Proposed resolution:

This wording is relative to N5014.

1. Modify [exec.as.awaitable] as indicated:

   -1- `as_awaitable` transforms an object into one that is awaitable within a particular coroutine. Subclause [exec.coro.util] makes use of the following exposition-only entities:

   namespace std::execution {
     template<class Sndr, class Promise>
       concept awaitable-sender =
         single-sender<Sndr, env_of_t<Promise>> &&
         sender_to<Sndr, typename sender-awaitable<Sndr, Promise>::awaitable-receiver> && // see below
         requires (Promise& p) {
           { p.unhandled_stopped() } -> convertible_to<coroutine_handle<>>;
         };
     […]
   }
   

In [exec.as.awaitable] p1 there is an exposition-only helper concept awaitable-sender defined as follows:

namespace std::execution {
  template<class Sndr, class Promise>
    concept awaitable-sender =
      single-sender<Sndr, env_of_t<Promise>> &&
      sender_to<Sndr, awaitable-receiver> && // see below
      requires (Promise& p) {
        { p.unhandled_stopped() } -> convertible_to<coroutine_handle<>>;
      };
}

The mention of the type awaitable-receiver here does not refer to any exposition-only type defined at namespace-scope. It seems to, instead, be referring to the nested member-type sender-awaitable<Sndr, Promise>::awaitable-receiver and so should be qualified as such.