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POIs this a compiler bug or a programmer bug?
 

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  1. POIs this a compiler bug or a programmer bug?
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    <p>I'm playing with tuples as compile time lists. In <a href="https://stackoverflow.com/questions/9831501/how-can-i-have-multiple-parameter-packs-in-a-variadic-template">How can I have multiple parameter packs in a variadic template?</a> I answered myself with some code that works in both GCC and Clang, but Clang wont compile now that I've added (what I think is) perfect forwarding. It complains that <code>As...</code> and <code>as...</code> have different lengths in <code>std::forward&lt;As&gt;(as)...</code>. How can this be true when <code>As...</code> is the type of <code>as...</code>? It's <code>As&amp;&amp;... as</code> in the parameters.</p> <pre><code>#include &lt;iostream&gt; #include &lt;tuple&gt; template &lt; typename ... &gt; struct two_impl {}; // Base case template &lt; typename F, typename ...Bs &gt; struct two_impl &lt; F, std::tuple &lt;&gt;, std::tuple&lt; Bs... &gt; &gt; { void operator()(F&amp;&amp; f, Bs&amp;&amp;... bs) { f(std::forward&lt;Bs&gt;(bs)...); } }; // Recursive case template &lt; typename F, typename A, typename ...As, typename ...Bs &gt; struct two_impl &lt; F, std::tuple&lt; A, As... &gt;, std::tuple&lt; Bs...&gt; &gt; { void operator()(F&amp;&amp; f, A&amp;&amp; a, As&amp;&amp;... as, Bs&amp;&amp;... bs) { auto impl = two_impl &lt; F, std::tuple &lt; As&amp;&amp;... &gt;, std::tuple &lt; Bs&amp;&amp;..., A&amp;&amp; &gt; &gt;(); impl(std::forward&lt;F&gt;(f), std::forward&lt;As&gt;(as)..., std::forward&lt;Bs&gt;(bs)..., std::forward&lt;A&gt;(a)); } }; template &lt; typename F, typename ...Ts &gt; void two(F&amp;&amp; f, Ts&amp;&amp; ...ts) { auto impl = two_impl&lt; F, std::tuple &lt; Ts... &gt;, std::tuple &lt;&gt; &gt;(); impl(std::forward&lt;F&gt;(f), std::forward&lt;Ts&gt;(ts)...); } struct Test { void operator()(int i, float f, double d) { std::cout &lt;&lt; i &lt;&lt; std::endl &lt;&lt; f &lt;&lt; std::endl &lt;&lt; d &lt;&lt; std::endl; } }; int main () { two(Test(), 1, 1.5f, 2.1); } </code></pre> <p>Compiling with <code>clang -lstdc++ -std=c++0x multiple_parameter_packs.cpp</code></p> <pre><code>clang -lstdc++ -std=c++0x multiple_parameter_packs.cpp multiple_parameter_packs.cpp:24:50: error: pack expansion contains parameter packs 'As' and 'as' that have different lengths (1 vs. 2) impl(std::forward&lt;F&gt;(f), std::forward&lt;As&gt;(as)..., std::forward&lt;Bs&gt;(bs)..., std::forward&lt;A&gt;(a)); ~~ ~~ ^ multiple_parameter_packs.cpp:24:5: note: in instantiation of member function 'two_impl&lt;Test, std::tuple&lt;float &amp;&amp;, double &amp;&amp;&gt;, std::tuple&lt;int &amp;&amp;&gt; &gt;::operator()' requested here impl(std::forward&lt;F&gt;(f), std::forward&lt;As&gt;(as)..., std::forward&lt;Bs&gt;(bs)..., std::forward&lt;A&gt;(a)); ^ multiple_parameter_packs.cpp:31:3: note: in instantiation of member function 'two_impl&lt;Test, std::tuple&lt;int, float, double&gt;, std::tuple&lt;&gt; &gt;::operator()' requested here impl(std::forward&lt;F&gt;(f), std::forward&lt;Ts&gt;(ts)...); ^ multiple_parameter_packs.cpp:41:3: note: in instantiation of function template specialization 'two&lt;Test, int, float, double&gt;' requested here two(Test(), 1, 1.5f, 2.1); ^ 1 error generated. Compilation exited abnormally with code 1 at Fri Mar 23 14:25:14 </code></pre>
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    1. USCommunity
    1. USSamuel Danielson
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      1. LTLinked
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    1. This table or related slice is empty.
    1. PO
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      1. VTUpMod
    1. COReduced test case: http://ideone.com/YA6ao
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      1. POIs this a compiler bug or a programmer bug?
      1. USkennytm
    2. COThe compiler doesn't like your Bs
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      1. POIs this a compiler bug or a programmer bug?
      1. USCaptain Obvlious
 

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