This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 112e. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.
Subclause 9.5.4 [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Suggested resolution [SUPERSEDED]: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 9.5.4 [dcl.fct.def.coroutine] paragraph 7 as follows:
The expression promise.get_return_object() is used to initialize thereturned reference or prvalue result object of a call to a coroutine . The call to get_return_objectis sequenced before the call to initial-suspend and 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 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.