Reverse empty simple values as an empty names sequence rather than empty name

This way we get:

config.import.foo =

Rather than:

config.import.foo = {}

3 files changed, 33 insertions, 12 deletions

diff --git a/build2/variable b/build2/variable
index 1248bd3..fb44bb4 100644
--- a/build2/variable
+++ b/build2/variable
@@ -476,6 +476,7 @@ namespace build2
     static void append (value&, bool); // OR.
     static name reverse (bool x) {return name (x ? "true" : "false", false);}
     static int compare (bool, bool);
+    static bool empty (bool) {return false;}
 
     static const char* const type_name;
     static const build2::value_type value_type;
@@ -491,6 +492,7 @@ namespace build2
     static void append (value&, uint64_t); // ADD.
     static name reverse (uint64_t x) {return name (to_string (x), false);}
     static int compare (uint64_t, uint64_t);
+    static bool empty (bool) {return false;}
 
     static const char* const type_name;
     static const build2::value_type value_type;
diff --git a/build2/variable.cxx b/build2/variable.cxx
index 7a13618..0134683 100644
--- a/build2/variable.cxx
+++ b/build2/variable.cxx
@@ -643,7 +643,8 @@ namespace build2
   static names_view
   name_reverse (const value& v, names&)
   {
-    return names_view (&v.as<name> (), 1);
+    const name& n (v.as<name> ());
+    return n.empty () ? names_view (nullptr, 0) : names_view (&n, 1);
   }
 
   const char* const value_traits<name>::type_name = "name";
@@ -726,19 +727,23 @@ namespace build2
   static names_view
   process_path_reverse (const value& v, names& s)
   {
-    auto& pp (v.as<process_path> ());
-    s.reserve (pp.effect.empty () ? 1 : 2);
+    const process_path& x (v.as<process_path> ());
 
-    s.push_back (name (pp.recall.directory (),
-                       string (),
-                       pp.recall.leaf ().string ()));
-
-    if (!pp.effect.empty ())
+    if (!x.empty ())
     {
-      s.back ().pair = '@';
-      s.push_back (name (pp.effect.directory (),
+      s.reserve (x.effect.empty () ? 1 : 2);
+
+      s.push_back (name (x.recall.directory (),
                          string (),
-                         pp.effect.leaf ().string ()));
+                         x.recall.leaf ().string ()));
+
+      if (!x.effect.empty ())
+      {
+        s.back ().pair = '@';
+        s.push_back (name (x.effect.directory (),
+                           string (),
+                           x.effect.leaf ().string ()));
+      }
     }
 
     return s;
diff --git a/build2/variable.txx b/build2/variable.txx
index f1335b4..95e4f0a 100644
--- a/build2/variable.txx
+++ b/build2/variable.txx
@@ -148,7 +148,21 @@ namespace build2
   names_view
   simple_reverse (const value& v, names& s)
   {
-    s.emplace_back (value_traits<T>::reverse (v.as<T> ()));
+    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:
+    //
+    // config.import.foo =
+    //
+    // Rather than:
+    //
+    // config.import.foo = {}
+    //
+    if (!value_traits<T>::empty (x))
+      s.emplace_back (value_traits<T>::reverse (x));
+
     return s;
   }