https://svn.boost.org/trac10/ticket/3720 <p> template&lt;class T&gt; inline bool operator == ( optional&lt;T&gt; const&amp; x, optional&lt;T&gt; const&amp; y ) ; </p> <p> and all other comparison operators should be replaced by </p> <p> template&lt;class T,class Y&gt; bool operator==(optional&lt;T&gt; const &amp;,optional&lt;Y&gt; const &amp;); </p> <p> so optionals of different but comparable types can be compared. </p> en-us /htdocs/site/boost.png https://svn.boost.org/trac10/ticket/3720 Trac 1.4.3