Byte-operator lowering: struct and union constants are not ID_constant

These are struct- and union expressions, or possible empty_union_exprt.
Creating constant_exprt resulted in a segmentation fault in simplifying
member expressions, as witnessed when working on
model-checking/kani#705.

src/util/lower_byte_operators.cpp

@@ -98,7 +98,8 @@ static struct_exprt bv_to_struct_expr(
 
     if(component_bits == 0)
     {
-      operands.push_back(constant_exprt{irep_idt{}, comp.type()});
+      operands.push_back(
+        bv_to_expr(bitvector_expr, comp.type(), endianness_map, ns));
       continue;
     }
 
@@ -123,17 +124,16 @@ static struct_exprt bv_to_struct_expr(
 
 /// Convert a bitvector-typed expression \p bitvector_expr to a union-typed
 /// expression. See \ref bv_to_expr for an overview.
-static union_exprt bv_to_union_expr(
+static exprt bv_to_union_expr(
   const exprt &bitvector_expr,
   const union_typet &union_type,
   const endianness_mapt &endianness_map,
   const namespacet &ns)
 {
   const union_typet::componentst &components = union_type.components();
 
-  // empty union, handled the same way as done in expr_initializert
   if(components.empty())
-    return union_exprt{irep_idt{}, nil_exprt{}, union_type};
+    return empty_union_exprt{union_type};
 
   const auto widest_member = union_type.find_widest_union_component(ns);
 
@@ -147,7 +147,7 @@ static union_exprt bv_to_union_expr(
   {
     return union_exprt{
       components.front().get_name(),
-      constant_exprt{irep_idt{}, components.front().type()},
+      bv_to_expr(bitvector_expr, components.front().type(), endianness_map, ns),
       union_type};
   }
 
@@ -371,13 +371,13 @@ static exprt bv_to_expr(
   }
   else if(target_type.id() == ID_union_tag)
   {
-    union_exprt result = bv_to_union_expr(
+    exprt result = bv_to_union_expr(
       bitvector_expr,
       ns.follow_tag(to_union_tag_type(target_type)),
       endianness_map,
       ns);
     result.type() = target_type;
-    return std::move(result);
+    return result;
   }
   else if(target_type.id() == ID_array)
   {

src/util/simplify_expr_struct.cpp

@@ -104,21 +104,20 @@ simplify_exprt::simplify_member(const member_exprt &expr)
       }
     }
   }
-  else if(op.id()==ID_struct ||
-          op.id()==ID_constant)
+  else if(op.id() == ID_struct)
   {
-    if(op_type.id()==ID_struct)
+    // pull out the right member
+    const struct_typet &struct_type = to_struct_type(op_type);
+    if(struct_type.has_component(component_name))
     {
-      // pull out the right member
-      const struct_typet &struct_type=to_struct_type(op_type);
-      if(struct_type.has_component(component_name))
-      {
-        std::size_t number=struct_type.component_number(component_name);
-        DATA_INVARIANT(
-          op.operands()[number].type() == expr.type(),
-          "member expression type must match component type");
-        return op.operands()[number];
-      }
+      std::size_t number = struct_type.component_number(component_name);
+      DATA_INVARIANT(
+        op.operands().size() > number,
+        "struct expression must have sufficiently many operands");
+      DATA_INVARIANT(
+        op.operands()[number].type() == expr.type(),
+        "member expression type must match component type");
+      return op.operands()[number];
     }
   }
   else if(op.id()==ID_byte_extract_little_endian ||