On 27/01/2018 17:11, Bjorn Reese via Boost wrote:

Are value_type_if_enabled and error_type_if_enabled supposed to be used by the library users? If so, how?

Purely a "self documenting typedef" there to remind the API reader that that overload becomes unavailable if there is any ambiguity between value and error types. This technique lets me avoid SFINAE around the in_place_type<T> overloads, saving some compile time load. Niall -- ned Productions Limited Consulting http://www.nedproductions.biz/ http://ie.linkedin.com/in/nialldouglas/