Proposed resolution:

This wording is relative to N4734.

1. Modify [async.exec.ctx] p1, as indicated:

   // service access
   template<class Service> typename Service::key_type&
   use_service(execution_context& ctx);
   template<class Service, class... Args> Service&
   make_service(execution_context& ctx, Args&&... args);
   template<class Service> bool has_service(const execution_context& ctx) noexcept;
   class service_already_exists : public logic_error { };
   {
   public:
     service_already_exists();
   };
   

2. Add a new subclause after [async.exec.ctx.globals]:

   13.7.6 Class service_already_exists [async.exec.ctx.except]

   -1- The class service_already_exists defines the type of objects thrown as exceptions to report an attempt to add an existing service to an execution_context.

   service_already_exists();
   

   -2- Postconditions: what() returns an implementation-defined NTBS.

[ 2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP. ]

[ 2020-04-18 Issue Prioritization ]

Status set to Tentatively Ready after six positive votes on the reflector.

Addresses: networking.ts

In the Networking TS, the service_already_exists exception type has no constructors declared. The logic_error base class is not default constructible, so service_already_exists's implicit default constructor is defined as deleted.

Implementations can add one or more private constructors that can be used by make_service, but there seems to be little benefit to that. The Boost.Asio type of the same name has a public default constructor.