1.0.0.tar.gz      -> liba {prefer {...} accept (...) reflect (...)},
-# |   |                            libb {prefer {...} accept (...) reflect (...)}
+# |   |-- biz-1.0.0.tar.gz      -> liba {prefer {...} accept (...) reflect {...}},
+# |   |                            libb {prefer {...} accept (...) reflect {...}}
+# |   |-- box-1.0.0.tar.gz      -> liba {prefer {...} accept (...) reflect {...}},
+# |   |                            libb {prefer {...} accept (...) reflect {...}}
 # |   `-- repositories.manifest
 # |
 # |-- t13h
@@ -335,7 +345,7 @@
 # |
 # |-- t13i
 # |   |-- liba-0.1.0.tar.gz
-# |   |-- bar-1.0.0.tar.gz      -> liba {require {...} reflect (...)}
+# |   |-- bar-1.0.0.tar.gz      -> liba {require {...} reflect {...}}
 # |   `-- repositories.manifest
 # |
 # |-- t13j
@@ -5023,6 +5033,227 @@ test.arguments += --sys-no-query
         $pkg_drop fax
       }
     }
+
+    : reconfigure-dependent
+    :
+    : Test some cases when a dependent needs to be reconfigured due to an
+    : upgraded dependency.
+    :
+    {
+      +$clone_cfg
+
+      test.arguments += --yes
+
+      : keep-alternative
+      :
+      {
+        $clone_cfg;
+
+        $* tax ?libfoo/1.0.0 2>!;
+
+        $pkg_status -r >>EOO;
+          !tax configured 1.0.0
+            libfoo configured !1.0.0 available 2.0.0
+          EOO
+
+        $* --upgrade --recursive 2>!; # Noop.
+
+        $pkg_status -r >>EOO;
+          !tax configured 1.0.0
+            libfoo configured !1.0.0 available 2.0.0
+          EOO
+
+        $* ?libfoo 2>!;               # Noop.
+
+        $pkg_status -r >>EOO;
+          !tax configured 1.0.0
+            libfoo configured !1.0.0 available 2.0.0
+          EOO
+
+        $* ?libfoo/2.0.0 2>>EOE != 0;
+          error: package libfoo doesn't satisfy its dependents
+            info: libfoo/2.0.0 doesn't satisfy tax/1.0.0
+          EOE
+
+        $pkg_status -r >>EOO;
+          !tax configured 1.0.0
+            libfoo configured !1.0.0 available 2.0.0
+          EOO
+
+        $pkg_drop tax
+      }
+
+      : re-evaluate-dependent
+      :
+      {
+        $clone_cfg;
+
+        $* tex ?libfoo/1.0.0 2>!;
+
+        $pkg_status -r >>EOO;
+          !tex configured 1.0.0
+            libfoo configured !1.0.0 available 2.0.0
+          EOO
+
+        cat cfg/tex-1.0.0/build/config.build >>~%EOO%;
+          %.*
+          config.tex.libfoo_protocol = 1
+          %.*
+          EOO
+
+        # @@ Strangely, if upgrade with -ur instead of ?libfoo, then status
+        #    prints 'libfoo configured !2.0.0' instead of
+        #    'libfoo configured 2.0.0'.
+        #
+        $* ?libfoo 2>>~%EOE%;
+          disfigured tex/1.0.0
+          disfigured libfoo/1.0.0
+          fetched libfoo/2.0.0
+          unpacked libfoo/2.0.0
+          configured libfoo/2.0.0
+          configured tex/1.0.0
+          %info: .+tex-1.0.0.+ is up to date%
+          updated tex/1.0.0
+          EOE
+
+        cat cfg/tex-1.0.0/build/config.build >>~%EOO%;
+          %.*
+          config.tex.libfoo_protocol = 2
+          %.*
+          EOO
+
+        $pkg_status -r >>EOO;
+          !tex configured 1.0.0
+            libfoo configured 2.0.0
+          EOO
+
+        $pkg_drop tex
+      }
+
+      : re-evaluate-reflect
+      :
+      {
+        $clone_cfg;
+
+        # @@ The fact that `$* tix` fails as follows but `$* tix ?libfoo`
+        #    doesn't looks confusing:
+        #
+        #    error: unable to select dependency alternative for package tix/1.0.0
+        #      info: explicitly specify dependency packages to manually select the alternative
+        #      info: alternative: libfoo
+        #      info: alternative: libfoo
+        #    info: while satisfying tix/1.0.0
+        #
+        #    Note:
+        #
+        #    tix -> libfoo>=2.0.0 reflect{...} | libfoo>=1.0.0 reflect{...}
+        #
+        $* tix ?libfoo/1.0.0 2>!;
+
+        $pkg_status -r >>EOO;
+          !tix configured 1.0.0
+            libfoo configured !1.0.0 available 2.0.0
+          EOO
+
+        cat cfg/tix-1.0.0/build/config.build >>~%EOO%;
+          %.*
+          config.tix.reflect = 1
+          %.*
+          EOO
+
+        $* ?libfoo 2>>~%EOE%;
+          disfigured tix/1.0.0
+          disfigured libfoo/1.0.0
+          fetched libfoo/2.0.0
+          unpacked libfoo/2.0.0
+          configured libfoo/2.0.0
+          configured tix/1.0.0
+          %info: .+tix-1.0.0.+ is up to date%
+          updated tix/1.0.0
+          EOE
+
+        $pkg_status -r >>EOO;
+          !tix configured 1.0.0
+            libfoo configured 2.0.0
+          EOO
+
+        cat cfg/tix-1.0.0/build/config.build >>~%EOO%;
+          %.*
+          config.tix.reflect = 2
+          %.*
+          EOO
+
+        $pkg_drop tix
+      }
+
+      : fail-select-alt-with-reflect
+      :
+      {
+        $clone_cfg;
+
+        $* tox ?libfoo/1.0.0 2>!;
+
+        $pkg_status -r >>EOO;
+          !tox configured 1.0.0
+            libfoo configured !1.0.0 available 2.0.0
+          EOO
+
+        cat cfg/tox-1.0.0/build/config.build >>~%EOO%;
+          %.*
+          config.tox.libfoo_protocol = '1 or 2'
+          %.*
+          EOO
+
+        # Note that the current behavior should actually be considered as a
+        # bug which we will fix eventually. The proper behaviour would be to
+        # re-evaluate this dependent rather than just to re-configure.
+        #
+        $* ?libfoo 2>>~%EOE% != 0;
+          error: unable to reconfigure dependent tox with reflect clause that refers to dependency configuration variables
+          info: while configuring tox
+          EOE
+
+        $pkg_status -r >>EOO;
+          !tox configured 1.0.0
+            libfoo configured !1.0.0 available 2.0.0
+          EOO
+
+        cat cfg/tox-1.0.0/build/config.build >>~%EOO%;
+          %.*
+          config.tox.libfoo_protocol = '1 or 2'
+          %.*
+          EOO
+
+        $pkg_drop tox
+      }
+
+      : fail-enable-banned-var
+      :
+      {
+        $clone_cfg;
+
+        $* tux ?libbox/0.1.0 2>!;
+
+        $pkg_status -r >>EOO;
+          !tux configured 1.0.0
+            libbox configured !0.1.0 available 1.0.0 0.1.1
+            libfoo configured 2.0.0
+          EOO
+
+        $* ?libbox 2>>EOE != 0;
+          error: unable to reconfigure dependent tux with enable clause that refers to dependency configuration variables
+          info: while configuring tux
+          EOE
+
+        $pkg_status -r >>EOO;
+          !tux configured 1.0.0
+            libbox configured !0.1.0 available 1.0.0 0.1.1
+            libfoo configured 2.0.0
+          EOO
+
+        $pkg_drop tux
+      }
+    }
   }
 
   : version-replacement