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    <p><strong>The essence is from the comment of WhozCraig</strong>:</p> <p>When you drop the T* in both templates the program will compile (with warnings -Wreorder) and give the expected output.</p> <pre><code>operator&lt;&lt;(std::ostream&amp; stream, X&lt;H*,T...&gt; const &amp; x) operator&lt;&lt;(std::ostream&amp; stream, X&lt;vector&lt;V*&gt;*, T...&gt; const &amp; x) </code></pre> <p>After going through [14] Templates without any enlightenment, I think there is a compiler bug.</p> <p>A modified test:</p> <pre><code>#include &lt;iostream&gt; #include &lt;vector&gt; #include &lt;boost/type_traits/is_fundamental.hpp&gt; #include &lt;boost/utility/enable_if.hpp&gt; using namespace std; #define USE_VARIADIC_TEMPLATE 1 #define USE_AMBIGUOUS 1 template &lt;typename ... T&gt; struct X; template &lt;&gt; struct X &lt;&gt; { }; template &lt;typename H, typename ... T&gt; struct X&lt;H*,T*...&gt; : public X&lt;T*...&gt; { H* value; X(H* value, T*... args) : X&lt;T*...&gt;(args...), value(value) {} }; #if USE_VARIADIC_TEMPLATE template &lt;typename H, typename ... T&gt; #if USE_AMBIGUOUS std::ostream&amp; operator&lt;&lt;(std::ostream&amp; stream, X&lt;H*,T*...&gt; const &amp; x) #else std::ostream&amp; operator&lt;&lt;(std::ostream&amp; stream, X&lt;H*,T...&gt; const &amp; x) #endif #else template &lt;typename H, typename T&gt; std::ostream&amp; operator&lt;&lt;(std::ostream&amp; stream, X&lt;H*, T*&gt; const &amp; x) #endif { return stream &lt;&lt; "specialized scalar pointer"; } #if USE_VARIADIC_TEMPLATE template &lt;typename V, typename ... T&gt; #if USE_AMBIGUOUS std::ostream&amp; operator&lt;&lt;(std::ostream&amp; stream, X&lt;vector&lt;V*&gt;*,T*...&gt; const &amp; x) #else std::ostream&amp; operator&lt;&lt;(std::ostream&amp; stream, X&lt;vector&lt;V*&gt;*,T...&gt; const &amp; x) #endif #else template &lt;typename V, typename T&gt; std::ostream&amp; operator&lt;&lt;(std::ostream&amp; stream, X&lt;vector&lt;V*&gt;*, T*&gt; const &amp; x) #endif { return stream &lt;&lt; "specialized vector pointer"; } int main() { double a,b; vector&lt;double *&gt; v; X&lt;double*,double*&gt; x (&amp;a,&amp;b); X&lt;vector&lt;double*&gt;*, double*&gt; y (&amp;v, &amp;b); cout &lt;&lt; x &lt;&lt; endl; cout &lt;&lt; y &lt;&lt; endl; // this line is ambiguous according to gcc 4.6 and later } </code></pre> <p>Even the case:</p> <pre><code>template &lt;typename H, typename T&gt; std::ostream&amp; operator&lt;&lt;(std::ostream&amp; stream, X&lt;H*, T*&gt; const &amp; x) template &lt;typename V, typename T&gt; std::ostream&amp; operator&lt;&lt;(std::ostream&amp; stream, X&lt;vector&lt;V*&gt;*, T*&gt; const &amp; x) </code></pre> <p>compiles nicely.</p> <p><strong>Edit</strong>: If the variadic template hold just one T passed as T* it should be the simple case shown above.</p>
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    1. COI prefer the modification from WhozCraig because to be consisten all T*... unpacking should be removed and not only the ones of the function overload templates. In your modified code you should also remove the last comment as it is confusing in that context.
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      1. USPatrick Fromberg
    2. CO...I agree now that there is a compiler error here but not a severe one because it should be a warning anyway. The right answer is as you and previously WhozCraig pointed out, that I had written something stupid and thanks to SO I am now aware.
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