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#include <tuple>
#include <iostream>
 
struct A { using data_type = int; };
struct B { using data_type = char; };
 
template < typename... Ts >
using cvt_2_data = std::tuple< Ts::data... >;
 
void main()
{
	using my_typelist = std::tuple<A, B>;
	cvt_2_data<my_typelist> data;
	std::cout 
		<< std::get<A::data_type>(data) << " "
		<< std::get<B::data_type>(data) << std::endl;
}

prog.cpp:8:32: error: template argument for template type parameter must be a type
using cvt_2_data = std::tuple< Ts::data... >;
                               ^~~~~~~~~~~
/usr/bin/../lib/gcc/x86_64-linux-gnu/8/../../../../include/c++/8/tuple:555:24: note: template parameter is declared here
  template<typename... _Elements>
                       ^
prog.cpp:10:1: error: 'main' must return 'int'
void main()
^~~~
int
prog.cpp:13:2: error: use of undeclared identifier 'cvt_2_data'
        cvt_2_data<my_typelist> data;
        ^
prog.cpp:13:13: error: unexpected type name 'my_typelist': expected expression
        cvt_2_data<my_typelist> data;
                   ^
prog.cpp:13:26: error: use of undeclared identifier 'data'
        cvt_2_data<my_typelist> data;
                                ^
prog.cpp:15:29: error: use of undeclared identifier 'data'
                << std::get<A::data_type>(data) << " "
                                          ^
prog.cpp:16:29: error: use of undeclared identifier 'data'
                << std::get<B::data_type>(data) << std::endl;
                                          ^
7 errors generated.

Standard output is empty