Issue 3460: Unimplementable noop_coroutine

3460. Unimplementable `noop_coroutine_handle` guarantees

Section: 17.12.5.2.4 [coroutine.handle.noop.resumption] Status: C++23 Submitter: Casey Carter Opened: 2020-07-01 Last modified: 2023-11-22

Priority: 2

View all issues with C++23 status.

Discussion:

17.12.5.2.4 [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.

17.12.5.2.6 [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<>.

[2020-07-12; Reflector prioritization]

Set priority to 2 after reflector discussions.

[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.

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

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

Proposed resolution:

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

Edit 17.12.4 [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.

Add the following subclause under 17.12.4 [coroutine.handle], immediately after 17.12.4.2 [coroutine.handle.con]:
?.?.?.? Conversion [coroutine.handle.conv]
```
    constexpr operator coroutine_handle<>() const noexcept;
```
-1- Effects: Equivalent to: return coroutine_handle<>::from_address(address());.
Edit 17.12.4.4 [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.

Edit 17.12.5.2 [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 
  };
}

Add the following subclause under 17.12.5.2 [coroutine.handle.noop], immediately before 17.12.5.2.3 [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());.

3460. Unimplementable noop_coroutine_handle guarantees

3460. Unimplementable `noop_coroutine_handle` guarantees