/**
     * Implement the cast expression.
     *
     * <p>TODO: check for overflow
     *
     * @return the rhs expression casted as the lhs type
     */
    public Expression implement() {
      // Check for invalid null assignment. Code generated afterwards
      // can assume null will never be assigned to a not null value.
      checkNotNull();

      // Check for an explicit rhs null value. Code generated
      // afterwards need never check for an explicit null.
      if (rhsType.getSqlTypeName() == SqlTypeName.NULL) {
        if (lhsType.isNullable()) {
          return castFromNull();
        } else {
          // NOTE jvs 27-Jan-2005:  this code will never actually
          // be executed do to previous checkNotNull test, but
          // it still has to compile!
          return rhsAsValue();
        }
      }

      // Case when left hand side is a nullable primitive
      if (translator.isNullablePrimitive(lhsType)) {
        if (SqlTypeUtil.isJavaPrimitive(rhsType)
            && (!rhsType.isNullable() || translator.isNullablePrimitive(rhsType))) {
          return castPrimitiveToNullablePrimitive();
        }
        return castToAssignableValue();
      }

      // Case when left hand side is a not nullable primitive
      if (SqlTypeUtil.isJavaPrimitive(lhsType)) {
        return castToNotNullPrimitive();
      }

      // Case when left hand side is a structure
      if (lhsType.isStruct()) {
        assert (rhsType.isStruct());

        // TODO jvs 27-May-2004:  relax this assert and deal with
        // conversions, null checks, etc.
        assert (lhsType.equals(rhsType));

        return getDirectAssignment();
      }

      // Default is to treat non-primitives as AssignableValue
      return castToAssignableValue();
    }
 /**
  * Gets the right hand expression as a valid value to be assigned to the left hand side. Usually
  * returns the original rhs. However, if the lhs is of a primitive type, and the rhs is an
  * explicit null, returns a primitive value instead.
  */
 private Expression rhsAsValue() {
   if (SqlTypeUtil.isJavaPrimitive(lhsType) && (rhsType.getSqlTypeName() == SqlTypeName.NULL)) {
     if (lhsType.getSqlTypeName() == SqlTypeName.BOOLEAN) {
       return Literal.constantFalse();
     } else {
       return Literal.constantZero();
     }
   }
   return rhsExp;
 }