Proposed resolution:

This wording is relative to N4861 and also resolves LWG issue 3469.

1. Edit [coroutine.handle] as indicated:

   namespace std {
   
     […]
   
     template<class Promise>
     struct coroutine_handle : coroutine_handle<>
     {
       // [coroutine.handle.con], construct/reset
       using coroutine_handle<>::coroutine_handle;
       constexpr coroutine_handle() noexcept;
       constexpr coroutine_handle(nullptr_t) noexcept;
       static coroutine_handle from_promise(Promise&);
       coroutine_handle& operator=(nullptr_t) noexcept;
     
       // [coroutine.handle.export.import], export/import
       constexpr void* address() const noexcept;
       static constexpr coroutine_handle from_address(void* addr);
     
       // [coroutine.handle.conv], conversion
       constexpr operator coroutine_handle<>() const noexcept;
     
       // [coroutine.handle.observers], observers
       constexpr explicit operator bool() const noexcept;
       bool done() const;
     
       // [coroutine.handle.resumption], resumption
       void operator()() const;
       void resume() const;
       void destroy() const;
     
       // [coroutine.handle.promise], promise access
       Promise& promise() const;
     
     private:
       void* ptr;  // exposition only 
     };
   }
   

   -1- An object of type coroutine_­handle<T> is called a coroutine handle and can be used to refer to a suspended or executing coroutine. A default-constructed coroutine_­handle object whose member address() returns a null pointer value does not refer to any coroutine. Two coroutine_handle objects refer to the same coroutine if and only if their member address() returns the same value.

2. Add the following subclause under [coroutine.handle], immediately after [coroutine.handle.con]:

   ?.?.?.? Conversion [coroutine.handle.conv]

       constexpr operator coroutine_handle<>() const noexcept;
   

   -1- Effects: Equivalent to: return coroutine_handle<>::from_address(address());.

3. Edit [coroutine.handle.export.import] as indicated, splitting the two versions:

   static constexpr coroutine_handle<> coroutine_handle<>::from_address(void* addr);
   

   -?- Preconditions: addr was obtained via a prior call to address on an object whose type is a specialization of coroutine_handle.

   -?- Postconditions: from_­address(address()) == *this.

   static constexpr coroutine_handle<Promise> coroutine_handle<Promise>::from_address(void* addr);
   

   -2- Preconditions: addr was obtained via a prior call to address on an object of type cv coroutine_handle<Promise>.

   -3- Postconditions: from_­address(address()) == *this.

4. Edit [coroutine.handle.noop] as indicated:

   namespace std {
     template<>
     struct coroutine_handle<noop_coroutine_promise> : coroutine_handle<>
     {
       // [coroutine.handle.noop.conv], conversion
       constexpr operator coroutine_handle<>() const noexcept;
   
       // [coroutine.handle.noop.observers], observers
       constexpr explicit operator bool() const noexcept;
       constexpr bool done() const noexcept;
   
       // [coroutine.handle.noop.resumption], resumption
       constexpr void operator()() const noexcept;
       constexpr void resume() const noexcept;
       constexpr void destroy() const noexcept;
   
       // [coroutine.handle.noop.promise], promise access
       noop_coroutine_promise& promise() const noexcept;
   
       // [coroutine.handle.noop.address], address
       constexpr void* address() const noexcept;
     
     private:
       coroutine_handle(unspecified);
       void* ptr; // exposition only 
     };
   }
   

5. Add the following subclause under [coroutine.handle.noop], immediately before [coroutine.handle.noop.observers]:

   ?.?.?.?.? Conversion [coroutine.handle.noop.conv]

       constexpr operator coroutine_handle<>() const noexcept;
   

   -1- Effects: Equivalent to: return coroutine_handle<>::from_address(address());.

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

[ 2020-08-21; Issue processing telecon: moved to Tentatively Ready ]

[ 2020-07-29 Tim adds PR and comments ]

The root cause for this issue as well as issue 3469 is the unnecessary public derivation from coroutine_handle<void>. The proposed resolution below replaces the derivation with a conversion function and adds explicit declarations for members that were previously inherited. It also modifies the preconditions on from_address with goal of making it impossible to obtain a coroutine_handle<P> to a coroutine whose promise type is not P in well-defined code.

[ 2020-07-12; Reflector prioritization ]

Set priority to 2 after reflector discussions.

[coroutine.handle.noop.resumption]/2 states "Remarks: If noop_coroutine_handle is converted to coroutine_handle<>, calls to operator(), resume and destroy on that handle will also have no observable effects." This suggests that e.g. in this function:

void f(coroutine_handle<> meow) 
{
  auto woof = noop_coroutine();
  static_cast<coroutine_handle<>&>(woof) = meow;
  static_cast<coroutine_handle<>&>(woof).resume();
}

the final call to coroutine_handle<>::resume must have no effect regardless of what coroutine (if any) meow refers to, contradicting the specification of coroutine_handle<>::resume. Even absent this contradiction, implementing the specification requires coroutine_handle<>::resume to determine if *this is a base subobject of a noop_coroutine_handle, which seems pointlessly expensive to implement.

[coroutine.handle.noop.address]/2 states "Remarks: A noop_coroutine_handle's ptr is always a non-null pointer value." Similar to the above case, a slicing assignment of a default-initialized coroutine_handle<> to a noop_coroutine_handle must result in ptr having a null pointer value — another contradiction between the requirements of noop_coroutine_handle and coroutine_handle<>.