Concepts

1. basic_outcome<T>

   A boolean concept matching types which are convertible to a basic_outcome<T, EC, EP, NoValuePolicy>.

2. basic_result<T>

   A boolean concept matching types which are convertible to a basic_result<T, E, NoValuePolicy>.

3. value_or_error<T>

   A boolean concept matching types with either a value or an error.

4. value_or_none<T>

   A boolean concept matching types with an optional value.