AMDG Robert Jones wrote:

Given a template class like

template < typename T > struct A { template < typename U > friend bool operator == ( const A &, const U & ); template < typename U > friend bool operator == ( const U &, const A & ); template < typename U > friend bool operator != ( const A &, const U & ); template < typename U > friend bool operator != ( const U &, const A & ); // ...other stuff... };

can I express it using equality_comparable?

There aren't any template operators in boost::operators, AFAICT. In Christ, Steven Watanabe