This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 117a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.
2025-06-02
Subclause 9.6.4 [dcl.fct.def.coroutine] paragraph 7 specifies:
The expression promise.get_return_object() is used to initialize the returned reference or prvalue result object of a call to a coroutine. The call to get_return_object is sequenced before the call to initial-suspend and is invoked at most once.
It is unclear:
There is implementation divergence.
Note that a user-defined conversion may be involved in the initialization of the coroutine's prvalue result object from get_return_object(). Note also that the return type of get_return_object might be non-copyable and non-movable. However, there are certain programming patterns that would benefit from a late-initialized return value.
See also compiler explorer.
Suggested resolution [SUPERSEDED]:
Change in 9.6.4 [dcl.fct.def.coroutine] paragraph 7 as follows:
The expression promise.get_return_object() is used to initialize theThe returned reference or prvalue result object of a call to a coroutine is copy-initialized with promise.get_return_object(). Thecall to get_return_objectinitialization is sequenced before the call to initial-suspendand is invoked at most once.
Additional notes (January, 2023)
See also clang bug report #56532.
Forwarded to EWG with paper issue 1414, by decision of the CWG chair.
EWG 2023-02-06
EWG agrees that get_return_object is invoked outside of the try-block and that, if a conversion is needed, the return value of get_return_object is considered an xvalue that is later converted to the result object.
Proposed resolution (May, 2025):
Change in 9.6.4 [dcl.fct.def.coroutine] paragraph 5 as follows:
A coroutine behaves as if the top-level cv-qualifiers in all parameter-declarations in the declarator of its function-definition were removed and its function-body were replaced by the following replacement body:{ promise-type promise promise-constructor-arguments ; get-return-object-invocation ; try { co_await promise .initial_suspend() ; function-body } catch ( ... ) { if (!initial-await-resume-called ) throw ; promise .unhandled_exception() ; } final-suspend : co_await promise .final_suspend() ; }where
- ...
- promise-constructor-arguments is determined as follows: ...
- get-return-object-invocation is as follows:
- if the return type R of promise.get_return_object() is cv void, then get-return-object-invocation is promise.get_return_object(), and the return type of the coroutine shall also be cv void;
- otherwise, if R is the same class type as the return type of the coroutine (ignoring cv-qualification), and the implementation does not create a temporary to hold the result object (6.7.7 [class.temporary]), then get-return-object-invocation initializes the prvalue result object of the call to the coroutine from promise.get_return_object();
- otherwise, get-return-object-invocation initializes a variable with the exposition-only name gro as if by
decltype(auto) gro = promise.get_return_object();- a coroutine is suspended at the initial suspend point if ...
Replace 9.6.4 [dcl.fct.def.coroutine] paragraph 8 as follows:
The expression promise.get_return_object() is used to initialize the returned reference or prvalue result object of a call to a coroutine. The call to get_return_object is sequenced before the call to initial-suspeend and is invoked at most once.
If get-return-object-invocation defines a variable gro and the coroutine returns to its caller, whether by suspending (7.6.2.4 [expr.await]) or completing without ever suspending (8.7.5 [stmt.return.coroutine]), it is as if by return gro. This return exits the scope of gro. It exits the scope of promise only if the coroutine completed without suspending. [Note: The operand of the return might be move-eligible (7.5.5.2 [expr.prim.id.unqual]). --end note]
Change in 9.6.4 [dcl.fct.def.coroutine] paragraph 11 as follows:
... If the allocation function returns nullptr, the coroutine transfers control to the caller of the coroutineand the return value is obtained by a call toas if by return T::get_return_object_on_allocation_failure();, where T is the promise type. ...