[Boost-users] [Concept] Ignoring return types

I have a template that I instantiate with a type that has to be nullary callable, so template <typename NullaryCallable> struct X { /*....*/ }; I'd like to be a good boy scout here, so I should constrain the template parameter, so template <typename NullaryCallablle> struct X { BOOST_CONCEPT_ASSERT(( Generator )); // ... }; However, that constrains it to be nullary callable and returning void, and I'd prefer to avoid the void condition. Is all that sensible, true, and fixable? Thx - Rob.