From 5607313a91e5ca0113b1f8b9acfd02c1fb105346 Mon Sep 17 00:00:00 2001
From: Boris Kolpackov <boris@codesynthesis.com>
Date: Thu, 19 Jan 2017 12:45:04 +0200
Subject: Get rid of project_name_pool

With small string optimizations this is most likely a hindrance rather
that an optimization.
---
 build2/algorithm        |  2 +-
 build2/algorithm.cxx    |  2 +-
 build2/algorithm.ixx    |  2 +-
 build2/cc/common        |  2 +-
 build2/cc/compile.cxx   |  2 +-
 build2/cc/link.cxx      |  7 +++++--
 build2/cc/pkgconfig.cxx |  8 ++++----
 build2/cc/utility.cxx   |  2 +-
 build2/context          |  1 -
 build2/context.cxx      |  2 --
 build2/dist/rule.cxx    |  2 +-
 build2/file.cxx         | 16 +++++++--------
 build2/install/rule.cxx |  2 +-
 build2/name             | 22 ++++++++++----------
 build2/name.cxx         |  4 ++--
 build2/name.ixx         |  8 +-------
 build2/parser           |  8 ++++----
 build2/parser.cxx       | 53 +++++++++++++++++++++++++++----------------------
 build2/prerequisite     | 28 ++++++++++++--------------
 build2/prerequisite.cxx |  9 ++++++---
 build2/target           |  6 ++++--
 build2/target.ixx       |  2 +-
 22 files changed, 96 insertions(+), 94 deletions(-)

diff --git a/build2/algorithm b/build2/algorithm
index 3dbef4c..9a91781 100644
--- a/build2/algorithm
+++ b/build2/algorithm
@@ -46,7 +46,7 @@ namespace build2
           const string& name,
           const string* ext,
           scope*,
-          const string* proj = nullptr);
+          const optional<string>& proj = nullopt);
 
   // As above but specify the target type as template argument.
   //
diff --git a/build2/algorithm.cxx b/build2/algorithm.cxx
index 8874e96..78af195 100644
--- a/build2/algorithm.cxx
+++ b/build2/algorithm.cxx
@@ -25,7 +25,7 @@ namespace build2
     // If this is a project-qualified prerequisite, then this is import's
     // business.
     //
-    if (pk.proj != nullptr)
+    if (pk.proj)
       return import (pk);
 
     if (target* t = pk.tk.type->search (pk))
diff --git a/build2/algorithm.ixx b/build2/algorithm.ixx
index 09d522f..84b0ee7 100644
--- a/build2/algorithm.ixx
+++ b/build2/algorithm.ixx
@@ -32,7 +32,7 @@ namespace build2
           const string& name,
           const string* ext,
           scope* scope,
-          const string* proj)
+          const optional<string>& proj)
   {
     return search (
       prerequisite_key {proj, {&type, &dir, &out, &name, ext}, scope});
diff --git a/build2/cc/common b/build2/cc/common
index 2c79fa6..f0476f9 100644
--- a/build2/cc/common
+++ b/build2/cc/common
@@ -229,7 +229,7 @@ namespace build2
                          bin::lib&,
                          bin::liba*,
                          bin::libs*,
-                         const string*,
+                         const optional<string>&,
                          const string&,
                          const dir_path&,
                          const dir_paths&) const; // pkgconfig.cxx
diff --git a/build2/cc/compile.cxx b/build2/cc/compile.cxx
index 3318f21..23499ad 100644
--- a/build2/cc/compile.cxx
+++ b/build2/cc/compile.cxx
@@ -277,7 +277,7 @@ namespace build2
             // we don't need to do match() in order to get options (if
             // any, they would be set by search_library()).
             //
-            if (p.proj () == nullptr ||
+            if (!p.proj () ||
                 search_library (
                   sys_lib_dirs, usr_lib_dirs, p.prerequisite) == nullptr)
             {
diff --git a/build2/cc/link.cxx b/build2/cc/link.cxx
index a6b67d5..8010598 100644
--- a/build2/cc/link.cxx
+++ b/build2/cc/link.cxx
@@ -153,7 +153,7 @@ namespace build2
 
             // Handle imported libraries.
             //
-            if (p.proj () != nullptr)
+            if (p.proj ())
               pt = search_library (sys_lib_dirs, usr_lib_dirs, p.prerequisite);
 
             if (pt == nullptr)
@@ -437,7 +437,10 @@ namespace build2
         {
           // Handle imported libraries.
           //
-          if (p.proj () != nullptr)
+          // Note that since the search is rule-specific, we don't cache the
+          // target in the prerequisite.
+          //
+          if (p.proj ())
             pt = search_library (sys_lib_dirs, usr_lib_dirs, p.prerequisite);
 
           // The rest is the same basic logic as in search_and_match().
diff --git a/build2/cc/pkgconfig.cxx b/build2/cc/pkgconfig.cxx
index 1b14960..f8d35af 100644
--- a/build2/cc/pkgconfig.cxx
+++ b/build2/cc/pkgconfig.cxx
@@ -41,7 +41,7 @@ namespace build2
                        lib& lt,
                        liba* at,
                        libs* st,
-                       const string* proj,
+                       const optional<string>& proj,
                        const string& stem,
                        const dir_path& libd,
                        const dir_paths& sysd) const
@@ -56,7 +56,7 @@ namespace build2
       // names).
       //
       path f;
-      auto search = [&f, proj, &stem, &libd] (const dir_path& dir) -> bool
+      auto search = [&f, &proj, &stem, &libd] (const dir_path& dir) -> bool
       {
         // Check if we have this directory inrelative to this library's
         // directory.
@@ -93,7 +93,7 @@ namespace build2
         if (exists (f))
           return true;
 
-        if (proj != nullptr)
+        if (proj)
         {
           f = pkgd;
           f /= *proj;
@@ -420,7 +420,7 @@ namespace build2
           const string* ext (nullptr);
 
           prerequisite_key pk {
-            nullptr, {&lib::static_type, &out, &out, &name, ext}, &s};
+            nullopt, {&lib::static_type, &out, &out, &name, ext}, &s};
 
           if (lib* lt = static_cast<lib*> (search_library (sysd, usrd, pk)))
           {
diff --git a/build2/cc/utility.cxx b/build2/cc/utility.cxx
index b786d11..f8a0afc 100644
--- a/build2/cc/utility.cxx
+++ b/build2/cc/utility.cxx
@@ -67,7 +67,7 @@ namespace build2
 
       if (r == nullptr)
         r = &search (ls ? libs::static_type : liba::static_type,
-                     prerequisite_key {nullptr, l.key (), nullptr});
+                     prerequisite_key {nullopt, l.key (), nullptr});
 
       return *r;
     }
diff --git a/build2/context b/build2/context
index dcf1b01..cb83105 100644
--- a/build2/context
+++ b/build2/context
@@ -17,7 +17,6 @@ namespace build2
   class file;
 
   extern string_pool extension_pool;
-  extern string_pool project_name_pool;
 
   // Current action (meta/operation).
   //
diff --git a/build2/context.cxx b/build2/context.cxx
index 106b7ec..a21de2a 100644
--- a/build2/context.cxx
+++ b/build2/context.cxx
@@ -24,7 +24,6 @@ using namespace butl;
 namespace build2
 {
   string_pool extension_pool;
-  string_pool project_name_pool;
 
   const string* current_mname;
   const string* current_oname;
@@ -58,7 +57,6 @@ namespace build2
     var_pool.clear ();
 
     extension_pool.clear ();
-    project_name_pool.clear ();
 
     // Reset meta/operation tables. Note that the order should match the id
     // constants in <build2/operation>.
diff --git a/build2/dist/rule.cxx b/build2/dist/rule.cxx
index e11f09f..6cbd7df 100644
--- a/build2/dist/rule.cxx
+++ b/build2/dist/rule.cxx
@@ -33,7 +33,7 @@ namespace build2
 
         // Skip prerequisites imported from other projects.
         //
-        if (p.proj () != nullptr)
+        if (p.proj ())
           continue;
 
         // If we can, go inside see-through groups. Note that here we are
diff --git a/build2/file.cxx b/build2/file.cxx
index cc1be77..9e957a9 100644
--- a/build2/file.cxx
+++ b/build2/file.cxx
@@ -826,19 +826,19 @@ namespace build2
     l5 ([&]{trace << target << " from " << ibase.out_path ();});
 
     // If there is no project specified for this target, then our run will be
-    // short and sweet: we simply return it as empty- project-qualified and
+    // short and sweet: we simply return it as empty-project-qualified and
     // let someone else (e.g., a rule) take a stab at it.
     //
     if (target.unqualified ())
     {
-      target.proj = &project_name_pool.find ("");
+      target.proj = string ();
       return names {move (target)};
     }
 
     // Otherwise, get the project name and convert the target to unqualified.
     //
-    const string& proj (*target.proj);
-    target.proj = nullptr;
+    string proj (move (*target.proj));
+    target.proj = nullopt;
 
     scope& iroot (*ibase.root_scope ());
 
@@ -873,7 +873,7 @@ namespace build2
           //
           if (out_root.empty ())
           {
-            target.proj = &proj;
+            target.proj = move (proj);
             l5 ([&]{trace << "skipping " << target;});
             return names {move (target)};
           }
@@ -924,7 +924,7 @@ namespace build2
           // config.import.build2.b=b-boot
           //
           if (p.relative ())
-            target.proj = &proj;
+            target.proj = move (proj);
 
           target.dir = p.directory ();
           target.value = p.leaf ().string ();
@@ -973,7 +973,7 @@ namespace build2
     //
     if (out_root.empty ())
     {
-      target.proj = &proj;
+      target.proj = move (proj);
       l5 ([&]{trace << "postponing " << target;});
       return names {move (target)};
     }
@@ -1098,7 +1098,7 @@ namespace build2
   {
     tracer trace ("import");
 
-    assert (pk.proj != nullptr);
+    assert (pk.proj);
     const string& p (*pk.proj);
 
     // Target type-specific search.
diff --git a/build2/install/rule.cxx b/build2/install/rule.cxx
index a368ac9..036f01f 100644
--- a/build2/install/rule.cxx
+++ b/build2/install/rule.cxx
@@ -157,7 +157,7 @@ namespace build2
         // Ignore unresolved targets that are imported from other projects.
         // We are definitely not installing those.
         //
-        if (p.proj () != nullptr)
+        if (p.proj ())
           continue;
 
         // Let a customized rule have its say.
diff --git a/build2/name b/build2/name
index 3bae2d0..2833a2b 100644
--- a/build2/name
+++ b/build2/name
@@ -19,9 +19,11 @@ namespace build2
   // A name is what we operate on by default. Depending on the context, it can
   // be interpreted as a target or prerequisite name. A name without a type
   // and directory can be used to represent any text. A name with directory
-  // and empty value represents a directory. A name may also be qualified with
-  // a project. If the project name is empty, then it means the name is in a
-  // project other than our own (e.g., it is installed).
+  // and empty value represents a directory.
+  //
+  // A name may also be qualified with a project. If the project name is
+  // empty, then it means the name is in a project other than our own (e.g.,
+  // it is installed).
   //
   // A type or project can only be specified if either directory or value are
   // not empty.
@@ -31,7 +33,7 @@ namespace build2
   //
   struct name
   {
-    const string* proj = nullptr; // Points to project_name_pool.
+    optional<string> proj;
     dir_path dir;
     string type;
     string value;
@@ -45,16 +47,14 @@ namespace build2
     name (dir_path d, string t, string v)
         : dir (move (d)), type (move (t)), value (move (v)) {}
 
-    // The first argument should be from project_name_pool.
-    //
-    name (const string* p, dir_path d, string t, string v)
-        : proj (p), dir (move (d)), type (move (t)), value (move (v)) {}
+    name (optional<string> p, dir_path d, string t, string v)
+        : proj (move (p)), dir (move (d)), type (move (t)), value (move (v)) {}
 
     bool
-    qualified () const {return proj != nullptr;}
+    qualified () const {return proj.has_value ();}
 
     bool
-    unqualified () const {return proj == nullptr;}
+    unqualified () const {return !qualified ();}
 
     bool
     typed () const {return !type.empty ();}
@@ -62,7 +62,7 @@ namespace build2
     bool
     untyped () const {return type.empty ();}
 
-    // Note: if dir and value are empty then so should be proj and type.
+    // Note: if dir and value are empty then there should be no proj or type.
     //
     bool
     empty () const {return dir.empty () && value.empty ();}
diff --git a/build2/name.cxx b/build2/name.cxx
index e4ecd52..0b3d00f 100644
--- a/build2/name.cxx
+++ b/build2/name.cxx
@@ -22,7 +22,7 @@ namespace build2
     if (n.empty ())
       return r;
 
-    if (n.proj != nullptr)
+    if (n.proj)
     {
       r += *n.proj;
       r += '%';
@@ -113,7 +113,7 @@ namespace build2
     if (quote && n.empty ())
       return os << "''";
 
-    if (n.proj != nullptr)
+    if (n.proj)
     {
       write_string (*n.proj);
       os << '%';
diff --git a/build2/name.ixx b/build2/name.ixx
index 4a8c86a..c77846a 100644
--- a/build2/name.ixx
+++ b/build2/name.ixx
@@ -7,13 +7,7 @@ namespace build2
   inline int name::
   compare (const name& x) const
   {
-    int r;
-
-    // Project string is pooled, so for equality can just compare pointers.
-    //
-    r = proj == x.proj
-      ? 0
-      : proj == nullptr || (x.proj != nullptr && *proj < *x.proj) ? -1 : 1;
+    int r (proj < x.proj ? -1 : (proj > x.proj ? 1 : 0));
 
     if (r == 0)
       r = dir.compare (x.dir);
diff --git a/build2/parser b/build2/parser
index 5909328..f91bbb6 100644
--- a/build2/parser
+++ b/build2/parser
@@ -199,7 +199,7 @@ namespace build2
     {
       names ns;
       parse_names (
-        t, tt, ns, chunk, what, separators, 0, nullptr, nullptr, nullptr);
+        t, tt, ns, chunk, what, separators, 0, nullopt, nullptr, nullptr);
       return ns;
     }
 
@@ -214,7 +214,7 @@ namespace build2
       names ns;
       pair<bool, const value_type*> p (
         parse_names (
-          t, tt, ns, chunk, what, separators, 0, nullptr, nullptr, nullptr));
+          t, tt, ns, chunk, what, separators, 0, nullopt, nullptr, nullptr));
 
       value r (p.second); // Potentially typed NULL value.
 
@@ -250,7 +250,7 @@ namespace build2
                  const char* what = "name",
                  const string* separators = &name_separators,
                  size_t pair = 0,
-                 const string* prj = nullptr,
+                 const optional<string>& prj = nullopt,
                  const dir_path* dir = nullptr,
                  const string* type = nullptr);
 
@@ -260,7 +260,7 @@ namespace build2
                          const char* what,
                          const string* separators,
                          size_t pair,
-                         const string* prj,
+                         const optional<string>& prj,
                          const dir_path* dir,
                          const string* type);
 
diff --git a/build2/parser.cxx b/build2/parser.cxx
index 57a5618..6025082 100644
--- a/build2/parser.cxx
+++ b/build2/parser.cxx
@@ -2163,7 +2163,7 @@ namespace build2
                        const char* what,
                        const string* separators,
                        size_t pairn,
-                       const string* pp,
+                       const optional<string>& pp,
                        const dir_path* dp,
                        const string* tp)
   {
@@ -2195,7 +2195,7 @@ namespace build2
 
       names x; // Parse into a separate list of names.
       parse_names_trailer (
-        t, tt, x, what, separators, 0, nullptr, nullptr, nullptr);
+        t, tt, x, what, separators, 0, nullopt, nullptr, nullptr);
 
       if (size_t n = x.size ())
       {
@@ -2219,12 +2219,12 @@ namespace build2
 
             // Move the project names.
             //
-            if (r.proj != nullptr)
+            if (r.proj)
             {
-              if (l.proj != nullptr)
+              if (l.proj)
                 fail (loc) << "nested project name " << *r.proj;
 
-              l.proj = r.proj;
+              l.proj = move (r.proj);
             }
 
             // Merge directories.
@@ -2285,7 +2285,7 @@ namespace build2
                const char* what,
                const string* separators,
                size_t pairn,
-               const string* pp,
+               const optional<string>& pp,
                const dir_path* dp,
                const string* tp)
   {
@@ -2379,7 +2379,6 @@ namespace build2
       }
     };
 
-
     // Number of names in the last group. This is used to detect when
     // we need to add an empty first pair element (e.g., @y) or when
     // we have a (for now unsupported) multi-name LHS (e.g., {x y}@z).
@@ -2558,31 +2557,33 @@ namespace build2
         // First take care of project. A project-qualified name is not very
         // common, so we can afford some copying for the sake of simplicity.
         //
-        const string* pp1 (pp);
+        optional<string> p1;
+        const optional<string>* pp1 (&pp);
 
         if (p != string::npos)
         {
           bool last (val[p] == '%');
-          string::size_type p1 (last ? p : val.rfind ('%', p - 1));
+          string::size_type q (last ? p : val.rfind ('%', p - 1));
 
-          if (p1 != string::npos)
+          if (q != string::npos)
           {
             string proj;
             proj.swap (val);
 
             // First fix the rest of the name.
             //
-            val.assign (proj, p1 + 1, string::npos);
-            p = last ? string::npos : p - (p1 + 1);
+            val.assign (proj, q + 1, string::npos);
+            p = last ? string::npos : p - (q + 1);
 
             // Now process the project name.
             //
-            proj.resize (p1);
+            proj.resize (q);
 
-            if (pp != nullptr)
+            if (pp)
               fail (t) << "nested project name " << proj;
 
-            pp1 = &project_name_pool.find (proj);
+            p1 = move (proj);
+            pp1 = &p1;
           }
         }
 
@@ -2629,7 +2630,7 @@ namespace build2
           }
 
           count = parse_names_trailer (
-            t, tt, ns, what, separators, pairn, pp1, dp1, tp1);
+            t, tt, ns, what, separators, pairn, *pp1, dp1, tp1);
           tt = peek ();
           continue;
         }
@@ -2665,7 +2666,7 @@ namespace build2
             if (dp != nullptr)
               dir = *dp / dir;
 
-            ns.emplace_back (pp1,
+            ns.emplace_back (*pp1,
                              move (dir),
                              (tp != nullptr ? *tp : string ()),
                              string ());
@@ -2673,7 +2674,7 @@ namespace build2
           }
         }
 
-        ns.emplace_back (pp1,
+        ns.emplace_back (*pp1,
                          (dp != nullptr ? *dp : dir_path ()),
                          (tp != nullptr ? *tp : string ()),
                          move (val));
@@ -2980,14 +2981,18 @@ namespace build2
           //
           for (const name& n: lv)
           {
-            const string* pp1 (pp);
+            const optional<string>* pp1 (&pp);
             const dir_path* dp1 (dp);
             const string* tp1 (tp);
 
-            if (n.proj != 0)
+            optional<string> p1;
+            if (n.proj)
             {
-              if (pp == nullptr)
-                pp1 = n.proj;
+              if (!pp)
+              {
+                p1 = n.proj;
+                pp1 = &p1;
+              }
               else
                 fail (loc) << "nested project name " << *n.proj << " in "
                            << what;
@@ -3032,7 +3037,7 @@ namespace build2
                 ns.push_back (ns[pairn - 1]);
             }
 
-            ns.emplace_back (pp1,
+            ns.emplace_back (*pp1,
                              (dp1 != nullptr ? *dp1 : dir_path ()),
                              (tp1 != nullptr ? *tp1 : string ()),
                              n.value);
@@ -3573,7 +3578,7 @@ namespace build2
 
     prerequisite& p (
       scope_->prerequisites.insert (
-        nullptr,
+        nullopt,
         dt.key (),
         *scope_,   // Doesn't matter which scope since dir is absolute.
         trace).first);
diff --git a/build2/prerequisite b/build2/prerequisite
index 1fe3c1a..b259557 100644
--- a/build2/prerequisite
+++ b/build2/prerequisite
@@ -27,22 +27,21 @@ namespace build2
   public:
     typedef build2::scope scope_type;
 
-    mutable const string* proj; // Can be NULL, from project_name_pool.
-    target_key tk;              // .dir and .out can be relative.
-    mutable scope_type* scope;  // Can be NULL if tk.dir is absolute.
+    const optional<string>& proj;
+    target_key tk;                // .dir and .out can be relative.
+    mutable scope_type* scope;    // Can be NULL if tk.dir is absolute.
 
     template <typename T>
     bool is_a () const {return tk.is_a<T> ();}
     bool is_a (const target_type& tt) const {return tk.is_a (tt);}
+
+    static const optional<string> nullproj;
   };
 
   inline bool
   operator== (const prerequisite_key& x, const prerequisite_key& y)
   {
     assert (x.scope == y.scope);
-
-    // Can compare project name pointers since they are from project_name_pool.
-    //
     return x.proj == y.proj && x.tk == y.tk;
   }
 
@@ -71,9 +70,7 @@ namespace std
     size_t
     operator() (const build2::prerequisite_key& k) const noexcept
     {
-      // Can hash project name pointers since they are from project_name_pool.
-      //
-      return build2::combine_hash (hash<const string*> () (k.proj),
+      return build2::combine_hash (hash<build2::optional<string>> () (k.proj),
                                    hash<build2::target_key> () (k.tk));
     }
   };
@@ -91,7 +88,7 @@ namespace build2
     // Note that unlike targets, for prerequisites an empty out directory
     // means undetermined rather than being definitely in the out tree.
     //
-    const string* const proj; // NULL if not project-qualified.
+    const optional<string>  proj;
     const target_type_type& type;
     const dir_path dir;      // Normalized absolute or relative (to scope).
     const dir_path out;      // Empty, normalized absolute, or relative.
@@ -103,14 +100,14 @@ namespace build2
                              // a target group.
 
   public:
-    prerequisite (const string* p,
+    prerequisite (optional<string> p,
                   const target_type_type& t,
                   dir_path d,
                   dir_path o,
                   string n,
                   const string* e,
                   scope_type& s)
-        : proj (p),
+        : proj (move (p)),
           type (t),
           dir (move (d)),
           out (move (o)),
@@ -186,7 +183,7 @@ namespace build2
   struct prerequisite_set: std::unordered_set<prerequisite>
   {
     pair<prerequisite&, bool>
-    insert (const string* proj,
+    insert (optional<string> proj,
             const target_type&,
             dir_path dir,
             dir_path out,
@@ -196,9 +193,10 @@ namespace build2
             tracer&);
 
     pair<prerequisite&, bool>
-    insert (const string* proj, const target_key& tk, scope& s, tracer& t)
+    insert (optional<string> proj, const target_key& tk, scope& s, tracer& t)
     {
-      return insert (proj, *tk.type, *tk.dir, *tk.out, *tk.name, tk.ext, s, t);
+      return insert (
+        move (proj), *tk.type, *tk.dir, *tk.out, *tk.name, tk.ext, s, t);
     }
   };
 }
diff --git a/build2/prerequisite.cxx b/build2/prerequisite.cxx
index 5bb2ddf..82d6ea2 100644
--- a/build2/prerequisite.cxx
+++ b/build2/prerequisite.cxx
@@ -15,10 +15,12 @@ namespace build2
 {
   // prerequisite_key
   //
+  const optional<string> prerequisite_key::nullproj = nullopt;
+
   ostream&
   operator<< (ostream& os, const prerequisite_key& pk)
   {
-    if (pk.proj != nullptr)
+    if (pk.proj)
       os << *pk.proj << '%';
     //
     // Don't print scope if we are project-qualified or the prerequisite's
@@ -49,7 +51,7 @@ namespace build2
   // prerequisite_set
   //
   auto prerequisite_set::
-  insert (const string* proj,
+  insert (optional<string> proj,
           const target_type& tt,
           dir_path dir,
           dir_path out,
@@ -64,7 +66,8 @@ namespace build2
 
     // Find or insert.
     //
-    auto r (emplace (proj, tt, move (dir), move (out), move (name), ext, s));
+    auto r (
+      emplace (move (proj), tt, move (dir), move (out), move (name), ext, s));
     prerequisite& p (const_cast<prerequisite&> (*r.first));
 
     // Update extension if the existing prerequisite has it unspecified.
diff --git a/build2/target b/build2/target
index dd9461c..48c5ebc 100644
--- a/build2/target
+++ b/build2/target
@@ -748,12 +748,14 @@ namespace build2
       return target != nullptr ? target->name : prerequisite.get ().name;
     }
 
-    const string*
+    const optional<string>&
     proj () const
     {
       // Target cannot be project-qualified.
       //
-      return target != nullptr ? nullptr : prerequisite.get ().proj;
+      return target != nullptr
+        ? prerequisite_key::nullproj
+        : prerequisite.get ().proj;
     }
 
     target_type&
diff --git a/build2/target.ixx b/build2/target.ixx
index e9d2d7a..2d4fff8 100644
--- a/build2/target.ixx
+++ b/build2/target.ixx
@@ -29,7 +29,7 @@ namespace build2
     // The use of the group's prerequisite scope is debatable.
     //
     scope& s (prerequisite.get ().scope);
-    return s.prerequisites.insert (nullptr, key ().tk, s, trace).first;
+    return s.prerequisites.insert (nullopt, key ().tk, s, trace).first;
   }
 
   // prerequisite_members
-- 
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