Function optionalToString

example/tutorial/introspection.cpp:43–45

//! [optionalToString.then]
template <typename T>
auto optionalToString(T const& obj)
  -> std::enable_if_t<decltype(has_toString(obj))::value, std::string>
{ return obj.toString(); }

template <typename T>
auto optionalToString(T const& obj)