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    <p>If you compile using <a href="http://clang.llvm.org/" rel="nofollow noreferrer">clang</a>, it helpfully reports that (2) and (3) are unusable. The warning for (3), which you expect to be selected, is as follows:</p> <blockquote> <p>warning: class template partial specialization contains a template parameter that can not be deduced; this partial specialization will never be used</p> <pre><code>struct alpha&lt;std::tuple&lt;Args..., T&gt;, T&gt; { ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ </code></pre> <p>note: non-deducible template parameter '<code>Args</code>'</p> <pre><code>template&lt;class T, class... Args&gt; ^ </code></pre> </blockquote> <p>Why is <code>Args</code> not deducible? The C++0x FDIS states at §14.8.2.5/9:</p> <blockquote> <p>If the template argument list of [a type that is specified in terms of template parameters] contains a pack expansion that is not the last template argument, the entire template argument list is a non-deduced context.</p> </blockquote> <p>In your specialization, the type <code>std::tuple&lt;Args..., T&gt;</code> is a type that is specified in terms of template parameters <code>Args</code> and <code>T</code>. It contains a pack expansion (<code>Args...</code>), but that pack expansion is not the last template argument (<code>T</code> is the last template argument). Thus, the entire template argument list of the <code>tuple</code> (the entirety of <code>&lt;Args..., T&gt;</code>) is a non-deduced context.</p> <p>The argument list of the <code>std::tuple</code> is the only place in the template specialization's argument list that <code>Args</code> appears; since it is not deducible from there, it is not deducible at all and the specialization will never be used.</p> <p>Matthieu M. <a href="https://stackoverflow.com/questions/6757691/partial-specialization-of-tuple-contents-with-variadic-arguments/6758377#6758377">provides a clever workaround in his answer</a>.</p>
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    1. USJames McNellis
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    1. COI don't know what the "solution" is or whether there is one at all. There are others here who would know, though.
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    2. COWell at least that tells me that there is no possible solution using this method, and lets me move on to others. If I come up with one that I like I'll post it.
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    3. COGiven that this is the issue, why not expand the pack on the left?
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