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#include <iostream>
#include <array>
 
template<typename T>
struct ValueType { using value_type = typename T::value_type; };
 
template<template<typename> typename A, typename T>
struct ValueType<A<T>> { using value_type = T; };
 
template<typename T>
using value_type_t = typename ValueType<T>::value_type;
 
template<typename...> struct Show;
 
template<typename T>
struct Array {};
 
int main() {
 
	Show<value_type_t<Array<int>>> _;
 
 
	return 0;
}

prog.cpp: In function ‘int main()’:
prog.cpp:20:33: error: aggregate ‘Show<int> _’ has incomplete type and cannot be defined
  Show<value_type_t<Array<int>>> _;
                                 ^

Standard output is empty