Improve empty simple value to empty list of names reduction heuristics

Specifically, do not reduce typed RHS empty simple values for prepend/append
and additionally for assignment provided LHS is typed and is a container.

19 files changed, 191 insertions, 67 deletions

diff --git a/libbuild2/adhoc-rule-buildscript.cxx b/libbuild2/adhoc-rule-buildscript.cxx
index 2cec3f3..ce4ab8b 100644
--- a/libbuild2/adhoc-rule-buildscript.cxx
+++ b/libbuild2/adhoc-rule-buildscript.cxx
@@ -47,7 +47,7 @@ namespace build2
       if (l)
       {
         storage.clear ();
-        names_view ns (reverse (*l, storage));
+        names_view ns (reverse (*l, storage, true /* reduce */));
 
         for (const name& n: ns)
           to_checksum (cs, n);
diff --git a/libbuild2/buildspec.cxx b/libbuild2/buildspec.cxx
index bd580ca..2eeaf31 100644
--- a/libbuild2/buildspec.cxx
+++ b/libbuild2/buildspec.cxx
@@ -53,7 +53,7 @@ namespace build2
       if (v)
       {
         names storage;
-        os << reverse (v, storage);
+        os << reverse (v, storage, true /* reduce */);
       }
       else
         os << "[null]";
@@ -86,7 +86,7 @@ namespace build2
       if (v)
       {
         names storage;
-        os << reverse (v, storage);
+        os << reverse (v, storage, true /* reduce */);
       }
       else
         os << "[null]";
diff --git a/libbuild2/cc/pkgconfig.cxx b/libbuild2/cc/pkgconfig.cxx
index d659a2c..c41a182 100644
--- a/libbuild2/cc/pkgconfig.cxx
+++ b/libbuild2/cc/pkgconfig.cxx
@@ -1927,7 +1927,7 @@ namespace build2
             const value& val (*b.val);
 
             names ns;
-            names_view nv (reverse (val, ns));
+            names_view nv (reverse (val, ns, true /* reduce */));
 
             os << *b.name << " =";
 
diff --git a/libbuild2/config/init.cxx b/libbuild2/config/init.cxx
index 46239d8..d42bace 100644
--- a/libbuild2/config/init.cxx
+++ b/libbuild2/config/init.cxx
@@ -39,7 +39,7 @@ namespace build2
     save_environment (const value& d, const value* b, names& storage)
     {
       if (b == nullptr)
-        return make_pair (reverse (d, storage), "=");
+        return make_pair (reverse (d, storage, true /* reduce */), "=");
 
       // The plan is to iterator over environment variables adding those that
       // are not in base to storage. There is, however, a complication: we may
diff --git a/libbuild2/config/operation.cxx b/libbuild2/config/operation.cxx
index 34ed402..9079bbf 100644
--- a/libbuild2/config/operation.cxx
+++ b/libbuild2/config/operation.cxx
@@ -181,7 +181,7 @@ namespace build2
         if (v)
         {
           storage.clear ();
-          dr << "'" << reverse (v, storage) << "'";
+          dr << "'" << reverse (v, storage, true /* reduce */) << "'";
         }
         else
           dr << "[null]";
@@ -531,7 +531,7 @@ namespace build2
             pair<names_view, const char*> p (
               sv.save != nullptr
               ? sv.save (v, base, storage)
-              : make_pair (reverse (v, storage), "="));
+              : make_pair (reverse (v, storage, true /* reduce */), "="));
 
             // Might becomes empty after a custom save function had at it.
             //
diff --git a/libbuild2/dump.cxx b/libbuild2/dump.cxx
index c28dd31..4ee75ee 100644
--- a/libbuild2/dump.cxx
+++ b/libbuild2/dump.cxx
@@ -49,7 +49,7 @@ namespace build2
     if (v)
     {
       names storage;
-      os << (a ? " " : "") << reverse (v, storage);
+      os << (a ? " " : "") << reverse (v, storage, true /* reduce */);
     }
   }
 
diff --git a/libbuild2/file.cxx b/libbuild2/file.cxx
index bf50a25..acfb5d7 100644
--- a/libbuild2/file.cxx
+++ b/libbuild2/file.cxx
@@ -1706,7 +1706,7 @@ namespace build2
         if (const value& v = *l)
         {
           storage.clear ();
-          auto ns (reverse (v, storage));
+          auto ns (reverse (v, storage, true /* reduce */));
 
           if (f == "multiline")
           {
diff --git a/libbuild2/function.cxx b/libbuild2/function.cxx
index ae69730..528b396 100644
--- a/libbuild2/function.cxx
+++ b/libbuild2/function.cxx
@@ -213,7 +213,7 @@ namespace build2
             if (f->arg_types[i]             &&
                 *f->arg_types[i] == nullptr &&
                 args[i].type != nullptr)
-              untypify (args[i]);
+              untypify (args[i], true /* reduce */);
           }
         }
 
diff --git a/libbuild2/function.test.cxx b/libbuild2/function.test.cxx
index 7ce7ad3..37ed5ff 100644
--- a/libbuild2/function.test.cxx
+++ b/libbuild2/function.test.cxx
@@ -115,7 +115,7 @@ namespace build2
             else if (!a.empty ())
             {
               names storage;
-              cout << reverse (a, storage);
+              cout << reverse (a, storage, true /* reduce */);
             }
             cout << endl;
           }
diff --git a/libbuild2/functions-builtin.cxx b/libbuild2/functions-builtin.cxx
index 6bf2264..378ffbc 100644
--- a/libbuild2/functions-builtin.cxx
+++ b/libbuild2/functions-builtin.cxx
@@ -83,7 +83,7 @@ namespace build2
       if (v->null)
         return string ();
 
-      untypify (*v); // Reverse to names.
+      untypify (*v, true /* reduce */); // Reverse to names.
 
       ostringstream os;
       to_stream (os,
diff --git a/libbuild2/functions-regex.cxx b/libbuild2/functions-regex.cxx
index ca7e229..922be18 100644
--- a/libbuild2/functions-regex.cxx
+++ b/libbuild2/functions-regex.cxx
@@ -21,7 +21,7 @@ namespace build2
     // Optimize for the string value type.
     //
     if (v.type != &value_traits<string>::value_type)
-      untypify (v);
+      untypify (v, true /* reduce */);
 
     return convert<string> (move (v));
   }
diff --git a/libbuild2/parser.cxx b/libbuild2/parser.cxx
index 812f201..b118cee 100644
--- a/libbuild2/parser.cxx
+++ b/libbuild2/parser.cxx
@@ -44,7 +44,10 @@ namespace build2
     {
       o << '=';
       names storage;
-      to_stream (o, reverse (a.value, storage), quote_mode::normal, '@');
+      to_stream (o,
+                 reverse (a.value, storage, true /* reduce */),
+                 quote_mode::normal,
+                 '@');
     }
 
     return o;
@@ -3692,7 +3695,7 @@ namespace build2
     if (value v = parse_value_with_attributes (t, tt, pattern_mode::expand))
     {
       names storage;
-      for (name& n: reverse (v, storage))
+      for (name& n: reverse (v, storage, true /* reduce */))
       {
         // @@ Could this be an out-qualified ad hoc import?
         //
@@ -3762,7 +3765,12 @@ namespace build2
       fail (l) << "null value in export";
 
     if (val.type != nullptr)
-      untypify (val);
+    {
+      // While feels far-fetched, let's preserve empty typed values in the
+      // result.
+      //
+      untypify (val, false /* reduce */);
+    }
 
     export_value = move (val).as<names> ();
 
@@ -4446,7 +4454,11 @@ namespace build2
     if (val && val.type != nullptr)
     {
       etype = val.type->element_type;
-      untypify (val);
+
+      // Note that here we don't want to be reducing empty simple values to
+      // empty lists.
+      //
+      untypify (val, false /* reduce */);
     }
 
     if (tt != type::newline)
@@ -4627,7 +4639,7 @@ namespace build2
     if (value v = parse_value_with_attributes (t, tt, pattern_mode::expand))
     {
       names storage;
-      cout << reverse (v, storage) << endl;
+      cout << reverse (v, storage, true /* reduce */) << endl;
     }
     else
       cout << "[null]" << endl;
@@ -4660,7 +4672,7 @@ namespace build2
     if (value v = parse_value_with_attributes (t, tt, pattern_mode::expand))
     {
       names storage;
-      dr << reverse (v, storage);
+      dr << reverse (v, storage, true /* reduce */);
     }
 
     if (tt != type::eos)
@@ -4843,7 +4855,12 @@ namespace build2
     // We store prepend/append values untyped (similar to overrides).
     //
     if (rhs.type != nullptr && kind != type::assign)
-      untypify (rhs);
+    {
+      // Our heuristics for prepend/append of a typed value is to preserve
+      // empty (see apply_value_attributes() for details) so do not reduce.
+      //
+      untypify (rhs, false /* reduce */);
+    }
 
     if (p.second)
     {
@@ -5152,6 +5169,13 @@ namespace build2
     bool rhs_type (false);
     if (rhs.type != nullptr)
     {
+      // Our heuristics is to not reduce typed RHS empty simple values for
+      // prepend/append and additionally for assign provided LHS is a
+      // container.
+      //
+      bool reduce (kind == type::assign &&
+                   (type == nullptr || !type->container));
+
       // Only consider RHS type if there is no explicit or variable type.
       //
       if (type == nullptr)
@@ -5162,7 +5186,7 @@ namespace build2
 
       // Reduce this to the untyped value case for simplicity.
       //
-      untypify (rhs);
+      untypify (rhs, reduce);
     }
 
     if (kind == type::assign)
@@ -6561,14 +6585,13 @@ namespace build2
       if (!vnull)
       {
         if (vtype != nullptr)
-          untypify (rhs);
+          untypify (rhs, true /* reduce */);
 
         names& d (rhs.as<names> ());
 
-        // If the value is empty, then untypify() will (typically; no pun
-        // intended) represent it as an empty sequence of names rather than
-        // a sequence of one empty name. This is usually what we need (see
-        // simple_reverse() for details) but not in this case.
+        // If the value is empty, then we asked untypify() to reduce it to
+        // an empty sequence of names rather than a sequence of one empty
+        // name.
         //
         if (!d.empty ())
         {
@@ -7422,7 +7445,8 @@ namespace build2
                   fail (loc) << "null variable/function name";
 
                 names storage;
-                vector_view<name_type> ns (reverse (v, storage)); // Movable.
+                vector_view<name_type> ns (
+                  reverse (v, storage, true /* reduce */)); // Movable.
                 size_t n (ns.size ());
 
                 // We cannot handle scope-qualification in the eval context as
@@ -7643,7 +7667,7 @@ namespace build2
                          ? value (move (result_data))
                          : value (*result));
 
-              untypify (val);
+              untypify (val, false /* reduce */);
 
               names& ns (val.as<names> ());
 
@@ -7754,7 +7778,11 @@ namespace build2
               fail (loc) << "no string conversion for " << t;
 
             result_data = move (p.first);
-            untypify (result_data); // Convert to untyped simple name.
+
+            // Convert to untyped simple name reducing empty string to empty
+            // names as an optimization.
+            //
+            untypify (result_data, true /* reduce */);
           }
 
           if ((concat && vtype != nullptr) || // LHS typed.
@@ -7855,7 +7883,7 @@ namespace build2
           // @@ Could move if nv is result_data; see untypify().
           //
           names nv_storage;
-          names_view nv (reverse (*result, nv_storage));
+          names_view nv (reverse (*result, nv_storage, true /* reduce */));
 
           count = splice_names (
             loc, nv, move (nv_storage), ns, what, pairn, pp, dp, tp);
diff --git a/libbuild2/script/parser.cxx b/libbuild2/script/parser.cxx
index 88e2f06..0be22a6 100644
--- a/libbuild2/script/parser.cxx
+++ b/libbuild2/script/parser.cxx
@@ -1028,7 +1028,7 @@ namespace build2
 
                 storage.clear ();
                 to_stream (os,
-                           reverse (a.value, storage),
+                           reverse (a.value, storage, true /* reduce */),
                            quote_mode::normal,
                            '@');
               }
@@ -2676,7 +2676,11 @@ namespace build2
                 if (val.type != nullptr)
                 {
                   etype = val.type->element_type;
-                  untypify (val);
+
+                  // Note that here we don't want to be reducing empty simple
+                  // values to empty lists.
+                  //
+                  untypify (val, false /* reduce */);
                 }
 
                 size_t fli (li);
diff --git a/libbuild2/target.cxx b/libbuild2/target.cxx
index 16aec49..6d083d9 100644
--- a/libbuild2/target.cxx
+++ b/libbuild2/target.cxx
@@ -605,7 +605,7 @@ namespace build2
         // name)? But then again we don't expect many such overrides. Plus
         // will complicate the diagnostics below.
         //
-        ns = reverse (*l, storage);
+        ns = reverse (*l, storage, true /* reduce */);
 
         if (ns.size () == 1)
         {
diff --git a/libbuild2/variable.cxx b/libbuild2/variable.cxx
index 0017633..fefea98 100644
--- a/libbuild2/variable.cxx
+++ b/libbuild2/variable.cxx
@@ -377,7 +377,7 @@ namespace build2
   }
 
   void
-  untypify (value& v)
+  untypify (value& v, bool reduce)
   {
     if (v.type == nullptr)
       return;
@@ -389,7 +389,7 @@ namespace build2
     }
 
     names ns;
-    names_view nv (v.type->reverse (v, ns));
+    names_view nv (v.type->reverse (v, ns, reduce));
 
     if (nv.empty () || nv.data () == ns.data ())
     {
@@ -493,6 +493,7 @@ namespace build2
     type_name,
     sizeof (bool),
     nullptr,                      // No base.
+    false,                        // Not container.
     nullptr,                      // No element.
     nullptr,                      // No dtor (POD).
     nullptr,                      // No copy_ctor (POD).
@@ -550,6 +551,7 @@ namespace build2
     type_name,
     sizeof (int64_t),
     nullptr,                          // No base.
+    false,                            // Not container.
     nullptr,                          // No element.
     nullptr,                          // No dtor (POD).
     nullptr,                          // No copy_ctor (POD).
@@ -607,6 +609,7 @@ namespace build2
     type_name,
     sizeof (uint64_t),
     nullptr,                          // No base.
+    false,                            // Not container.
     nullptr,                          // No element.
     nullptr,                          // No dtor (POD).
     nullptr,                          // No copy_ctor (POD).
@@ -701,6 +704,7 @@ namespace build2
     type_name,
     sizeof (string),
     nullptr,                      // No base.
+    false,                        // Not container.
     nullptr,                      // No element.
     &default_dtor<string>,
     &default_copy_ctor<string>,
@@ -768,6 +772,7 @@ namespace build2
     type_name,
     sizeof (path),
     nullptr,                    // No base.
+    false,                      // Not container.
     nullptr,                    // No element.
     &default_dtor<path>,
     &default_copy_ctor<path>,
@@ -835,6 +840,7 @@ namespace build2
     sizeof (dir_path),
     &value_traits<path>::value_type, // Base (assuming direct cast works for
                                      // both).
+    false,                           // Not container.
     nullptr,                         // No element.
     &default_dtor<dir_path>,
     &default_copy_ctor<dir_path>,
@@ -883,6 +889,7 @@ namespace build2
     sizeof (abs_dir_path),
     &value_traits<dir_path>::value_type, // Base (assuming direct cast works
                                          // for both).
+    false,                               // Not container.
     nullptr,                             // No element.
     &default_dtor<abs_dir_path>,
     &default_copy_ctor<abs_dir_path>,
@@ -908,10 +915,10 @@ namespace build2
   }
 
   static names_view
-  name_reverse (const value& v, names&)
+  name_reverse (const value& v, names&, bool reduce)
   {
     const name& n (v.as<name> ());
-    return n.empty () ? names_view (nullptr, 0) : names_view (&n, 1);
+    return reduce && n.empty () ? names_view (nullptr, 0) : names_view (&n, 1);
   }
 
   const char* const value_traits<name>::type_name = "name";
@@ -921,6 +928,7 @@ namespace build2
     type_name,
     sizeof (name),
     nullptr,                    // No base.
+    false,                      // Not container.
     nullptr,                    // No element.
     &default_dtor<name>,
     &default_copy_ctor<name>,
@@ -975,13 +983,13 @@ namespace build2
   }
 
   static names_view
-  name_pair_reverse (const value& v, names& ns)
+  name_pair_reverse (const value& v, names& ns, bool reduce)
   {
     const name_pair& p (v.as<name_pair> ());
     const name& f (p.first);
     const name& s (p.second);
 
-    if (f.empty () && s.empty ())
+    if (reduce && f.empty () && s.empty ())
       return names_view (nullptr, 0);
 
     if (f.empty ())
@@ -1003,6 +1011,7 @@ namespace build2
     type_name,
     sizeof (name_pair),
     nullptr,                         // No base.
+    false,                           // Not container.
     nullptr,                         // No element.
     &default_dtor<name_pair>,
     &default_copy_ctor<name_pair>,
@@ -1133,10 +1142,14 @@ namespace build2
   }
 
   static names_view
-  process_path_reverse (const value& v, names& s)
+  process_path_reverse (const value& v, names& s, bool)
   {
     const auto& x (v.as<process_path> ());
 
+    // Note that strictly speaking process_path doesn't have empty
+    // representation (see convert() above). Thus we always return reduced
+    // representation.
+    //
     if (!x.empty ())
     {
       s.reserve (x.effect.empty () ? 1 : 2);
@@ -1153,6 +1166,7 @@ namespace build2
     type_name,
     sizeof (process_path),
     nullptr,                         // No base.
+    false,                           // Not container.
     nullptr,                         // No element.
     &default_dtor<process_path>,
     &process_path_copy_ctor<process_path>,
@@ -1301,10 +1315,13 @@ namespace build2
   }
 
   static names_view
-  process_path_ex_reverse (const value& v, names& s)
+  process_path_ex_reverse (const value& v, names& s, bool)
   {
     const auto& x (v.as<process_path_ex> ());
 
+    // Note that process_path_ex only has reduced empty representation (see
+    // convert() above).
+    //
     if (!x.empty ())
     {
       s.reserve ((x.effect.empty () ? 1 : 2) +
@@ -1348,6 +1365,7 @@ namespace build2
     sizeof (process_path_ex),
     &value_traits<                   // Base (assuming direct cast works
       process_path>::value_type,     // for both).
+    false,                           // Not container.
     nullptr,                         // No element.
     &default_dtor<process_path_ex>,
     &process_path_ex_copy_ctor,
@@ -1389,6 +1407,7 @@ namespace build2
     type_name,
     sizeof (target_triplet),
     nullptr,                         // No base.
+    false,                           // Not container.
     nullptr,                         // No element.
     &default_dtor<target_triplet>,
     &default_copy_ctor<target_triplet>,
@@ -1433,6 +1452,7 @@ namespace build2
     type_name,
     sizeof (project_name),
     nullptr,                         // No base.
+    false,                           // Not container.
     nullptr,                         // No element.
     &default_dtor<project_name>,
     &default_copy_ctor<project_name>,
@@ -1544,7 +1564,7 @@ namespace build2
   }
 
   static names_view
-  cmdline_reverse (const value& v, names&)
+  cmdline_reverse (const value& v, names&, bool)
   {
     const auto& x (v.as<cmdline> ());
     return names_view (x.data (), x.size ());
@@ -1565,7 +1585,8 @@ namespace build2
     type_name,
     sizeof (cmdline),
     nullptr,                           // No base.
-    &value_traits<string>::value_type,
+    true,                              // Container.
+    &value_traits<string>::value_type, // Element type.
     &default_dtor<cmdline>,
     &default_copy_ctor<cmdline>,
     &default_copy_assign<cmdline>,
diff --git a/libbuild2/variable.hxx b/libbuild2/variable.hxx
index 400aaf1..2942f1c 100644
--- a/libbuild2/variable.hxx
+++ b/libbuild2/variable.hxx
@@ -48,7 +48,11 @@ namespace build2
 
     template <typename T> const value_type* is_a () const;
 
-    // Element type, if this is a vector.
+    // True if the type is a container.
+    //
+    bool container;
+
+    // Element type, if this is a container and the element type is named.
     //
     const value_type* element_type;
 
@@ -75,9 +79,11 @@ namespace build2
     void (*const prepend) (value&, names&&, const variable*);
 
     // Reverse the value back to a vector of names. Storage can be used by the
-    // implementation if necessary. Cannot be NULL.
+    // implementation if necessary. If reduce is true, then for an empty
+    // simple value return an empty list rather than a list of one empty name.
+    // Note that the value cannot be NULL.
     //
-    names_view (*const reverse) (const value&, names& storage);
+    names_view (*const reverse) (const value&, names& storage, bool reduce);
 
     // Cast value::data_ storage to value type so that the result can be
     // static_cast to const T*. If it is NULL, then cast data_ directly. Note
@@ -363,8 +369,8 @@ namespace build2
     //
   public:
     // Assign/append/prepend a typed value. For assign, LHS should be either
-    // of the same type or untyped. For append, LHS should be either of the
-    // same type or untyped and NULL.
+    // of the same type or untyped. For append/prepend, LHS should be either
+    // of the same type or untyped and NULL.
     //
     template <typename T> value& operator= (T);
     template <typename T> value& operator+= (T);
@@ -497,18 +503,20 @@ namespace build2
   typify_atomic (context&, value&, const value_type&, const variable*);
 
   // Remove value type from the value reversing it to names. This is similar
-  // to reverse() below except that it modifies the value itself.
+  // to reverse() below except that it modifies the value itself. Note that
+  // the reduce semantics applies to empty but not null.
   //
-  LIBBUILD2_SYMEXPORT void untypify (value&);
+  LIBBUILD2_SYMEXPORT void untypify (value&, bool reduce);
 
   // Reverse the value back to names. The value should not be NULL and storage
-  // should be empty.
+  // should be empty. If reduce is true, then for an empty simple value return
+  // an empty list rather than a list of one empty name.
   //
   vector_view<const name>
-  reverse (const value&, names& storage);
+  reverse (const value&, names& storage, bool reduce);
 
   vector_view<name>
-  reverse (value&, names& storage);
+  reverse (value&, names& storage, bool reduce);
 
   // Variable lookup result, AKA, binding of a variable to a value.
   //
diff --git a/libbuild2/variable.ixx b/libbuild2/variable.ixx
index c9dfad4..c7b1f35 100644
--- a/libbuild2/variable.ixx
+++ b/libbuild2/variable.ixx
@@ -326,18 +326,21 @@ namespace build2
   }
 
   inline vector_view<const name>
-  reverse (const value& v, names& storage)
+  reverse (const value& v, names& storage, bool reduce)
   {
     assert (v &&
             storage.empty () &&
             (v.type == nullptr || v.type->reverse != nullptr));
-    return v.type == nullptr ? v.as<names> () : v.type->reverse (v, storage);
+
+    return v.type == nullptr
+      ? v.as<names> ()
+      : v.type->reverse (v, storage, reduce);
   }
 
   inline vector_view<name>
-  reverse (value& v, names& storage)
+  reverse (value& v, names& storage, bool reduce)
   {
-    names_view cv (reverse (static_cast<const value&> (v), storage));
+    names_view cv (reverse (static_cast<const value&> (v), storage, reduce));
     return vector_view<name> (const_cast<name*> (cv.data ()), cv.size ());
   }
 
diff --git a/libbuild2/variable.txx b/libbuild2/variable.txx
index fda3486..2950ea0 100644
--- a/libbuild2/variable.txx
+++ b/libbuild2/variable.txx
@@ -229,13 +229,13 @@ namespace build2
 
   template <typename T>
   names_view
-  simple_reverse (const value& v, names& s)
+  simple_reverse (const value& v, names& s, bool reduce)
   {
     const T& x (v.as<T> ());
 
-    // Represent an empty simple value as empty name sequence rather than
-    // a single empty name. This way, for example, during serialization we
-    // end up with a much saner looking:
+    // Unless requested otherwise, represent an empty simple value as empty
+    // name sequence rather than a single empty name. This way, for example,
+    // during serialization we end up with a much saner looking:
     //
     // config.import.foo =
     //
@@ -245,6 +245,8 @@ namespace build2
     //
     if (!value_traits<T>::empty (x))
       s.emplace_back (value_traits<T>::reverse (x));
+    else if (!reduce)
+      s.push_back (name ());
 
     return s;
   }
@@ -590,7 +592,7 @@ namespace build2
 
   template <typename T>
   static names_view
-  vector_reverse (const value& v, names& s)
+  vector_reverse (const value& v, names& s, bool)
   {
     auto& vv (v.as<vector<T>> ());
     s.reserve (vv.size ());
@@ -651,7 +653,8 @@ namespace build2
     nullptr,                          // Patched above.
     sizeof (vector<T>),
     nullptr,                          // No base.
-    &value_traits<T>::value_type,
+    true,                             // Container.
+    &value_traits<T>::value_type,     // Element type.
     &default_dtor<vector<T>>,
     &default_copy_ctor<vector<T>>,
     &default_copy_assign<vector<T>>,
@@ -702,7 +705,7 @@ namespace build2
 
   template <typename K, typename V>
   static names_view
-  pair_vector_reverse (const value& v, names& s)
+  pair_vector_reverse (const value& v, names& s, bool)
   {
     auto& vv (v.as<vector<pair<K, V>>> ());
     s.reserve (2 * vv.size ());
@@ -803,7 +806,8 @@ namespace build2
     nullptr,                     // Patched above.
     sizeof (vector<pair<K, V>>),
     nullptr,                     // No base.
-    nullptr,                     // No element.
+    true,                        // Container.
+    nullptr,                     // No element (not named).
     &default_dtor<vector<pair<K, V>>>,
     &default_copy_ctor<vector<pair<K, V>>>,
     &default_copy_assign<vector<pair<K, V>>>,
@@ -882,7 +886,7 @@ namespace build2
 
   template <typename K, typename V>
   static names_view
-  map_reverse (const value& v, names& s)
+  map_reverse (const value& v, names& s, bool)
   {
     auto& vm (v.as<map<K, V>> ());
     s.reserve (2 * vm.size ());
@@ -983,7 +987,8 @@ namespace build2
     nullptr,             // Patched above.
     sizeof (map<K, V>),
     nullptr,             // No base.
-    nullptr,             // No element.
+    true,                // Container.
+    nullptr,             // No element (not named).
     &default_dtor<map<K, V>>,
     &default_copy_ctor<map<K, V>>,
     &default_copy_assign<map<K, V>>,
diff --git a/tests/value/reverse.testscript b/tests/value/reverse.testscript
index 9f73981..921d14b 100644
--- a/tests/value/reverse.testscript
+++ b/tests/value/reverse.testscript
@@ -89,3 +89,58 @@
     EOO
   }
 }
+
+: reduce
+:
+: Test empty simple value reduction heuristics.
+:
+{
+  : typed
+  :
+  $* <<EOI >>"EOO"
+  x = [string]
+  n = [string,null]
+  y = [strings] $x
+  y += $x
+  y += $n
+  print $size($y)
+
+  file{*}: y += $x
+  file{x}:
+  print $size($(file{x}: y))
+
+  for i: $x
+    print iteration
+
+  print $null($x[0])
+  EOI
+  2
+  3
+  iteration
+  false
+  EOO
+
+  : untyped
+  :
+  $* <<EOI >>"EOO"
+  x =
+  n = [null]
+  y = $x
+  y += $x
+  y += $n
+  print $size($y)
+
+  file{*}: y += $x
+  file{x}:
+  print $size($(file{x}: y))
+
+  for i: $x
+    print iteration
+
+  print $null($x[0])
+  EOI
+  0
+  0
+  true
+  EOO
+}