#include <type_traits>

template <typename T,typename = typename std::enable_if<std::is_arithmetic<T>::value,T>::type>
class foo
{
public:
    void bar();
};

template <typename T>
void foo<T>::bar ()
{
}

error: invalid use of incomplete type 'class foo<T>'
void foo<T>::bar ()
^
error: declaration of 'class foo<T>'
class foo
^

#include <type_traits>

template <typename T,typename = typename std::enable_if<std::is_arithmetic<T>::value,T>::type>
class foo
{
public:
    void bar()
    {
    }
};