The intent of this snippet is to illustrate the use and proscription of the typename keyword.

typename_proscript.cc

// The example I've chosen is from here:
// http://en.cppreference.com/w/cpp/language/template_specialization
#include <iostream>
template<class T>   // primary template
struct is_void : /*typename*/ std::false_type // Despite having a qualified name, typename is now allowed here.
// It leads to the error, error: keyword 'typename' not allowed in this context (the base class is implicitly a type)
{
    struct s
    {
    };
    
};
template<>  // explicit specialization for T = void
struct is_void<void> : std::true_type
{
};

template <typename X>
void foo(void)
{
    typedef typename is_void<X>::s innser_s;   // Without typename this will cause a compiler error.
    // main.cpp: In function 'void foo()':
    // main.cpp:18:13: error: need 'typename' before 'is_void<X>::s' because 'is_void<X>' is a dependent scope
    // typedef is_void<X>::s innser_s;    
}

int main()
{
    // for any type T other than void, the 
    // class is derived from false_type
    std::cout << is_void<char>::value << '\n'; 
    // but when T is void, the class is derived
    // from true_type
    std::cout << is_void<void>::value << '\n';
}