`run_loop` should not have a `set_error` completion signature

[ericniebler]

When run_loop was proposed, the only implementation we had synchronized with a mutex and a condition variable. Operations on those can theoretically throw exceptions, so run_loop got a set_error_t(exception_ptr) completion signature.

Since then, a lock-free implementation of run_loop has been found*. Atomic operations cannot fail with an exception, so an atomic run_loop can never complete with an error.

Proposed Wording

change the class synopsis of run_loop ([exec.run.loop.general]) as follows:

 namespace std::execution {
   class run_loop {
     // [exec.run.loop.types], associated types
     class run-loop-scheduler;                                   // exposition only
     class run-loop-sender;                                      // exposition only
     struct run-loop-opstate-base {                              // exposition only
-      virtual void execute() = 0;                               // exposition only
+      virtual void execute() noexcept = 0;                      // exposition only
       run_loop* loop;                                           // exposition only
       run-loop-opstate-base* next;                              // exposition only
     };
     template<class Rcvr>
       using run-loop-opstate = unspecified;                     // exposition only
 
     // [exec.run.loop.members], member functions
-    run-loop-opstate-base* pop-front();                         // exposition only
-    void push-back(run-loop-opstate-base*);                     // exposition only
+    run-loop-opstate-base* pop-front() noexcept;                // exposition only
+    void push-back(run-loop-opstate-base*) noexcept;            // exposition only
 
   public:
     // [exec.run.loop.ctor], constructor and destructor
     run_loop() noexcept;
     run_loop(run_loop&&) = delete;
     ~run_loop();
 
     // [exec.run.loop.members], member functions
-    run-loop-scheduler get_scheduler();
-    void run();
-    void finish();
+    run-loop-scheduler get_scheduler() noexcept;
+    void run() noexcept;
+    void finish() noexcept;
   };
 }

Change [exec.run.loop.types]/p5 as follows:

 run-loop-sender is an exposition-only type that satisfies sender.
 completion_signatures_of_t<run-loop-sender> is

-  completion_signatures<set_value_t(), set_error_t(exception_ptr), set_stopped_t()>
+  completion_signatures<set_value_t(), set_stopped_t()>

Change [exec.run.loop.types]/p9.3 as follows:

 The expression start(o) is equivalent to:

-  try {
     o.loop->push-back(addressof(o));
-  } catch(...) {
-    set_error(std::move(REC(o)), current_exception());
-  }

In [exec.run.loop.members], change the prototypes of the following member functions to add noexcept:

• pop-front
• push-back
• get_scheduler
• run
• finish