Add well-formedness check for code_typet

Author: danpoe

src/util/std_types.h

@@ -981,6 +981,19 @@ class code_typet:public typet
     }
     return parameter_indices;
   }
+
+  static void check(
+    const typet &type,
+    const validation_modet vm = validation_modet::INVARIANT)
+  {
+    DATA_CHECK(
+      type.find(ID_parameters) != get_nil_irep(),
+      "code type should indicate parameters");
+
+    DATA_CHECK(
+      type.find(ID_return_type) != get_nil_irep(),
+      "code type should indicate return type");
+  }
 };
 
 /// Check whether a reference to a typet is a \ref code_typet.
@@ -1003,13 +1016,15 @@ inline bool can_cast_type<code_typet>(const typet &type)
 inline const code_typet &to_code_type(const typet &type)
 {
   PRECONDITION(can_cast_type<code_typet>(type));
+  code_typet::check(type);
   return static_cast<const code_typet &>(type);
 }
 
 /// \copydoc to_code_type(const typet &)
 inline code_typet &to_code_type(typet &type)
 {
   PRECONDITION(can_cast_type<code_typet>(type));
+  code_typet::check(type);
   return static_cast<code_typet &>(type);
 }