3469. Precondition of coroutine_handle::promise may be insufficient

Section: 17.12.4.7 [coroutine.handle.promise] Status: Resolved Submitter: Jiang An Opened: 2020-07-25 Last modified: 2020-11-09

Priority: 2

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Discussion:

The issue is related to LWG 3460.

Because the coroutine_handle<> base subobject of a coroutine_handle<P1> can be assigned from the one of a coroutine_handle<P2>, a coroutine_handle<P1> may refer to a coroutine whose promise type is P2. If a coroutine_handle<P> refers to a coroutine with difference, a call to promise() should result in undefined behavior IMO.

I think that 17.12.4.7 [coroutine.handle.promise]/1 should be changed to: "Preconditions: *this refers to a coroutine whose promise type is Promise.", and the same precondition should be added to 17.12.5.2.5 [coroutine.handle.noop.promise], and hence noexcept should be removed from coroutine_handle<noop_coroutine_promise>::promise.

[2020-08-21; Reflector prioritization]

Set priority to 2 after reflector discussions.

Previous resolution [SUPERSEDED]:

This wording is relative to N4861.

1. Modify 17.12.4.7 [coroutine.handle.promise] as indicated:

   Promise& promise() const;
   

   -1- Preconditions: *this refers to a coroutine whose promise type is Promise.

   -2- Returns: A reference to the promise of the coroutine.

2. Modify 17.12.5.2 [coroutine.handle.noop], class coroutine_handle<noop_coroutine_promise> synopsis, as indicated:

   […]
   // 17.12.5.2.5 [coroutine.handle.noop.promise], promise access
   noop_coroutine_promise& promise() const noexcept;
   […]
   

3. Modify 17.12.5.2.5 [coroutine.handle.noop.promise] as indicated:

   noop_coroutine_promise& promise() const noexcept;
   

   -?- Preconditions: *this refers to a coroutine whose promise type is noop_coroutine_promise.

   -1- Returns: A reference to the promise object associated with this coroutine handle.

[2020-11-09 Resolved by acceptance of 3460. Status changed: Tentatively Resolved → Resolved.]

Proposed resolution:

This issue is resolved by the resolution of issue 3460.