https://svn.boost.org/trac10/ticket/11602 <p> There is one "converting constructor" in boost.variant which accepts any type - even those which are not convertible to instantiation types of boost.variant. This causes that writing function accepting some specific variant instantiation cause that this function is on candidate list of any type in your program. </p> <p> The problem is with this constructor: </p> <pre class="wiki"> template &lt;typename T&gt; variant(const T&amp; operand) { convert_construct(operand, 1L); } </pre><p> And the small code presenting the issue: </p> <pre class="wiki">#include &lt;iostream&gt; #include &lt;boost/variant.hpp&gt; // types not related in any way class A {}; class B {}; class C {}; class D {}; using ABC_variant = boost::variant&lt;A,B,C&gt;; std::ostream&amp; operator &lt;&lt; (std::ostream&amp; os, const ABC_variant&amp;) { return os &lt;&lt; "ABC"; } int main() { D d; std::cout &lt;&lt; d; } </pre><p> As described in <a class="ext-link" href="http://stackoverflow.com/questions/32275725/boostvariant-construction-weirdness-its-ctor-accepts-everything"><span class="icon">​</span>http://stackoverflow.com/questions/32275725/boostvariant-construction-weirdness-its-ctor-accepts-everything</a> </p> <p> compiler (gcc4.9) tries to use ostream operator with this diagnostic: </p> <p> /usr/include/boost/variant/variant.hpp:1591:38: error: no matching function for call to 'boost::variant&lt;A, B, C&gt;::initializer::initialize(void*, D&amp;)' </p> <blockquote> <p> initializer::initialize( </p> </blockquote> <p> However it should simple report that ostream operator for D is missing. </p> <p> One simple way is to declare this boost.variant constructor as explicit (what probably would impact to many client code) or to make some restriction in this constructor like this C++11 way: </p> <pre class="wiki">template &lt;typename T, typename ...C&gt; struct IsAnyOf; template &lt;typename T, typename ...C&gt; struct IsAnyOf&lt;T,T,C...&gt; : std::true_type {}; template &lt;typename T&gt; struct IsAnyOf&lt;T&gt; : std::false_type {}; template &lt;typename T, typename C1, typename ...C&gt; struct IsAnyOf&lt;T,C1,C...&gt; : IsAnyOf&lt;T, C...&gt; {}; template &lt;typename T, typename EnableIf = typename std::enable_if&lt;IsAnyOf&lt;VariantType...&gt;::value&gt;::type&gt; variant(const T&amp; operand) </pre><p> BR, Piotr Nycz (piotrwn1 @ gmail com) </p> en-us /htdocs/site/boost.png https://svn.boost.org/trac10/ticket/11602 Trac 1.4.3 piotrwn1@… Tue, 01 Sep 2015 08:31:17 GMT https://svn.boost.org/trac10/ticket/11602#comment:2 https://svn.boost.org/trac10/ticket/11602#comment:2 <ul> <li><strong>cc</strong> <span class="trac-author">raad@…</span> added </li> </ul> Ticket raad@… Thu, 14 Apr 2016 21:39:01 GMT https://svn.boost.org/trac10/ticket/11602#comment:4 https://svn.boost.org/trac10/ticket/11602#comment:4 <ul> <li><strong>owner</strong> changed from <span class="trac-author">ebf</span> to <span class="trac-author">Antony Polukhin</span> </li> <li><strong>status</strong> <span class="trac-field-old">new</span> → <span class="trac-field-new">assigned</span> </li> <li><strong>milestone</strong> <span class="trac-field-old">To Be Determined</span> → <span class="trac-field-new">Boost 1.62.0</span> </li> </ul> Ticket Antony Polukhin Fri, 27 May 2016 22:39:24 GMT https://svn.boost.org/trac10/ticket/11602#comment:6 https://svn.boost.org/trac10/ticket/11602#comment:6 <ul> <li><strong>status</strong> <span class="trac-field-old">assigned</span> → <span class="trac-field-new">closed</span> </li> <li><strong>resolution</strong> → <span class="trac-field-new">fixed</span> </li> </ul> Ticket