What is boost-dev's opinion on this proposed change in semantics?
On the basis that nobody argued against the proposed breaking interface change, I just sent in a commit which prints the following compile time error if an attempt is made to wide observe a result/std_result/boost_result, or outcome/std_outcome/boost_outcome, where it has an EC or EP which is a UDT: "Attempt to wide observe value, error or exception for a result/outcome given an EC or E type which is not void, and for whom trait::has_error_code_v<EC>, trait::has_exception_ptr_v<EC>, and trait::has_exception_ptr_v<E> are all false. Please specify a NoValuePolicy to tell result/outcome what to do, or else use a more specific convenience type alias such as unchecked<T, E> to indicate you want the wide observers to be narrow, or checked<T, E> to indicate you always want an exception throw etc." Which is a bit laboured as far as compile time errors go, but is certainly helpful. Thanks to everybody who contributed discussion on this question. Niall