#include <iostream>
#include <string>
#include <type_traits>

template < typename T, typename ...Ts> T f(const T &a, Ts&&... args)
{
    return a;
}

template < typename R, typename T, typename... Ts>
typename std::enable_if<!std::is_same<R, T>::value, R>::type
f(typename T, Ts&&... args)
//^^^^^^^^^^ --->HERE
{
    return f<R>(std::forward<Ts>(args)...);
}

int main() {
    std::cout << f<int>('a', std::string{ "string" }, 12);
    return 0;
}

f(typename T, Ts&&... args)
//^^^^^^^^^^

error : 'template<class R, class T, class ... Ts> typename std::enable_if<(! std::is_same< <template-parameter-1-1>, <template-parameter-1-2> >::value), R>::type f' conflicts with a previous declaration
typename std::enable_if<!std::is_same<R, T>::value, R>::type f(typename T, Ts&&... args)
^ ~~~~~~~
note : previous declaration 'T f(T, Ts&& ...)'

template < typename T, typename ...Ts> T f(T a, Ts&&... args)
^
error : expected nested - name - specifier before 'T'
typename std::enable_if<!std::is_same<R, T>::value, R>::type f(typename T, Ts&&... args)
^
error : expected '(' before 'T'
error : expected primary - expression before '&&' token
typename std::enable_if<!std::is_same<R, T>::value, R>::type f(typename T, Ts&&... args)
^ ~
warning : variable templates only available with - std = c++14 or -std = gnu++14
typename std::enable_if<!std::is_same<R, T>::value, R>::type f(typename T, Ts&&... args)
^
error : expected ';' before '{' token
{
^

<source>:13 : 74 : error : expected a qualified name after 'typename'
typename std::enable_if<!std::is_same<R, T>::value, R>::type f(typename T, Ts&&... args)
^
<source> : 13 : 74 : error : expected a qualified name after 'typename'
<source> : 13 : 74 : error : expected ')'
<source> : 13 : 64 : note : to match this '('
typename std::enable_if<!std::is_same<R, T>::value, R>::type f(typename T, Ts&&... args)
^
<source> : 15 : 34 : error : use of undeclared identifier 'args'; did you mean 'abs' ?
return f<R>(std::forward<Ts>(args)...);
^~~~
abs
/ usr / include / stdlib.h:837 : 12 : note : 'abs' declared here
extern int abs(int __x) __THROW __attribute__((__const__)) __wur;
^