Store symbol type instead of followed type when clean casting

Don't store followed type as type of superclass when doing clean cast as it may later change since we can now convert more methods after typechecking and doing this may change the type

Author: NathanJPhillips

src/java_bytecode/java_pointer_casts.cpp

@@ -45,16 +45,20 @@ bool find_superclass_with_type(
     if(base_struct.components().empty())
       return false;
 
-    const typet &first_field_type=
-        ns.follow(base_struct.components()[0].type());
+    const typet &first_field_type=base_struct.components()[0].type();
     ptr=clean_deref(ptr);
+    // Careful not to use the followed type here, as stub types may be
+    // extended by later method conversion adding fields (e.g. an access
+    // against x->y might add a new field `y` to the type of `*x`)
     ptr=member_exprt(
       ptr,
       base_struct.components()[0].get_name(),
       first_field_type);
     ptr=address_of_exprt(ptr);
 
-    if(first_field_type==target_type)
+    // Compare the real (underlying) type, as target_type is already a non-
+    // symbolic type.
+    if(ns.follow(first_field_type)==target_type)
       return true;
   }
 }