Cleanup group "see through" design

15 files changed, 350 insertions, 316 deletions

diff --git a/build/algorithm b/build/algorithm
index 2547466..1cfd15b 100644
--- a/build/algorithm
+++ b/build/algorithm
@@ -60,25 +60,38 @@ namespace build
   void
   match (action, target&);
 
-  // The default prerequisite search and match implementation. It calls
+  // The standard prerequisite search and match implementations. They call
+  // search_and_match_*() versions below passing non-empty directory for
+  // the clean operation.
+  //
+  void
+  search_and_match_prerequisites (action, target&);
+
+  // If we are cleaning, this function doesn't go into group members,
+  // as an optimization (the group should clean everything up).
+  //
+  void
+  search_and_match_prerequisite_members (action, target&);
+
+  // The actual prerequisite search and match implementations. They call
   // search() and then match() for each prerequisite in a loop. If this
-  // target is a member of a group, then it first does this to the group's
+  // target is a member of a group, then they first do this to the group's
   // prerequisites.
   //
+  // If the directory argument is not empty, then they ignore (do not
+  // match) prerequisites that are not in the same or its subdirectory.
+  //
   void
-  search_and_match (action, target&);
+  search_and_match_prerequisites (action, target&, const dir_path&);
 
-  // As above but ignores (does not match) prerequisites that are not
-  // in the same or a subdirectory of dir.
-  //
   void
-  search_and_match (action, target&, const dir_path&);
+  search_and_match_prerequisite_members (action, target&, const dir_path&);
 
   // Unless already available, match, and, if necessary, execute
   // the group in order to obtain its members list.
   //
   group_view
-  resolve_group_members (action, target_group&);
+  resolve_group_members (action, target&);
 
   // Inject dependency on the parent directory's fsdir{}, unless it is
   // the project's out_root (or is outside of any project; say, for
diff --git a/build/algorithm.cxx b/build/algorithm.cxx
index 5c7805f..40d4b0b 100644
--- a/build/algorithm.cxx
+++ b/build/algorithm.cxx
@@ -175,7 +175,7 @@ namespace build
   }
 
   group_view
-  resolve_group_members_impl (action a, target_group& g)
+  resolve_group_members_impl (action a, target& g)
   {
     group_view r;
 
@@ -186,7 +186,7 @@ namespace build
     {
       auto p (match_impl (a, g, false));
 
-      r = g.members (a);
+      r = g.group_members (a);
       if (r.members != nullptr)
         return r;
 
@@ -196,42 +196,48 @@ namespace build
       g.recipe (a, p.first->apply (a, g, p.second));
     }
 
-    // Note that we use execute_direct() rather than execute() here
-    // to sidestep the dependents count logic. In this context,
-    // this is by definition the first attempt to execute this
-    // rule (otherwise we would have already known the members
-    // list) and we really do need to execute it now.
+    // Note that we use execute_direct() rather than execute() here to
+    // sidestep the dependents count logic. In this context, this is by
+    // definition the first attempt to execute this rule (otherwise we
+    // would have already known the members list) and we really do need
+    // to execute it now.
     //
     execute_direct (a, g);
 
-    r = g.members (a);
+    r = g.group_members (a);
     assert (r.members != nullptr); // What "next step" did the group expect?
     return r;
   }
 
   void
-  search_and_match (action a, target& t)
+  search_and_match_prerequisites (action a, target& t, const dir_path& d)
   {
-    group_prerequisites gp (t);
-    size_t i (t.prerequisite_targets.size ());
-    t.prerequisite_targets.resize (gp.size () + i);
+    const bool e (d.empty ());
 
-    for (prerequisite& p: gp)
+    for (prerequisite p: group_prerequisites (t))
     {
       target& pt (search (p));
-      match (a, pt);
-      t.prerequisite_targets[i++] = &pt;
+
+      if (e || pt.dir.sub (d))
+      {
+        match (a, pt);
+        t.prerequisite_targets.push_back (&pt);
+      }
     }
   }
 
   void
-  search_and_match (action a, target& t, const dir_path& d)
+  search_and_match_prerequisite_members (action a,
+                                         target& t,
+                                         const dir_path& d)
   {
-    for (prerequisite& p: group_prerequisites (t))
+    const bool e (d.empty ());
+
+    for (prerequisite_member p: group_prerequisite_members (a, t))
     {
-      target& pt (search (p));
+      target& pt (p.search ());
 
-      if (pt.dir.sub (d))
+      if (e || pt.dir.sub (d))
       {
         match (a, pt);
         t.prerequisite_targets.push_back (&pt);
diff --git a/build/algorithm.ixx b/build/algorithm.ixx
index bd1f9c7..ddd63f6 100644
--- a/build/algorithm.ixx
+++ b/build/algorithm.ixx
@@ -58,15 +58,35 @@ namespace build
   }
 
   group_view
-  resolve_group_members_impl (action, target_group&);
+  resolve_group_members_impl (action, target&);
 
   inline group_view
-  resolve_group_members (action a, target_group& g)
+  resolve_group_members (action a, target& g)
   {
-    group_view r (g.members (a));
+    group_view r (g.group_members (a));
     return r.members != nullptr ? r : resolve_group_members_impl (a, g);
   }
 
+  inline void
+  search_and_match_prerequisites (action a, target& t)
+  {
+    search_and_match_prerequisites (
+      a, t, a.operation () != clean_id ? dir_path () : t.dir);
+  }
+
+  inline void
+  search_and_match_prerequisite_members (action a, target& t)
+  {
+    if (a.operation () != clean_id)
+      search_and_match_prerequisite_members (a, t, dir_path ());
+    else
+      // Note that here we don't iterate over members even for see
+      // through groups since the group target should clean eveything
+      // up. A bit of an optimization.
+      //
+      search_and_match_prerequisites (a, t, t.dir);
+  }
+
   target_state
   execute_impl (action, target&);
 
diff --git a/build/bin/rule.cxx b/build/bin/rule.cxx
index d2928ca..0fc0e40 100644
--- a/build/bin/rule.cxx
+++ b/build/bin/rule.cxx
@@ -34,7 +34,7 @@ namespace build
     // lib
     //
     // The whole logic is pretty much as if we had our two group
-    // members as prerequisites.
+    // members as our prerequisites.
     //
     match_result lib_rule::
     match (action, target& t, const std::string&) const
diff --git a/build/bin/target.cxx b/build/bin/target.cxx
index 66eadb7..1190bfc 100644
--- a/build/bin/target.cxx
+++ b/build/bin/target.cxx
@@ -29,7 +29,8 @@ namespace build
       &file::static_type,
       &obja_factory,
       nullptr,
-      &search_file
+      &search_file,
+      false
     };
 
     static target*
@@ -51,7 +52,8 @@ namespace build
       &file::static_type,
       &objso_factory,
       nullptr,
-      &search_file
+      &search_file,
+      false
     };
 
     static target*
@@ -77,7 +79,8 @@ namespace build
       &target::static_type,
       &obj_factory,
       nullptr,
-      &search_target
+      &search_target,
+      false
     };
 
     const target_type exe::static_type
@@ -87,7 +90,8 @@ namespace build
       &file::static_type,
       &target_factory<exe>,
       nullptr,
-      &search_file
+      &search_file,
+      false
     };
 
     static target*
@@ -109,7 +113,8 @@ namespace build
       &file::static_type,
       &liba_factory,
       nullptr,
-      &search_file
+      &search_file,
+      false
     };
 
     static target*
@@ -131,7 +136,8 @@ namespace build
       &file::static_type,
       &libso_factory,
       nullptr,
-      &search_file
+      &search_file,
+      false
     };
 
     static target*
@@ -157,7 +163,8 @@ namespace build
       &target::static_type,
       &lib_factory,
       nullptr,
-      &search_target
+      &search_target,
+      false
     };
   }
 }
diff --git a/build/cli/rule.cxx b/build/cli/rule.cxx
index 09ef4f3..2d001c4 100644
--- a/build/cli/rule.cxx
+++ b/build/cli/rule.cxx
@@ -163,15 +163,9 @@ namespace build
         //
         inject_parent_fsdir (a, t);
 
-        // Search and match prerequisites.
+        // Search and match prerequisite members.
         //
-        switch (a.operation ())
-        {
-        case default_id:
-        case update_id: search_and_match (a, t); break;
-        case clean_id: search_and_match (a, t, t.dir); break;
-        default: assert (false); // We didn't register for this.
-        }
+        search_and_match_prerequisite_members (a, t);
 
         switch (a)
         {
diff --git a/build/cli/target b/build/cli/target
index e5bf16d..e72d4e0 100644
--- a/build/cli/target
+++ b/build/cli/target
@@ -23,10 +23,10 @@ namespace build
       static const target_type static_type;
     };
 
-    class cli_cxx: public target_group
+    class cli_cxx: public target
     {
     public:
-      using target_group::target_group;
+      using target::target;
 
       target* m[3] {nullptr, nullptr, nullptr};
 
@@ -39,7 +39,7 @@ namespace build
       void i (cxx::ixx& t) {m[2] = &t;}
 
       virtual group_view
-      members (action) const;
+      group_members (action) const;
 
     public:
       virtual const target_type& type () const {return static_type;}
diff --git a/build/cli/target.cxx b/build/cli/target.cxx
index 2854818..aded1ff 100644
--- a/build/cli/target.cxx
+++ b/build/cli/target.cxx
@@ -20,13 +20,14 @@ namespace build
       &file::static_type,
       &target_factory<cli>,
       &target_extension_fix<cli_ext>,
-      &search_file
+      &search_file,
+      false
     };
 
     // cli.cxx
     //
     group_view cli_cxx::
-    members (action) const
+    group_members (action) const
     {
       return m[0] != nullptr
         ? group_view {m, (m[2] != nullptr ? 3U : 2U)}
@@ -37,10 +38,11 @@ namespace build
     {
       typeid (cli_cxx),
       "cli.cxx",
-      &target_group::static_type,
+      &target::static_type,
       &target_factory<cli_cxx>,
       nullptr,
-      &search_target
+      &search_target,
+      true // See through default semantics.
     };
   }
 }
diff --git a/build/cxx/rule.cxx b/build/cxx/rule.cxx
index 84e2f91..0812922 100644
--- a/build/cxx/rule.cxx
+++ b/build/cxx/rule.cxx
@@ -128,19 +128,8 @@ namespace build
       // When cleaning, ignore prerequisites that are not in the same
       // or a subdirectory of ours.
       //
-      const auto& ps (group_prerequisite_members (a, t));
-      for (auto i (ps.begin ()); i != ps.end (); ++i)
+      for (prerequisite_member p: group_prerequisite_members (a, t))
       {
-        prerequisite_member p (*i);
-
-        // See through the group unless it is one that we recognize.
-        //
-        if (p.is_a<target_group> ())
-        {
-          if (!p.is_a<lib> ())
-            continue;
-        }
-
         target& pt (p.search ());
 
         if (a.operation () == clean_id && !pt.dir.sub (t.dir))
@@ -155,10 +144,7 @@ namespace build
         // populated; see append_lib_options() above.
         //
         if (pt.is_a<lib> () || pt.is_a<liba> () || pt.is_a<libso> ())
-        {
-          i.skip_group (); // Don't go inside the lib{} group.
           continue;
-        }
 
         t.prerequisite_targets.push_back (&pt);
       }
@@ -855,11 +841,8 @@ namespace build
       bool seen_cxx (false), seen_c (false), seen_obj (false),
         seen_lib (false);
 
-      const auto& ps (group_prerequisite_members (a, t));
-      for (auto i (ps.begin ()); i != ps.end (); ++i)
+      for (prerequisite_member p: group_prerequisite_members (a, t))
       {
-        prerequisite_member p (*i);
-
         if (p.is_a<cxx> ())
         {
           seen_cxx = seen_cxx || true;
@@ -880,14 +863,12 @@ namespace build
                  p.is_a<obj> ())
         {
           seen_obj = seen_obj || true;
-          i.skip_group (); // Don't go inside the obj{} group.
         }
         else if (p.is_a<liba> ()  ||
                  p.is_a<libso> () ||
                  p.is_a<lib> ())
         {
           seen_lib = seen_lib || true;
-          i.skip_group (); // Don't go inside the lib{} group.
         }
         else if (p.is_a<h> ()   ||
                  p.is_a<hxx> () ||
@@ -895,8 +876,6 @@ namespace build
                  p.is_a<txx> () ||
                  p.is_a<fsdir> ())
           ;
-        else if (p.is_a<target_group> ())
-          ; // See through.
         else
         {
           level3 ([&]{trace << "unexpected prerequisite type " << p.type ();});
@@ -953,22 +932,9 @@ namespace build
       // Process prerequisites: do rule chaining for C and C++ source
       // files as well as search and match.
       //
-      const auto& ps (group_prerequisite_members (a, t));
-      for (auto i (ps.begin ()); i != ps.end (); ++i)
+      for (prerequisite_member p: group_prerequisite_members (a, t))
       {
-        prerequisite_member p (*i);
-
-        // See through the group unless it is one that we recognize.
-        //
-        if (p.is_a<target_group> ())
-        {
-          if (!p.is_a<lib> () &&
-              !p.is_a<obj> ())
-            continue;
-        }
-
         bool group (!p.prerequisite.belongs (t)); // Group's prerequisite.
-
         target* pt (nullptr);
 
         if (!p.is_a<c> () && !p.is_a<cxx> ())
@@ -982,7 +948,6 @@ namespace build
 
           // If this is the obj{} or lib{} target group, then pick the
           // appropriate member and make sure it is searched and matched.
-          // In both cases, skip going over the group's members.
           //
           if (obj* o = pt->is_a<obj> ())
           {
@@ -991,8 +956,6 @@ namespace build
             if (pt == nullptr)
               pt = &search (so ? objso::static_type : obja::static_type,
                             p.key ());
-
-            i.skip_group ();
           }
           else if (lib* l = pt->is_a<lib> ())
           {
@@ -1028,8 +991,6 @@ namespace build
             if (pt == nullptr)
               pt = &search (lso ? libso::static_type : liba::static_type,
                             p.key ());
-
-            i.skip_group ();
           }
 
           build::match (a, *pt);
@@ -1120,19 +1081,9 @@ namespace build
         // searching and matching speculatively doesn't really hurt.
         //
         bool found (false);
-        const auto& ps (reverse_group_prerequisite_members (a, *pt));
-        for (auto i (ps.begin ()); i != ps.end (); ++i)
+        for (prerequisite_member p1:
+               reverse_group_prerequisite_members (a, *pt))
         {
-          prerequisite_member p1 (*i);
-
-          // See through the group unless it is one that we recognize.
-          //
-          if (p1.is_a<target_group> ())
-          {
-            if (!p1.is_a<lib> ())
-              continue;
-          }
-
           // Ignore some known target types (fsdir, headers, libraries).
           //
           if (p1.is_a<fsdir> () ||
@@ -1144,7 +1095,6 @@ namespace build
               p1.is_a<liba> () ||
               p1.is_a<libso> ())
           {
-            i.skip_group (); // Skip going inside lib{}.
             continue;
           }
 
diff --git a/build/cxx/target.cxx b/build/cxx/target.cxx
index 9fd5487..c23fdd0 100644
--- a/build/cxx/target.cxx
+++ b/build/cxx/target.cxx
@@ -18,7 +18,8 @@ namespace build
       &file::static_type,
       &target_factory<hxx>,
       &target_extension_var<hxx_ext_var>,
-      &search_file
+      &search_file,
+      false
     };
 
     constexpr const char ixx_ext_var[] = "ixx.ext";
@@ -29,7 +30,8 @@ namespace build
       &file::static_type,
       &target_factory<ixx>,
       &target_extension_var<ixx_ext_var>,
-      &search_file
+      &search_file,
+      false
     };
 
     constexpr const char txx_ext_var[] = "txx.ext";
@@ -40,7 +42,8 @@ namespace build
       &file::static_type,
       &target_factory<txx>,
       &target_extension_var<txx_ext_var>,
-      &search_file
+      &search_file,
+      false
     };
 
     constexpr const char cxx_ext_var[] = "cxx.ext";
@@ -51,7 +54,8 @@ namespace build
       &file::static_type,
       &target_factory<cxx>,
       &target_extension_var<cxx_ext_var>,
-      &search_file
+      &search_file,
+      false
     };
 
     constexpr const char h_ext_var[] = "h.ext";
@@ -62,7 +66,8 @@ namespace build
       &file::static_type,
       &target_factory<h>,
       &target_extension_var<h_ext_var>,
-      &search_file
+      &search_file,
+      false
     };
 
     constexpr const char c_ext_var[] = "c.ext";
@@ -73,7 +78,8 @@ namespace build
       &file::static_type,
       &target_factory<c>,
       &target_extension_var<c_ext_var>,
-      &search_file
+      &search_file,
+      false
     };
   }
 }
diff --git a/build/rule.cxx b/build/rule.cxx
index e834426..ce8ef01 100644
--- a/build/rule.cxx
+++ b/build/rule.cxx
@@ -80,7 +80,7 @@ namespace build
 
     // Search and match all the prerequisites.
     //
-    search_and_match (a, t);
+    search_and_match_prerequisites (a, t);
 
     return a == perform_update_id
       ? &perform_update
@@ -134,18 +134,7 @@ namespace build
   recipe dir_rule::
   apply (action a, target& t, const match_result&) const
   {
-    // When cleaning, ignore prerequisites that are not in the same
-    // or a subdirectory of ours. For default, we don't do anything
-    // other than letting our prerequisites do their thing.
-    //
-    switch (a.operation ())
-    {
-    case default_id:
-    case update_id: search_and_match (a, t); break;
-    case clean_id: search_and_match (a, t, t.dir); break;
-    default: assert (false);
-    }
-
+    search_and_match_prerequisites (a, t);
     return default_recipe;
   }
 
@@ -162,26 +151,30 @@ namespace build
   {
     switch (a.operation ())
     {
-    // For default, we don't do anything other than letting our
-    // prerequisites do their thing.
-    //
     case default_id:
     case update_id:
+      // For default, we don't do anything other than letting our
+      // prerequisites do their thing.
+      //
+
       // Inject dependency on the parent directory. Note that we
       // don't do it for clean since we shouldn't be removing it.
       //
       inject_parent_fsdir (a, t);
 
-      search_and_match (a, t);
+      search_and_match_prerequisites (a, t, dir_path ());
       break;
-    // For clean, ignore prerequisites that are not in the same or a
-    // subdirectory of ours (if t.dir is foo/bar/, then "we" are bar
-    // and our directory is foo/). Just meditate on it a bit and you
-    // will see the light.
-    //
+
     case clean_id:
-      search_and_match (a, t, t.dir.root () ? t.dir : t.dir.directory ());
+      // For clean, ignore prerequisites that are not in the same or a
+      // subdirectory of ours (if t.dir is foo/bar/, then "we" are bar
+      // and our directory is foo/). Just meditate on it a bit and you
+      // will see the light.
+      //
+      search_and_match_prerequisites (
+        a, t, t.dir.root () ? t.dir : t.dir.directory ());
       break;
+
     default:
       assert (false);
     }
diff --git a/build/target b/build/target
index b86ea94..9587c6a 100644
--- a/build/target
+++ b/build/target
@@ -30,7 +30,6 @@ namespace build
 {
   class scope;
   class target;
-  class target_group;
 
   target&
   search (prerequisite&); // From <build/algorithm>.
@@ -99,6 +98,14 @@ namespace build
     belongs (const target&) const;
   };
 
+  // A view of target group members.
+  //
+  struct group_view
+  {
+    target* const* members; // NULL means not yet known.
+    std::size_t count;
+  };
+
   // Target.
   //
   class target
@@ -118,13 +125,52 @@ namespace build
     const std::string* ext;  // Extension, NULL means unspecified,
                              // empty means no extension.
 
-    //@@ Make target_group.
-    target* group {nullptr}; // Target group to which this target belongs,
-                             // if any. Note that we assume that the group
-                             // and all its members are in the same scope
-                             // (see, for example, variable lookup).
-                             // We also currently assume that there are
-                             // no multi-level groups.
+    // Target group to which this target belongs, if any. Note that
+    // we assume that the group and all its members are in the same
+    // scope (for example, in variable lookup). We also don't support
+    // nested groups.
+    //
+    // The semantics of the interaction between the group and its
+    // members and what it means to, say, update the group, is
+    // unspecified and determined by the group's type. In particular,
+    // a group can be created out of member types that have no idea
+    // they are part of this group (e.g., cli.cxx{}).
+    //
+    // Normally, however, there are two kinds of groups: "alternatives"
+    // and "combination". In an alternatives group, normally one of the
+    // members is selected when the group is mentioned as a prerequisite
+    // with, perhaps, an exception for special rules, like aliases, where
+    // it makes more sense to treat the group as a whole. In this case we
+    // say that the rule "semantically recognizes" the group and picks
+    // some of its members.
+    //
+    // Updating an alternative group as a whole can mean updating some
+    // subset of its members (e.g., lib{}). Or the group may not support
+    // this at all (e.g., obj{}).
+    //
+    // In a combination group, when a group is updated, normally all
+    // members are updates (and usually with a single command), though
+    // there could be some members that are omitted, depending on the
+    // configuration (e.g., an inline file not/being generated). When
+    // a combination group is mentioned as a prerequisite, the rule
+    // is usually interested in the individual members rather than
+    // the whole group. For example, a C++ compile rule would like to
+    // "see" the ?xx{} members when it gets a cli.cxx{} group.
+    //
+    // Which brings us to the group iteration mode. The target type
+    // contains a member called see_through that indicates whether the
+    // default iteration mode for the group should be "see through";
+    // that is, whether we see the members or the group itself. For
+    // the iteration support itself, see the *_prerequisite_members()
+    // machinery below.
+    //
+    target* group {nullptr};
+
+    // You should not call this function directly; rather use
+    // resolve_group_members() from <build/algorithm>.
+    //
+    virtual group_view
+    group_members (action) const;
 
     target_key
     key () const {return target_key {&type (), &dir, &name, &ext};}
@@ -400,110 +446,6 @@ namespace build
     target& t_;
   };
 
-  //
-  //
-  struct target_set
-  {
-    typedef std::map<target_key, std::unique_ptr<target>> map;
-    typedef butl::map_iterator_adapter<map::const_iterator> iterator;
-
-    iterator
-    find (const target_key& k, tracer& trace) const;
-
-    iterator
-    find (const target_type& type,
-          const dir_path& dir,
-          const std::string& name,
-          const std::string* ext,
-          tracer& trace) const
-    {
-      return find (target_key {&type, &dir, &name, &ext}, trace);
-    }
-
-    // As above but ignore the extension and return the target or
-    // nullptr instead of the iterator.
-    //
-    template <typename T>
-    T*
-    find (const dir_path& dir, const std::string& name) const
-    {
-      const std::string* e (nullptr);
-      auto i (map_.find (target_key {&T::static_type, &dir, &name, &e}));
-      return i != map_.end () ? static_cast<T*> (i->second.get ()) : nullptr;
-    }
-
-    iterator begin () const {return map_.begin ();}
-    iterator end () const {return map_.end ();}
-
-    std::pair<target&, bool>
-    insert (const target_type&,
-            dir_path dir,
-            std::string name,
-            const std::string* ext,
-            tracer&);
-
-    template <typename T>
-    T&
-    insert (const dir_path& dir, const std::string& name, tracer& t)
-    {
-      return static_cast<T&> (
-        insert (T::static_type, dir, name, nullptr, t).first);
-    }
-
-    void
-    clear () {map_.clear ();}
-
-  private:
-    map map_;
-  };
-
-  extern target_set targets;
-
-  using target_type_map_base = std::map<
-    const char*,
-    std::reference_wrapper<const target_type>,
-    butl::compare_c_string>;
-
-  class target_type_map: public target_type_map_base
-  {
-  public:
-    void
-    insert (const target_type& tt) {emplace (tt.name, tt);}
-
-    using target_type_map_base::find;
-
-    // Given a name, figure out its type, taking into account extensions,
-    // special names (e.g., '.' and '..'), or anything else that might be
-    // relevant. Also process the name (in place) by extracting the
-    // extension, adjusting dir/value, etc (note that the dir is not
-    // necessarily normalized). Return NULL if not found.
-    //
-    const target_type*
-    find (name&, const std::string*& ext) const;
-  };
-
-  extern target_type_map target_types;
-
-  // Target group.
-  //
-  struct group_view
-  {
-    target* const* members; // NULL means not yet known.
-    std::size_t count;
-  };
-
-  class target_group: public target
-  {
-  public:
-    using target::target;
-
-    virtual group_view
-    members (action) const = 0;
-
-  public:
-    static const target_type static_type;
-  };
-
   // A member of a prerequisite. If 'target' is NULL, then this is the
   // prerequisite itself. Otherwise, it is its member. In this case
   // 'prerequisite' still refers to the prerequisite.
@@ -557,10 +499,11 @@ namespace build
 
   // A "range" that presents a sequence of prerequisites (e.g., from
   // group_prerequisites()) as a sequence of prerequisite_member's. For
-  // each prerequisite you will first "see" the prerequisite itself
-  // followed by all its members, if it resolves to a target group.
-  // You can skip the group members with the skip_group() iterator
-  // function. Usage:
+  // each group prerequisite you will "see" either the prerequisite
+  // itself or all its members, depending on the default iteration
+  // mode of the target group type. You can skip the rest of the
+  // group members with leave_group() and you can force iteration
+  // over the members with enter_group(). Usage:
   //
   // for (prerequisite_member pm: prerequisite_members (a, ...))
   //
@@ -593,31 +536,35 @@ namespace build
   {
   public:
     prerequisite_members_range (action a, T&& r)
-        : a_ (a), r_ (std::forward<T> (r)) {}
+        : a_ (a), r_ (std::forward<T> (r)), e_ (r_.end ()) {}
+
+    using base_iterator = decltype (std::declval<T> ().begin ());
 
     struct iterator
     {
-      using base_iterator = decltype (std::declval<T> ().begin ());
-
       typedef prerequisite_member value_type;
       typedef const value_type* pointer;
       typedef const value_type& reference;
       typedef typename base_iterator::difference_type difference_type;
       typedef std::forward_iterator_tag iterator_category;
 
-      iterator (): a_ (0, 0) {}
-      iterator (action a, base_iterator i): a_ (a), i_ (i), g_ {nullptr, 0} {}
+      iterator (): r_ (nullptr) {}
+      iterator (const prerequisite_members_range* r, const base_iterator& i)
+          : r_ (r), i_ (i), g_ {nullptr, 0}
+      {
+        if (i_ != r_->e_ && i_->get ().type.see_through)
+          switch_members ();
+      }
 
       iterator& operator++ ();
       iterator  operator++ (int) {iterator r (*this); return ++r;}
 
-      // Skip iterating over this group's members, if any. Note that
-      // the only valid operation after this call is to increment the
-      // iterator.
-      //
+      // Skip iterating over the rest of this group's members, if any.
+      // Note that the only valid operation after this call is to
+      // increment the iterator.
       //
       void
-      skip_group ()
+      leave_group ()
       {
         // Pretend we are on the last member of some group.
         //
@@ -625,18 +572,19 @@ namespace build
         g_.count = 1;
       }
 
-      /*
-      reference operator* () const
+      // Iterate over this group's members. Similar to leave_group(),
+      // you should increment the iterator after calling this function.
+      //
+      void
+      enter_group ()
       {
-        m_.prerequisite = *i;
-        m_.target = g_.count != 0 ? g_.members[j_] : nullptr;
-        return m_;
+        switch_members ();
+        --j_; // Compensate for the increment that will follow.
       }
-      */
 
       value_type operator* () const
       {
-        return value_type {*i_, g_.count != 0 ? g_.members[j_] : nullptr};
+        return value_type {*i_, g_.count != 0 ? g_.members[j_ - 1] : nullptr};
       }
 
       pointer operator-> () const
@@ -646,7 +594,7 @@ namespace build
           "prerequisite_member is not trivially destructible");
 
         return new (&m_)
-          value_type {*i_, g_.count != 0 ? g_.members[j_] : nullptr};
+          value_type {*i_, g_.count != 0 ? g_.members[j_ - 1] : nullptr};
       }
 
       friend bool
@@ -661,23 +609,28 @@ namespace build
       operator!= (const iterator& x, const iterator& y) {return !(x == y);}
 
     private:
-      action a_;
+      void
+      switch_members ();
+
+    private:
+      const prerequisite_members_range* r_;
       base_iterator i_;
       group_view g_;
-      std::size_t j_;
+      std::size_t j_; // 1-based index, to support enter_group().
       mutable std::aligned_storage<sizeof (prerequisite_member),
                                    alignof (prerequisite_member)>::type m_;
     };
 
     iterator
-    begin () const {return iterator (a_, r_.begin ());}
+    begin () const {return iterator (this, r_.begin ());}
 
     iterator
-    end () const {return iterator (a_, r_.end ());}
+    end () const {return iterator (this, e_);}
 
   private:
     action a_;
     T r_;
+    base_iterator e_;
   };
 
   // prerequisite_members(t.prerequisites)
@@ -713,6 +666,90 @@ namespace build
       a, butl::reverse_iterate (group_prerequisites (t)));
   }
 
+  //
+  //
+  struct target_set
+  {
+    typedef std::map<target_key, std::unique_ptr<target>> map;
+    typedef butl::map_iterator_adapter<map::const_iterator> iterator;
+
+    iterator
+    find (const target_key& k, tracer& trace) const;
+
+    iterator
+    find (const target_type& type,
+          const dir_path& dir,
+          const std::string& name,
+          const std::string* ext,
+          tracer& trace) const
+    {
+      return find (target_key {&type, &dir, &name, &ext}, trace);
+    }
+
+    // As above but ignore the extension and return the target or
+    // nullptr instead of the iterator.
+    //
+    template <typename T>
+    T*
+    find (const dir_path& dir, const std::string& name) const
+    {
+      const std::string* e (nullptr);
+      auto i (map_.find (target_key {&T::static_type, &dir, &name, &e}));
+      return i != map_.end () ? static_cast<T*> (i->second.get ()) : nullptr;
+    }
+
+    iterator begin () const {return map_.begin ();}
+    iterator end () const {return map_.end ();}
+
+    std::pair<target&, bool>
+    insert (const target_type&,
+            dir_path dir,
+            std::string name,
+            const std::string* ext,
+            tracer&);
+
+    template <typename T>
+    T&
+    insert (const dir_path& dir, const std::string& name, tracer& t)
+    {
+      return static_cast<T&> (
+        insert (T::static_type, dir, name, nullptr, t).first);
+    }
+
+    void
+    clear () {map_.clear ();}
+
+  private:
+    map map_;
+  };
+
+  extern target_set targets;
+
+  using target_type_map_base = std::map<
+    const char*,
+    std::reference_wrapper<const target_type>,
+    butl::compare_c_string>;
+
+  class target_type_map: public target_type_map_base
+  {
+  public:
+    void
+    insert (const target_type& tt) {emplace (tt.name, tt);}
+
+    using target_type_map_base::find;
+
+    // Given a name, figure out its type, taking into account extensions,
+    // special names (e.g., '.' and '..'), or anything else that might be
+    // relevant. Also process the name (in place) by extracting the
+    // extension, adjusting dir/value, etc (note that the dir is not
+    // necessarily normalized). Return NULL if not found.
+    //
+    const target_type*
+    find (name&, const std::string*& ext) const;
+  };
+
+  extern target_type_map target_types;
+
   // Modification time-based target.
   //
   class mtime_target: public target
diff --git a/build/target-key b/build/target-key
index d791d61..54813b0 100644
--- a/build/target-key
+++ b/build/target-key
@@ -28,6 +28,7 @@ namespace build
     target* (*const factory) (dir_path, std::string, const std::string*);
     const std::string& (*const extension) (const target_key&, scope&);
     target* (*const search) (const prerequisite_key&);
+    bool see_through; // A group with the default "see through" semantics.
 
     bool
     is_a (const std::type_index&) const; // Defined in target.cxx
diff --git a/build/target.cxx b/build/target.cxx
index ec96779..7958a85 100644
--- a/build/target.cxx
+++ b/build/target.cxx
@@ -48,6 +48,14 @@ namespace build
 
   // target
   //
+
+  group_view target::
+  group_members (action) const
+  {
+    assert (false); // Not a group or doesn't expose its members.
+    return group_view {nullptr, 0};
+  }
+
   scope& target::
   base_scope () const
   {
@@ -388,16 +396,7 @@ namespace build
     nullptr,
     nullptr,
     &search_target,
-  };
-
-  const target_type target_group::static_type
-  {
-    typeid (target_group),
-    "target_group",
-    &target::static_type,
-    nullptr,
-    nullptr,
-    &search_target
+    false
   };
 
   const target_type mtime_target::static_type
@@ -407,7 +406,8 @@ namespace build
     &target::static_type,
     nullptr,
     nullptr,
-    &search_target
+    &search_target,
+    false
   };
 
   const target_type path_target::static_type
@@ -417,7 +417,8 @@ namespace build
     &mtime_target::static_type,
     nullptr,
     nullptr,
-    &search_target
+    &search_target,
+    false
   };
 
   static target*
@@ -439,7 +440,8 @@ namespace build
     &path_target::static_type,
     &file_factory,
     nullptr, // Factory always assigns an extension.
-    &search_file
+    &search_file,
+    false
   };
 
   const target_type dir::static_type
@@ -449,7 +451,8 @@ namespace build
     &target::static_type,
     &target_factory<dir>,
     nullptr, // Should never need.
-    &search_alias
+    &search_alias,
+    false
   };
 
   const target_type fsdir::static_type
@@ -459,6 +462,7 @@ namespace build
     &target::static_type,
     &target_factory<fsdir>,
     nullptr, // Should never need.
-    &search_target
+    &search_target,
+    false
   };
 }
diff --git a/build/target.ixx b/build/target.ixx
index b6bf509..16616b5 100644
--- a/build/target.ixx
+++ b/build/target.ixx
@@ -32,7 +32,7 @@ namespace build
   // prerequisite_members
   //
   group_view
-  resolve_group_members (action, target_group&); // <build/algorithm>
+  resolve_group_members (action, target&); // <build/algorithm>
 
   template <typename T>
   inline auto prerequisite_members_range<T>::iterator::
@@ -40,35 +40,36 @@ namespace build
   {
     if (g_.count != 0)
     {
-      // Member iteration.
+      if (++j_ <= g_.count)
+        return *this;
+
+      // Switch back to prerequisite iteration mode.
       //
-      if (++j_ == g_.count)
-      {
-        // Switch back to prerequisite iteration.
-        //
-        g_.count = 0;
-        ++i_;
-      }
+      g_.count = 0;
     }
-    else
-    {
-      // Prerequisite iteration.
-      //
-      if (i_->get ().template is_a<target_group> ())
-      {
-        // Switch to member iteration.
-        //
-        target_group& g (static_cast<target_group&> (search (*i_)));
-        j_ = 0;
-        g_ = resolve_group_members (a_, g);
 
-        if (g_.count == 0)
-          ++i_; // Empty group.
-      }
-      else
-        ++i_;
-    }
+    ++i_;
+
+    // Switch to member iteration mode.
+    //
+    if (i_ != r_->e_ && i_->get ().type.see_through)
+      switch_members ();
 
     return *this;
   }
+
+  template <typename T>
+  inline void prerequisite_members_range<T>::iterator::
+  switch_members ()
+  {
+    j_ = 1;
+
+    do
+    {
+      g_ = resolve_group_members (r_->a_, search (*i_));
+    }
+    while (g_.count == 0  && // Skip empty groups.
+           ++i_ != r_->e_ &&
+           i_->get ().type.see_through);
+  }
 }