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copied!<p>There are two problems here:</p> <ol> <li><code>enable_if</code> works with partial specialization, not primary templates.</li> <li>The number of externally-visible arguments is determined by the primary template, of which there may be only one.</li> </ol> <h1>Answer 1.</h1> <p>As you suggested in chat, a linked list of templates can emulate the variadic parameter pack.</p> <pre><code>template<int I> class X{ template<class list, class = void> struct Y; template<class list> struct Y< list, typename std::enable_if<I==1>::type > { typedef typename list::type t1; }; template<class list> struct Y< list, typename std::enable_if<I==2>::type > { typedef typename list::type t1; typedef typename list::next::type t2; }; }; </code></pre> <p>If you end up with <code>next::next::next</code> garbage, it's easy to write a metafunction, or use Boost MPL.</p> <hr> <h1>Answer 2.</h1> <p>The different-arity templates can be named similarly but still stay distinct if they are nested inside the SFINAE-controlled type.</p> <pre><code>template<int I> class X{ template<typename = void, typename = void> struct Z; template<typename v> struct Z< v, typename std::enable_if<I==1>::type > { template<class T1> struct Y{}; }; template<typename v> struct Z< v, typename std::enable_if<I==2>::type > { template<class T1, class T2> struct Y{}; }; }; X<1>::Z<>::Y< int > a; X<2>::Z<>::Y< char, double > b; </code></pre>

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