protect typet::subtype() const

Protecting this method enables the use of type-specific accessor methods,
which can exploit type invariants about the existence of the subtype.

Author: kroening

src/util/type.h

@@ -45,13 +45,17 @@ class typet:public irept
   }
 #endif
 
+  // Deliberately protected -- use type-specific accessor methods instead.
+protected:
   const typet &subtype() const
   {
     if(get_sub().empty())
       return static_cast<const typet &>(get_nil_irep());
     return static_cast<const typet &>(get_sub().front());
   }
 
+public:
+  // This method will be protected eventually.
   typet &subtype()
   {
     subt &sub=get_sub();