Long story short, right now you (I?) have to provide a specific U: template <template <class> class HKT, class T, class U = T> concept HKTWithTemplateMemberFunctionF { return requires(HKT<T> h) { // HKT<T> is a type, h is an object h.F(std::declval<U>()) -> HKT<U>; } } since the compiler cannot prove for all types U that … Read more
Yes, concepts lite basically dresses up SFINAE. Plus it allows deeper introspection to allow for better overloading. However that only works if the concept predicates are defined as concept bool. The improved overloading does not work with the current concept predicates, but conditional overloading can be used. Lets look how we can define predicates, constrain … Read more
A constraint on the deduced auto type doesn’t mean it needs to be a specific type, it means it needs to be one of a set of types that satisfy the constraint. Note that a constraint and a type are not the same thing, and they’re not interchangeable. e.g. a concept like std::integral constrains the … Read more
The following information is out of date. It needs to be updated according to the latest Concepts Lite draft. Section 3 of the constraints proposal covers this in reasonable depth. The concepts proposal has been put on the back burners for a short while in the hope that constraints (i.e. concepts-lite) can be fleshed out … Read more
It is because the grammar requires it. It does. A requires constraint does not have to use a requires expression. It can use any more-or-less arbitrary boolean constant expression. Therefore, requires (foo) must be a legitimate requires constraint. A requires expression (that thing that tests whether certain things follow certain constraints) is a distinct construct; … Read more