Direct inheritance from the same type cpp2::store_as_base<T> multiple times is ill-formed. I suggest its API follows https://www.boost.org/doc/libs/release/libs/utility/doc/html/utility/utilities/base_from_member.html.

Minimal reproducer:

x: type = { }
w: type = {
  a: std::string;
  b: std::string;
  this: x;
}

Commands:

cppfront x.cpp2
clang++17 -std=c++2b -I $CPPFRONT_INCLUDE_DIR x.cpp

Expected result: A well-formed program.

Actual result and error:

Generated C++1

//=== Cpp2 type declarations ====================================================
#include "cpp2util.h"
#line 1 "x.cpp2"
class x;
class w;
//=== Cpp2 type definitions and function declarations ===========================
#line 1 "x.cpp2"
class x {};
class w: private cpp2::store_as_base<std::string>, private cpp2::store_as_base<std::string>, public x {
#line 6 "x.cpp2"
};
//=== Cpp2 function definitions =================================================

x.cpp2:2:52: error: base class 'cpp2::store_as_base<std::string>' (aka 'store_as_base<basic_string<char>>') specified more than once as a direct base class
class w: private cpp2::store_as_base<std::string>, private cpp2::store_as_base<std::string>, public x {
                                                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.