introduce expr_protectedt

This will serve as the interface that we aspire exprt to offer; in
particular, access to opX is protected.

Author: Daniel Kroening

src/util/expr.h

@@ -319,6 +319,28 @@ class exprt:public irept
   const_unique_depth_iteratort unique_depth_cend() const;
 };
 
+/// Base class for all expressions. This protects low-level methods in
+/// exprt that are not type safe. Depcrecated constructors are removed.
+/// This API will eventually replace exprt.
+class expr_protectedt : public exprt
+{
+protected:
+  // constructors
+  expr_protectedt(const irep_idt &_id, const typet &_type) : exprt(_id, _type)
+  {
+  }
+
+  // protect these low-level methods
+  using exprt::add;
+  using exprt::make_bool;
+  using exprt::make_typecast;
+  using exprt::op0;
+  using exprt::op1;
+  using exprt::op2;
+  using exprt::op3;
+  using exprt::remove;
+};
+
 class expr_visitort
 {
 public:

src/util/std_expr.cpp

@@ -167,8 +167,8 @@ extractbits_exprt::extractbits_exprt(
   const exprt &_src,
   const std::size_t _upper,
   const std::size_t _lower,
-  const typet &_type):
-  exprt(ID_extractbits, _type)
+  const typet &_type)
+  : expr_protectedt(ID_extractbits, _type)
 {
   PRECONDITION(_upper >= _lower);
   operands().resize(3);