diff --git a/data/c++.m4 b/data/c++.m4
index 4fb28051..855099f7 100644
--- a/data/c++.m4
+++ b/data/c++.m4
@@ -306,7 +306,7 @@ m4_define([b4_public_types_define],
   {
     this->type = s.type_get ();]b4_locations_if([
     location = s.location;])[
-    ]b4_variant_if([b4_symbol_variant([s.type_get ()], [value], [build],
+    ]b4_variant_if([b4_symbol_variant([s.type_get ()], [value], [move],
                                       [s.value])],
                    [value = s.value;])[
   }
diff --git a/data/variant.hh b/data/variant.hh
index 4317abaf..41286c5b 100644
--- a/data/variant.hh
+++ b/data/variant.hh
@@ -161,7 +161,12 @@ m4_define([b4_variant_define],
     }
 
     /// Swap the content with \a other, of same type.
-    /// Both variants must be built beforehand.
+    /// Both variants must be built beforehand, because swapping the actual
+    /// data requires reading it (with as()), and this is not possible on
+    /// unconstructed variants: it would require some dynamic testing, which
+    /// should not be the variant's responsability.
+    /// Swapping between built and ((possibly) non-built is done with
+    /// variant::move ().
     template <typename T>
     inline void
     swap (variant<S>& other)
@@ -176,8 +181,9 @@ m4_define([b4_variant_define],
     /// Destroys \a other.
     template <typename T>
     inline void
-    build (variant<S>& other)
-    {
+    move (variant<S>& other)
+    {]b4_parse_assert_if([
+      YYASSERT (! built);])[
       build<T>();
       swap<T>(other);
       other.destroy<T>();