On 4/13/19 7:26 PM, Bjorn Reese via Boost wrote:

Better compiler error messages would be desirable. For instance, if we use an illegal visitor that has different return types, then the compiler error does not give us any hint about the different return types.

A possible solution is to declare a tag and use this in the else-part of mp_if. E.g. struct return_types_must_be_the_same; template<class L> using front_if_same = mp11::mp_if, mp11::mp_front<L>, return_types_must_be_the_same>;