4 files changed, 52 insertions, 12 deletions
diff --git a/libbuild2/variable.cxx b/libbuild2/variable.cxx
index a4c3ef3..91b25bd 100644
--- a/libbuild2/variable.cxx
+++ b/libbuild2/variable.cxx
@@ -410,6 +410,24 @@ namespace build2
     v.assign (move (ns), nullptr); // Assign new data.
   }
 
+  [[noreturn]] void
+  convert_throw (const value_type* from, const value_type& to)
+  {
+    string m ("invalid ");
+    m += to.name;
+    m += " value: ";
+
+    if (from != nullptr)
+    {
+      m += "conversion from ";
+      m += from->name;
+    }
+    else
+      m += "null";
+
+    throw invalid_argument (move (m));
+  }
+
   // Throw invalid_argument for an invalid simple value.
   //
   [[noreturn]] static void
diff --git a/libbuild2/variable.hxx b/libbuild2/variable.hxx
index 6c78c95..3152c71 100644
--- a/libbuild2/variable.hxx
+++ b/libbuild2/variable.hxx
@@ -47,6 +47,8 @@ namespace build2
     //
     const value_type* base_type;
 
+    template <typename T> const value_type* is_a () const;
+
     // Element type, if this is a vector.
     //
     const value_type* element_type;
@@ -628,6 +630,10 @@ namespace build2
   //
   template <typename T> T convert (value&&);
 
+  // As above but preserving the value.
+  //
+  template <typename T> T convert (const value&);
+
   // Default implementations of the dtor/copy_ctor/copy_assing callbacks for
   // types that are stored directly in value::data_ and the provide all the
   // necessary functions (copy/move ctor and assignment operator).
diff --git a/libbuild2/variable.ixx b/libbuild2/variable.ixx
index a11ee12..c8f9541 100644
--- a/libbuild2/variable.ixx
+++ b/libbuild2/variable.ixx
@@ -7,6 +7,19 @@
 
 namespace build2
 {
+  // value_type
+  //
+  template <typename T>
+  inline const value_type* value_type::
+  is_a () const
+  {
+    const value_type* b (this);
+    for (;
+         b != nullptr && b != &value_traits<T>::value_type;
+         b = b->base_type) ;
+    return b;
+  }
+
   // value
   //
   inline bool value::
@@ -169,10 +182,7 @@ namespace build2
     //
     // Note that here we use the value type address as type identity.
     //
-    const value_type* b (v.type);
-    for (;
-         b != nullptr && b != &value_traits<T>::value_type;
-         b = b->base_type) ;
+    const value_type* b (v.type->is_a<T> ());
     assert (b != nullptr);
 
     return *static_cast<const T*> (v.type->cast == nullptr
diff --git a/libbuild2/variable.txx b/libbuild2/variable.txx
index 51176ae..0e10e87 100644
--- a/libbuild2/variable.txx
+++ b/libbuild2/variable.txx
@@ -55,6 +55,9 @@ namespace build2
       (n == 0 ? " value: empty" : " value: multiple names"));
   }
 
+  [[noreturn]] void
+  convert_throw (const value_type* from, const value_type& to);
+
   template <typename T>
   T
   convert (value&& v)
@@ -67,19 +70,22 @@ namespace build2
         return move (v).as<T> ();
     }
 
-    string m ("invalid ");
-    m += value_traits<T>::value_type.name;
-    m += " value: ";
+    convert_throw (v ? v.type : nullptr, value_traits<T>::value_type);
+  }
 
+  template <typename T>
+  T
+  convert (const value& v)
+  {
     if (v)
     {
-      m += "conversion from ";
-      m += v.type->name;
+      if (v.type == nullptr)
+        return convert<T> (names (v.as<names> ()));
+      else if (v.type == &value_traits<T>::value_type)
+        return v.as<T> ();
     }
-    else
-      m += "null";
 
-    throw invalid_argument (move (m));
+    convert_throw (v ? v.type : nullptr, value_traits<T>::value_type);
   }
 
   template <typename T>