Forbid redefenition of builtin types

boot/symbol.cc

@@ -247,6 +247,11 @@ namespace boot
         return tag == type_tag::empty;
     }
 
+    alias_type::alias_type(const std::string& name)
+        : name(name), reference()
+    {
+    }
+
     pointer_type::pointer_type(type base)
         : base(base)
     {