...

I think so far I have failed to make my use case sufficiently clear, so I'll simplify and try to follow the Process. Take the following simple algorithm sum_ints:

template<class First, class Second> int sum_ints_impl( std::pair<First, Second> const & p ) { return sum_ints_impl(p.first) + sum_ints_impl(p.second); }

int sum_ints_impl( int i ) { return i; } template<class T> int sum_ints( T const & t ) { return sum_ints_impl( t ); }

Hi, how about the below. Concept IntOrPair is empty because you do not require it to have any operations. Keyword "explicit" indicates that we do not want it to be an auto-concept: concept map is > always required. The second concept_map implements the recursion.

Regards, &rzej

explicit concept< typename T > IntOrPair { }

concept_map IntOrPair<int> { }

template< IntOrPair type1, IntOrPair type2 > concept_map IntOrPair< std::pair<type1, type2> > { }

template<IntOrPair First, IntOrPair Second> int sum_ints_impl( std::pair<First, Second> const & p ) { return sum_ints_impl(p.first) + sum_ints_impl(p.second); }

int sum_ints_impl( int i ) { return i; }

template<IntOrPair T> int sum_ints( T const & t ) { return sum_ints_impl( t ); }

Well, I believe I got it wrong. It is concept maps where the specializtion is to tke plce. It should read: explicit concept< typename T > IntOrPair { int sum_ints_impl( T const& ); } concept_map IntOrPair<int> { int sum_ints_impl( int const& i ) { return i; } } template< IntOrPair type1, IntOrPair type2 > concept_map IntOrPair< std::pair<type1, type2> > { int sum_ints_impl( std::pair<First, Second> const& p ) { return sum_ints_impl(p.first) + sum_ints_impl(p.second); } } template<IntOrPair T> int sum_ints( T const & t ) { return IntOrPair::sum_ints_impl( t ); } Regrds, &rzej